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Introduction to Visual Otthymo

This guide explores Visual Otthymo and its features


Welcome to Visual OTTHYMO v6.0 (VO6), the sixth version of the INTERHYMOOTTHYMO hydrologic model simulation software package designed for Microsoft Windows OS.

OTTHYMO is a successful hydrologic management model that has been used for various simulation analyses such as: Watershed Studies, Sub-watershed Studies, Master Drainage Plans, Functional Stormwater Management Plans, Site Plans, and Stormwater Management Pond Designs.


The manual is divided into chapters and does not necessarily have to be read from start to finish. Users that are familiar with previous releases of Visual OTTHYMO can probably learn how to navigate around the model on their own and need only refer to the guide for new additional features. The User’s Manual is organized as follows:


Chapter 1 – IntroductionThis chapter gives an introduction to the model including new features, how the Help System and documentation is organized, and how to install and uninstall the program.
Chapter 2 – Quick Start TutorialA tutorial to help new users to understand the basic steps to create and run a model.
Chapter 3 – Conceptual ModelAn explanation of the conceptual model used in Visual OTTHYMO and describes all hydrologic objects.
Chapter 4 – Visual OTTHYMO Main WindowA description of the main interface and some windows are explained.
Chapter 5 – Working with Projects and ScenariosDescription of how to manage the concept of projects and scenarios in VO.
Chapter 6 – Working with CanvasThe usage of the canvas to create a model in Schematic View.
Chapter 7 – Working with the MapDescribes the usage of the map to create a model in Map View.
Chapter 8 – Working with Resource LibraryThe Resource Library’s concept and usage is described as it is the hub for climate data.
Chapter 9 – Running a SimulationInstructs users to change simulation engine parameter and then create and run simulations.
Chapter 10 – Working with OutputGuides users to view simulation outputs with various tools.
Chapter 11 – Visual OTTHYMO FilesCovers and reviews all the files used in Visual OTTHYMO including importing from previous versions.
Chapter 11 – TroubleshootingInstructs users through some common troubleshooting situations.
Chapter 12 – Appendix A – Useful ToolsIntroduces to and guides users some useful tools.
Chapter 13 – ConclusionConclusion of the manual and contact information.

VO Help Support

VO has a comprehensive Help System and supporting documentation that will assist both beginners and advanced users. The primary goal in designing this Help System was to empower users with the tools and information so that almost every question can be answered in a timely manner, without having to call technical support. Should a question arise that is not addressed in the user manual, please contact technical support at


Two separate documents are accessible for VO, a User’s Manual and a Reference Guide. This current document, the User’s Manual, contains information on how to use the program and complete descriptions of all features. This manual does not concern the background theory. The Reference Guide contains all of the hydrologic theory behind the program and gives guidance for users to select or measure object parameters. The history of the development of the model is also addressed for advanced users who need to know “why” and from “where”.

Context-Sensitive Help (F1)

Many parts of VO are context-sensitive. Context-sensitive means you can get Help on these parts directly without having to go through the Help menu. For example, to get Help on Resource Library in VO, press F1 while Resource Library is opened. You can press F1 from any context-sensitive part of the VO interface to display Help information about that part.

Seminars and Workshops

Smart Water City Inc. hosts seminars and workshops that allow users the opportunity to learn the basics of VO and use all its features to their full potential. Seminars and workshops are organized by need. You can find more information from our website.

Customer Support

Users requiring support should first consult the User’s Manual and Reference Manual to answer their question. Should a question not be addressed, or further assistance is required, users should contact Smart City Water’s VO Technical Support or +1 (905) 417-9792. Live technical support is also available for all registered users regarding program installation and troubleshooting. A nominal fee will be charged to users requiring technical support pertaining to the use of the model in an engineering application.

Software License Agreement

Visual OTTHYMO is a complex computational software program (Visual OTTHYMO Software Program) for stormwater management owned by Smart City Water Inc.

Although the Visual OTTHYMO Software Program has been thoroughly tested by Smart City Water Inc. and has endeavoured to make this program error free, this program is not and cannot be warranted as infallible and there remains the possibility of program errors. Further, the Visual OTTHYMO Software Program is complex requiring professional engineering expertise and professional engineering judgment to input information and to interpret the information generated by the program. Therefore, Smart City Water Inc. can make no warranty either implicit or explicit as to the correct performance or accuracy of the Visual OTTHYMO Software Program to process or implement the information supplied.

As a result, Smart City Water Inc. disclaims all liability including, without limitation, special, collateral, incidental or consequential damages in connection with or arising out of the purchase and use of the Visual OTTHYMO Software Program.

Smart City Water Inc. reserves the right, from time to time, to revise and improve its documentation, program and software as they may deem necessary. The information in this program describes the state of the software at the time of its publication. It may not, however, accurately reflect the state of future revisions to the software.

You should read carefully the following terms and conditions before continuing with the installation and use of the Visual OTTHYMO software for stormwater management (the “software”). By installing the software, you are agreeing to be bound by the terms and conditions of this license. If you do not agree to the terms of this license, please permanently remove all copies of the installation (with all the original contents) to the place of purchase for a full refund within 10 days of purchase.

The software and the related documentation are licensed to you by Smart City Water Inc. (“LICENSOR”) as owner and also as distributor (“DISTRIBUTOR”). You will own the media on which the Software is stored and provided to you herewith, but LICENSOR retains all rights, including the copyright, in the Software and the related documentation. You may install and maintain the Software (the “Installed Copy”) on either a: (i) single computer for use by one person at a time (without sharing); or (ii) network server for use on an internal network, provided that the number of users concurrently using or sharing the Software does not exceed the number of valid licenses of the Software you have purchased from the LICENSOR. You may not assign or otherwise transfer any of your rights under this License to any third party. YOU AGREE TO ENSURE THAT ANYONE WHO USES THE SOFTWARE DOES SO ONLY FOR YOUR AUTHORIZED USE AND COMPLIES WITH THE TERMS OF THIS AGREEMENT.


3. TERM:
This License is effective until terminated. You may terminate this License at any time by destroying all copies (in any format and including the Installed Copy) of the Software and related documentation. This License will terminate immediately, without notice from LICENSOR, if you fail to comply with any provision of this License. Upon termination, you must destroy all copies (in any format and including the Installed Copy) of the Software and related documentation, and you must notify LICENSOR in writing that all such copies have been destroyed.

LICENSOR warrants that the disk(s) or compact disc(s) provided to you by LICENSOR on which the Software is stored, shall be free from defects in materials and workmanship under normal use for ninety (90) days from the date of delivery to you.

You expressly acknowledge and agree that use of the Software is at your sole risk. Although the SOFTWARE has been thoroughly tested and LICENSOR has endeavored to make this program error free, the SOFTWARE is not and can not be warranted as infallible, and there remains the possibility of program errors. Further, the SOFTWARE is complex, requiring professional engineering expertise and professional engineering judgment to input information into the SOFTWARE and to interpret the information generated thereby. Therefore, LICENSOR and DISTRIBUTOR can make no warranty either implicit or explicit as to the correct performance or accuracy of the SOFTWARE to process or implement the information required. THE SOFTWARE AND RELATED DOCUMENTATION ARE PROVIDED “AS IS” AND WITHOUT WARRANTY OF ANY KINDEXPRESSED OR IMPLIEDINCLUDINGBUT NOT LIMITED TO, ANY IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSELICENSOR AND DISTRIBUTOR DO NOT WARRANT THAT THE SOFTWARE WILL MEET YOUR REQUIREMENTS, OR THAT THE OPERATION OF THE SOFTWARE WILL BE INTERRUPTED OR ERRORFREE, OR THAT DEFECTS IN THE SOFTWARE WILL BE CORRECTED. Furthermore, LICENSOR and DISTRIBUTOR do not warrant or make any representations regarding the use or the results of the use of the Software or related materials in terms of their correctness, accuracy, reliability or otherwise.

No oral or written information or advice given by LICENSOR or DISTRIBUTOR shall create a warranty or in any way increase the scope of the warranty contained in this License

Upon the expiration of the initial one-year maintenance period, the Licensee agrees to be charged for maintenance support in the amount of the LICENSOR’s regular list price for maintenance and support for the SOFTWARE as published from time to time by LICENSOR. Licensee shall notify LICENSOR in writing if it decides to decline mandatory maintenance. If Licensee fails to renew maintenance and later elects to receive it, LICENSOR reserves the right to charge Licensee its maintenance fees for the period(s) of the lapsed maintenance. Should the Licensee allow a lapse in maintenance, the Licensee forfeits access to technical support, updates and upgrades that may be available for the SOFTWARELICENSOR may elect to discontinue maintenance at any time upon written notice to Licensee.


This License shall be governed by and construed in accordance with the laws of the province of Ontario and adjudicated in a court of that province. If, for any reason, a court of competent jurisdiction finds any provision of the License, or portion thereof, to be unenforceable, that provision of the License shall be enforced to the maximum extent permissible in order to effect the intention of the parties, and the remainder of this License shall continue in full force and effect.

This License constitutes the entire agreement between the parties with respect to the use of the Software and related materials, and supersedes all prior or contemporaneous understandings or agreements, written or oral, regarding such subject matter.

Visual OTTHYMO Software may include software under license from third parties (“Third Party Software” and “Third Party License”). Any Third Party Software is licensed to you is subject to the terms and conditions of the corresponding Third Party License. The Third Party License(s) is located in the license.txt file. Please contact Visual OTTHYMO support if you cannot find a Third Party License.

Purchase of Visual OTTHYMO Software program is country specific. License sharing is permissible within the country of purchase and not internationally. Separate licenses specific to the country will be required for the use of the program in multiple countries.


Hardware and Software Requirements

Before you install VO, make sure that your computer meets the minimum requirements listed below. The minimum system requirements for all versions of the program are given in Table 1-2.


Minimum Requirements: 
Operating system:Microsoft Windows XP SP3/ Vista / 7 (32 or 64-bit)
Processor:Intel Pentium 4 1.5 GHz
RAM:1.0 GB
Hard disk space:500 MB
Recommended Requirements: 
Operating system:Microsoft 7/10 (32 or 64-bit)
Processor:Intel Core i7 2.0 GHz
RAM:1.5 GB
Hard disk space:1.0 GB

Licensing System

VO has a simple yet effective cloud-based licensing protection system to ensure that users comply with the terms of their license agreement. As set out in the license agreement, VO may be installed in multiple computers, but only the computer securing a license from cloud will be able to run the application.

A customer portal is provided to track the usage of all licenses. For more information, please refer to Customer Portal for Cloud-based Licensing document.

Installing VO

Before installing VO, make sure that you have closed all other programs and that any virus protection software is disabled. To install VO on your computer, please follow the directions below.

Step 1: Download the installation file from VO Download page. The download link is updated once a new version is available.

Step 2: Click on the installation file and follow the instructions in the installation wizard as seen below.

Step 3: Accept the License Agreement

Step 4: By default, VO6 is installed on C:\Program Files (x86)\Visual OTTHYMO 6.0

Step 5: Setup will create a shortcut in the Start Menu folder

Step 6: Choose to Create a Desktop Icon AND UNCHECK USB Key License driver as we now utilize a cloud-based licensing system.

Step 7: Click Install

Step 8: To complete the installation, you need to restart your computer.

Step 9: To activate VO6, copy and paste VO cloud license file (vo.lic) into Smart City Water folder or Use Selected License button on the License Manager Window. The default location is C:\Program Files (x86)\Visual OTTHYMO 6.0. 

Running VO

To start VO, simply double-click on the VO desktop icon  or find the VO item from your Start menu. Once VO starts, you will first see the splash screen as below

Then, you will see the main window.

Uninstall VO

You may be required to uninstall VO in the future. The following procedure should be followed to uninstall VO from your system:

  • Launch Control Panel and double-click Add/Remove Programs.
  • Scroll down the list until you find VO.
  • Select item and click OK button.
  • Re-boot computer.


What's New in Version 6.0

For Visual OTTHYMO 6.0 (VO) we have added low impact development (LID), Water Quality calculations to the model, in-built TatukGIS map, Compound Channel, Erosion Index as well as various bug fixes. LID design tools are built to assist engineers in meeting the requirements laid out by the MOECP and GTA Conservation Authorities/Municipalities. Changes have been made to adapt to continuous simulations as summarized below.

  1. Eight (8) different LID commands has been added to have been added in Version 6.0. These commands allow users to model both large-scale planning studies and complete detailed LID designs. The new LIDs are:
  • -Bioretention systems
  • -Rain gardens
  • -Permeable pavement
  • -Underground storage chambers
  • -Soakaway pits
  • -Greenroof
  • -Enhanced Swales
  • -Filter
  1. Water quality calculations have been added in Version 6.0. Total suspended solids (TSS) and total phosphorus (TP) removal rates can be determined by the model. The user can input loading rates based on existing or proposed land use.
  1. VO 6.0 now has an in-built TatukGIS map that will require no separate license.
  1. Compound Channel command has been added to provide flow analysis for main channel and floodplains. Users can now control the number of lines in the travel timetable.
  1. Assess the impact of your treatment train design by viewing removal efficiency in the model schematic.
  1. Erosion index calculations are added in VO6 and users can now determine erosion threshold flow rate as targets to assess pre and post development conditions.
  1. Water balance calculations can be generated throughout the model, allowing users to determine how individual LIDs are performing, review groups of LIDs or investigate the impact of the LIDs on the downstream watercourse.
  1. LIDs assessment includes key information review agencies require in the detailed output. For example, the model will calculate; drawdown time, volume reduction, TSS and TP removal rates and maximum storage used.
  1. GIS interface allows users to automatically assign water quality parameters allowing automated calculation of loading rates.
  1. Bug fixes include:
  • -Fixed problems with copying and pasting groups of catchments and other commands,
  • -Duplicating scenarios,
  • -Time-step issues when adding to project manager and,
  • -Improvements to the plot calibration window.

What's New in Version 6.1

For Visual OTTHYMO 6.1 (VO) we have added scenario comparison to single-event and continuous models., upgraded continuous modelling by including GIS tool and hydrograph commands of ScsHyd and NasHyd, improved LID package, expanded the data exporting/importing function between project manager and resource library, as well as various bug fixes.

  1. Single-event GIS tools are now available in Continuous modelling.
  1. Added ScsHyd and WilHyd commands in Continuous modelling.
  1. LID chamber: add Triton and new “Other” company in the list of LID chambers
  1. Export storm data from Project Manager to Resource Library.
  1. Add scenario comparison for single-event models. Now, this tool is available for both single-event and continuous models. For more details, please refer to the section of Scenario Comparison. You can also learn how to use it by looking into tutorials
  1. Create online documentations: User’s Manual (this current document), Reference Guide and Tutorials.
  1. Bug fixes include:
  • -Fixed error when importing VO3 model.
  • -Fixes for groudwater group in Continuous model.
  • -Fixes for DuHyd label in Single-event model.
  • -Fixes in rainfall intensity for manual input design storm.
  • -Fixes to existing RouteChannel in certain scenarios and new RouteChannel
  • -Fixes to the time step for a file-based design storm exported from Project Manager to Resource Library;
  • -Fixes to saving changes in Resource Library

What's New in Version 6.2

The biggest change of VO6.2 from VO6.1 is to include another engine VO-SWMM. Therefore, with one installer and platform, user can have access to two engines: OTTHYMO engine and VO-SWMM engine. For more details, please refer to VO-SWMM Manual and VO-SWMM Tutorials.


Example Study Area

In this tutorial, we will create a single-event VO model for the watershed shown below. It has two urban catchments (1003 and 1005) and three rural catchments (1001, 1002 and 1004). We will then run the simulation with 2-100yr design storms. The model is then converted to a continuous model to run the simulation with 10-year precipitation and temperature data.

Project Setup for Single-Event Model

To create a single-event OTTHYMO project, select File -> New Project -> New Otthymo Project.

To have a reference to place the hydrologic objects, a background picture can be added to the canvas. To add the background, switch to Schematic View and choose Background -> Change Background … from the context menu. Change the position of the background with mouse.

Creating Drainage Network on Canvas

All available hydrologic objects are list in Tool Box on the left. To add one hydrologic object on canvas in Schematic View, drag and drop it onto the canvas. Then it can be moved to any location.

The study area has two urban catchments, three rural catchments, three channels and two con-fluence points. They can be modeled with StandHyd NasHyd RouteChannel  and AddHyd  respectively. Drag and drop them from Tool Box to Canvas and move them to the proper locations based on the background. At this point, your canvas should look like the one shown below. Note that the ID of each command is labeled at the bottom and a few hydrologic object icons have red outlines indicating errors.

Then the drainage system components (hydrologic objects) will be connected to form a connected system. The connection between hydrologic objects tells where the flow come from and where the flow will go. On canvas, it’s represented by an arrow line pointing from the source to the destination.

In the example study area, the flow generated at catchment 1003 flows to channel 2002. The relationship is represented by a connection (or link) from StandHyd 1003 to RouteChannel 2002. To create this connection, move the cursor on top of 1003. Notice that the curve changes to a cross. Then hold the left mouse button, move to 2002 and release the left mouse button.

The connection between other hydrologic objects can also be created. At this point your canvas should look like that shown below. Note that the red outline disappears.

Setting Hydrologic Object Properties

Default parameter values are used for newly created hydrologic objects, which may need to be changed to represent the working project. Parameters can be edited with the Properties window or the Parameters Tables window.

The Properties window is used to edit the parameters of selected objects. If more than one objects are selected, only the common parameters are editable and changes will be applied to all selected objects.

The Parameter Tables window shows all parameters of each type of objects in a table. The table can be sorted by any columns. Besides editing single parameter value, data can be copied from a spreadsheet software as along as the columns are in the same order.

To edit the value for a property, select the hydrologic object(s) on the canvas and find the property in Properties window or Parameter Tables window. Type in the new value in the text box or select proper options from the combo box.

Adding Design Storm

The design storm is added from Resource Library to Project Manager and then used in the simulation.

Resource Library

The Resource Library is a library of climate data including design storm and long-term measured precipitation data. The climate data for the model simulation should be first added to the Resource Library before it can be used in model simulation.

To open the Resource Library, click Resource Library button  in Simulation tab. Some de-sign storms and reginal storms used in TRCA are shipped with VO, which can be a good starting point. If the required climate data is not available in the library, it can be added from different sources.

Project Manager

Project Manager is where the scenarios and climate data are managed. It’s located at the right side of the main window.

Adding Design Storm from Resource Library to Project Manager

To add a design storm to the project, drag and drop the design storm node from Library Explorer to the Rain Data section in Project Manager. A new rain group will be added in Project Manager. Design storms for other return periods can be added by the same method. Note that design storms of different return periods should be added to different rain groups.

Running Single-Event Simulations

A simulation is to apply a rainfall to a drainage network to calculate hydrograph. A simulation run can be created by combining the rainfall (design storm) and drainage network (scenario).

To create a simulation, click the Run button  located at the Simulation tab to open the Batch Run window. A default simulation has been created with the default rain group (a Chicago design storm). The simulation can be renamed and changed to use another rain group. New simulation can be added by clicking the Add button  in the toolbar.

To run simulations, check the simulations in the first column and then click the Run button at the bottom. A window will appear to show the simulation run progress. The Batch Run window will be closed after the simulation run is finished.

Viewing Single-Event Simulation Outputs

The main output from a single-event simulation is hydrograph. The hydrographs can be displayed in graph, table and summary.


To plot hydrographs with rainfall, select the hydrologic objects and then click the Hydrograph button  in Simulation tab. The Hydrograph window will appear. The appearance of the plot can be changed using the control panel on the left.


To view the hydrograph data in a table, click the Flow Data button  in Simulation tab. The data can be exported to a file.


The summary of a hydrograph includes drainage area (AREA), peak flow (PKFW), time to peak (TP), runoff volume (RV) and dry weather flow (DWF). To view the summary of all hydrographs, use the Hydrograph Result window located at the bottom.

The summaries can be labeled on canvas beside the hydrologic objects as shown in the figure below. Refer to section 7.7 to apply labels onto the canvas.


The classic OTTHYMO detail and summary output is available through the Detail Output  and Summary Output  button.

Converting to Continuous OTTHYMO Model

The single-event OTTHYMO model can be converted to continuous OTTHYMO model to run continuous simulation. To do this, first create a Continuous OTTHYMO project using menu File -> New Project -> New Continuous Otthymo Project.

Then the single-event model can be imported to the Continuous project using the menu File -> Import -> Import VH Scenario (Current Project). Extra parameters (e.g. land cover and soil parameters) are added to hydrologic objects to enable continuous simulation.

Adding Long-term Precipitation and Temperature

Same as design storm, long-term precipitation and temperature can be added from Resource Library to Project Manager by drag-and-drop.

Running Continuous Simulations

Besides rainfall data, a continuous simulation can also use temperature and evaporation data. It’s also required to setup the starting and ending date.

To create a continuous simulation, click the Run button  located at the Simulation tab to open the Batch Run window. And then click the Add button  in the toolbar to create a simulation. If precipitation and/or temperature is available in Project Manager, it will be automatically selected in the new simulation. The starting and ending date is also automatically based on the precipitation and temperature data.

To run simulations, check the simulations in the first column and then click the Run button at the bottom. A window will appear to show the simulation run progress. The Batch Run window will be closed after the simulation run is finished.

Note that the default time step for a continuous simulation is 5 minutes. The simulation run may take a while when the climate data covers long time period. To use a longer time step, change it in the Simulation Engine window (Engine Options  button in Simulation tab).

Viewing Continuous Simulation Outputs

The continuous simulation models the water balance in snowpack and active soil zone. All the water balance components are available as time-series data from the outputs. Similar to hydro-graph summary, these water balance components are also summarized to help get the big picture.

Time Series

There are two ways to plot the time series data. The Hydrograph  button is similar to the one for single-event simulation, which will open the Hydrograph window plotting flow versus precipitation.

Another tool is Plot Results , which will plot all available water balance components from a hydrological object. The time series data can be plotted with the original time interval or with higher values (year, month and week).


The average annual summary of the water balance components is shown in the Water Balance Results window located at the bottom.

To view the yearly and monthly summary for each catchment, choose Water Balance from the canvas context menu. The Water Balance window will appear.

The average annual summaries can also be labeled on canvas.



Visual OTTHYMO models flows generated from rainfall (or snow melt) on a drainage system. The drainage system first receives water from rainfall or snow melt and transform it to flow. The flow is then routed from upstream to the outlet. Structures may exist to 1) merge multiple flows together or 2) split one single flow to multiple parts.
Visual OTTHYMO conceptualized the drainage system as a collection of hydrologic processes. A hydrologic process is a unit process to 1) generate flow from rainfall, 2) route flow, 3) merge flow or 4) split flow. Each hydrologic process is modeled with a Hydrologic Object, a visual object represented with an icon on canvas or a feature on map. The hydrologic objects are then connected to simulate the sequence of hydrologic processes to simulate the whole drainage system.

An example of the conceptual model is given in the figure above. The drainage system consists of five (5) catchments, three (3) channels and two (2) confluence points. The system can be broken down into five catchments, three channels and two confluence points where catchments transform the rainfall to flow, channels route flow, and confluence points merge flow. Each of the components is a hydrologic process and can be represented by a Hydrologic Object (the icon at the right bottom corner of each component). The drainage system can be simulated with these hydrologic objects in Visual OTTHYMO by creating the hydrologic objects first, and then linking them as a connected system.

The behavior of a hydrologic process (e.g. flow generation) may be different. The behavior can be characterized with parameters (e.g. area and slope) and algorithms (e.g. different unit hydrograph). Visual OTTHYMO uses different Hydrologic Objects for different algorithms of same hydrologic process. For example, the flow from a catchment can be calculated using the Nash unit hydrograph or William unit hydrograph. So two different types of hydrologic objects, NasHyd and WilHyd, are provided in Visual OTTHYMO. As different algorithms may require different parameters, the parameters of different hydrologic objects will also be different.

Visual OTTHYMO Hydrologic Objects Overview

The drainage system component and the corresponding Visual OTTHYMO hydrologic objects are listed in Table 3-1. The hydrologic processes modelled are given below each hydrologic object, which are 1) flow generation, 2) flow routing, 3) flow separation and 4) flow merging. Each hydrologic object is represented with a unique icon.

1) Flow Generation Process: Generates flow from rainfall or snow melt on catchments. Flows from rural and urban catchments significantly differ due to decreased infiltration caused by urbanization. With the same amount of rainfall, the hydrograph from an urban catchment has larger and earlier peak flows and more runoff volume. Visual OTTHYMO provides one hydrologic object for urban catchments and three hydrologic objects for rural catchments. Often rural hydrologic objects in existing condition need to be converted to urban hydrologic objects for post-development condition.

2) Flow Routing Process: Routes flow through a certain structure. The hydrograph is usually changed (delay and attenuation). The structures supported are channels, reservoirs (ponds), and pipes. Ponds are important as they are usually required for a new development to control the flow to the allowable rates. Visual OTTHYMO can help size the ponds by determining the rating curve (storage-discharge relationship).

3) Flow Separation: Separates flow to multiple receiving structures such as flow diversion or catch basin. The latter is commonly used in new developments to have part of the runoff flow into the sewer system. In the planning stage, the number of catch basins can be estimated and Visual OTTHYMO can model it with minimal parameters.

4) Flow Merging: Merges flow from different sources to one single flow which typically happens at the confluence points. The outlet of the study area is usually a confluence point.


Drainage System ComponentVO Hydrologic ObjectDrainage System ComponentVO Hydrologic Object
Urban Catchment
StandHyd (Flow Generation)
Channel, River



CompoundChannel (Flow Routing)
Rural Catchment


ScsHyd (Flow Generation)
Reservoir, SWM Pond
RoutePipe (Flow Routing)
AddHyd (Flow Merging)
Flow Diversion
DiverHyd (Flow Separation)
Catch basin
DuHyd (Flow Separation)
RouteWetland (Flow Routing) Only available in continuous mode
Soakaway Pit
SoakawayPit (LID)
Underground Chambers
Underground Chambers (LID)
Rain Garden (LID)
Permeable Pavement (LID)
Enhanced Swales (LID)
Bio retention (LID)
Green Roof (LID)

Note that only NasHyd, StandHyd, AddHyd, RouteChannel, RouteReservoir, Route Wetland, SoakawayPit, Chambers, Pavement, Rain Garden, Green roof, Filter, Bio-retention and Enhanced Swales are available in continuous mode.

Common Parameters

Some parameters are available for all or most of the hydrologic objects. These parameters are given in Table 3-2.


Parameter NameDescriptionDefault Value ApplicableHydrologic Object
NHYDHydrograph numberNext available numberALL
NAMEA descriptive nameHydrograph object type name and NHYDALL
COMMENTS 1Any text descriptionEmptyALL
COMMENTS 2Any text descriptionEmptyALL
COMMENTS 3Any text descriptionEmptyALL
OUTLETThe NHYD of downstream hydrologic objectEmptyALL except ShiftHyd and DuHyd
DWFA constant Dry Weather Flow or baseflow (m³/s or ft³/s)0All flow generation hydrologic objects
DTSimulation time step increment (min).5ALL except AddHyd, ShiftHyd, DiverHyd, DuHyd and ReadHyd
AREACatchment area (ha or acre)10All flow generation hydrologic objects and StoreHyd
STORM INDEXThe index of the rainfall in the rain group1All flow generation hydrologic objects
RAINOptional list of rainfall intersities (mm/hr or in/hr) entered at the time steps equals to DT. If the list is not given, the model will use the rainfall data assigned by STORM INDEX.EmptyAll flow generation hydrologic objects



StandHyd is used to simulate runoff flows from urban watersheds. Two parallel standard instantaneous unit hydrographs are used to convolute the effective rainfall intensity over the pervious and impervious surfaces.

The losses over the pervious surfaces are calculated by one of three methods: 1) Horton’s soil infiltration equation; 2) SCS modified CN procedure; or 3) Proportional Loss Coefficient.

A baseflow can also be added to the total simulated hydrograph.

To obtain adequate results, the hydrologic object should be applied to areas with impervious ratios larger than 20%. For smaller impervious ratios, the watershed should be sub-divided into urban and rural basins.


Parameter NameDescriptionDefault Value
TIMPRatio of total impervious area. The value must be in the range of 0 to 1 and greater than or equal to XIMP.0.50
XIMPRatio of total area directly connected impervious areas are those form a continuous pathway from the point of runoff generation to the outlet point. For example, the area directly connected to the sewer system. The value must be in the range of 0 and 1.0.35
LOSSRainfall loss method to be applied to the pervious area. It can be Modified SCS Curve MethodHorton’s Equation or Proportional Loss Method.Modified SCS Curve Method
CNSoil’s SCS or Modified Curve Number for the pervious area. Available when LOSS is set to Modified SCS Curve Method.85
IAInitial Abstraction (mm or in). Available when LOSS is set to Modified SCS Curve Method or Proportional Loss Method.1.5
FoInitial infiltration rate (mm/hr or in/hr). Available when LOSS is set to Horton’s Equation.50
FcFinal infiltration rate (mm/hr or in/hr). Available when LOSS is set to Horton’s Equation.7.5
DCAYDecay constant (1/hr). Available when LOSS is set to Horton’s Equation.2
FAccumulated moisture in the soil at the beginning of the storm (mm or in). Available when LOSS is set to Horton’s Equation.0
DPSPDepression storage available over the pervious area (mm or in). Available when LOSS is set to Horton’s Equation.1.5
CProportional loss coefficient ration (between 0 and 1). Available when LOSS is set to Proportional Loss Method.0.5
SLPPAverage slope of the pervious area (%). Value must be greater than 0.0.2
LGPOverland flow length of the pervious area (m or ft)40
MNPManning’s roughness coefficient for pervious surfaces. Note that coefficient should be selected based on sheet flow, not channel flow.0.25
SCPStorage coefficient for the linear reservoir of the pervious area (hr). Enter 0 to allow the program to internally select the value.0
DPSIAvailable depression storage over the impervious area (mm or in).1
SLPIAverage slope of impervious area (%)1
LGI TypeLGI calculation method. It can be Auto and ManualAuto will calculate the LGI from AREA assuming AREA = Manual will read the LGI value from user input.Auto
LGIThe overland flow length of impervious area (m or ft)Calculated from AREA
MNIManning’s roughness coefficient for pervious surfaces. Note that coefficient should be selected based on channel flow (i.e. sewer and/or road flow).0.013
SCIStorage coefficient for the linear reservoir of the impervious area (hr). Enter 0 to allow the program to internally select the value.0


NasHyd is used to simulate runoff flows with Nash instantaneous unit hydrograph. This hydro-graph is made of cascade of “N” linear reservoirs. The command is mainly used for rural areas but can also be used for very large urban watersheds and to simulate the effects of infiltration/inflow in sanitary sewers. Rainfall losses can be computed by SCS Modified CN Procedure or Proportional Loss Coefficient.


Parameter NameDescriptionDefault Value
CNSCS Modified Curve Number or Proportional Loss Coefficient (if negative value between 0 and -1 entered).80
IAInitial abstraction (mm or in). If IA is negative, the program uses the SCS method where IA = 0.2 × S and S is a function of Curve Number.5
NNumber of linear reservoirs used for the derivations of Nash Unit Hydrograph.3
TPUnit Hydrograph time to peak (hr). It is approximately equal to (N-1)/N × TC where TC is the Time of Concentration.0.2


WilHyd is used to simulate hydrographs from rural watersheds with long recession periods. The program uses the Williams and Hann’s unit hydrographs developed in the original HYMO program and the Modified SCS Curve Number Procedure to calculate the rainfall losses.


Parameter NameDescriptionDefault Value
AA/DWFPrintout parameter or if less than 0, to enter a constant Dry Weather Flow or baseflow (m³/s or ft³/s). If AA is positive, the unit hydro-graph will be printed. If AA is 0, neither happens.0
BBPrintout parameter. If BB is positive the rainfall excess ordinates will be printed. If BB is 0, excess ordinates will not be printed.3
CNSCS Modified Curve Number80
IAInitial abstraction (mm or in). If IA is negative, the program uses the SCS method where IA = 0.2 × S and S is a function of Curve Num-ber.5
KRecession constant in the William and Hann unit hydrograph equation (hr)4
TPUnit hydrograph time to peak (hr). The time step DT should be smaller than TP.0.2


ScsHyd is essentially the same as the NasHyd with the exception that it uses parameters for the SCS Procedure (i.e. initial abstraction is a function of the SCS Curve Number, and the number of linear reservoir “N” is set to 5). This command can be used when the SCS procedure is required by agencies or for comparison with other options.


Parameter NameDescriptionDefault Value
CNSCS Modified Curve Number. The initial abstraction is calculated as 0.2 × S, and S is a function of Curve Number.80
TPUnit Hydrograph time to peak (hr). It is approximately equal to (N-1)/N × TC where TC is the Time of Concentration. DT should be smaller than TP.0.2



RouteChannel is used to route hydrographs through typical channel cross-sections using the Variable Storage Coefficient (VSC) Method. The open channel cross-sections are described with X and Y co-ordinates. The COMPOUNDCHANNEL command in VO6 has been split to calculate channel routing in both a “low flow channel” and “floodplain” portions of the cross section with the routing effects then being combined for the final output. The user can define channel and flood-plain lengths and slopes.

The ROUTECHANNEL and COMPOUNDCHANNEL have an option for Erosion Index calculations. Users could determine erosion threshold flow rate as targets to assess pre and post development conditions.

Other inputs are the average longitudinal slope and the variation of Manning’s roughness coefficient across the width. The hydrologic object computes a rating curve and travel time prior to routing with the VSC method.

Parameter NameDescriptionDefault Value
CHLGTHLength of channel reach (m or ft)500
FPLGTHLength of floodplain reach (m or ft)500
CHSLOPEAverage longitudinal channel slope (%)0.2
FPSLOPEAverage flood plain slope (%)0.2
VSNValley Section Number used for identification and printing purposes.1.1
NSEGNumber of segments in the channel cross-section with constant Manning’s roughness coefficients.
The channel and floodplain sections of the cross section are defined here. A maximum of six
across the section are permitted. NOTE: The Manning’s roughness coefficient that describes
the main channel must be entered as a negative (e.g. –0.025)
ROUGHSEGDISTPaired values describing the roughness over the segment distance (X co-ordinate).
Each roughness value, ROUGH, is applied over the distance specified by SEGDIST which should
also be one of the distance co-ordinates found in DIST/ELEVSEGDIST has units (m or ft).
1.5, 0.050
4.5, -0.03
6.5, 0.050
DIST/ELEVCo-ordinates describing the shape of the cross section as (X, Y). A maximum of 100 points can be
entered. Units area (mm or ft).
0.0, 101.5
1.0, 100.7
1.5, 100.5
2.0, 99.50
3.5, 99.60
4.5, 100.65
6.0, 101.45
DividerNumber of lines in travel time table1
Critical flow (m³/s)Critical flow to calculate erosion index0.1


MuskingumCunge is used to route hydrographs through typical channel cross-sections using the Muskingum-Cunge routing method. This method is based on the continuity equation and the storage-discharge relation. The open channel cross-section is described with X and Y co-ordinates. Other inputs are the average longitudinal slope, the variation of Manning’s roughness coefficient across the width and a constant, and Beta of the stage-discharge curve which is also a function of the kinematic wave celerity.

MuskingumCunge shares same parameters as RouteChannel except for BETABETA is a function of the kinematic wave celerity and is a constant of the stage-discharge curve. Beta is a reflection of the channel shape. Beta has an upper limit of 1.67 and a lower limit of 1. Beta equals 1.67 for natural and wide rectangular channels, 1.5 for trapezoidal channels, 1.33 for triangular channels, 1.5 for rectangular channels.


RoutePipe is used to route hydrographs in circular or rectangular pipes. It uses a simplified form of the RouteChannel input.

Only the pipe diameter or width and heights are required and only one Manning’s roughness coefficient is allowed.

The hydrologic object automatically resizes the pipe cross-section if the dimensions entered are not sufficient to accommodate the peak flow without surcharging.


Parameter NameDescriptionDefault Value
PIPEPipe identifier used for identification and printing purposes.1
PLENGTHThe length of the pipe (m or ft)500
ROUGHThe Manning’s roughness coefficient0.013
PSLOPEThe average slope of the pipe (m/m or ft/ft)0.005
TYPEThe pipe section type. It can be Circular or Rectangular.Circular
DIAMThe pipe diameter (mm or in). Used when TYPE is Circular.1650
WIDTHHEIGHTThe width and height of the pipe (mm or in). Use when TYPE is Rectangular.2400, 1200


RouteReservoir is used to route hydrographs through reservoirs using the Storage-Indication method.

RouteReservoir has only one parameter, the Discharge-Storage Curve (Rating Curve). It has pairs of discharge-storage values to describe the Discharge-Storage relationship of the reservoir (m³/s & ha.m. or ft³/s & ac.ft.). A maximum of 20 co-ordinates can be entered. The first set of co-ordinates must be 0,0. If the outlet to the pond is not at the bottom of the facility, the curve can have multiple 0 values for discharge as shown in the example below:

In this example the reservoir would fill up to a volume of 0.100 ha-m before discharging any run-off.

The RouteReservoir command has the option to model an overflow hydrograph if the discharge-storage curve is exceeded during a model run. Selecting this option will limit the outflow from the reservoir to the maximum discharge value and will generate an overflow hydrograph equal to in-flow minus maximum outflow. If the overflow option is not selected and the inflow exceeds the capacity of the reservoir, the Discharge-Storage Curve will be automatically extended to contain the inflow.



ShiftHyd is used as an alternate routing method when the peak flow attenuation expected is negligible. The command shifts the entire hydrograph forward to the nearest equal number of time steps specified by user-entered time shift, TLAG (min).


DuHyd is used to separate the major (street flow) and the minor (pipe flow) hydrographs from a total hydrograph.


Parameter NameDescriptionDefault Value
MajorNHYD of major system connectionEmpty
MinorNHYD of minor system connectionEmpty
CINLETThe peak flow capture rate per inlet (m³/s or ft³/s)0.06
NINLETThe number of inlets in the drainage system which have the capture rate of CINLET. Note: The maximum minor system capture equals CINLET × NINLET.10


RouteWetland command is used to route flows through a wetland. This command is only available for continuous modeling, which calculates the hydrologic cycle of a wetland. The wetland command includes a dry area (similar to a wetland) and a wet area (similar to a route reservoir).

Parameter NameDescriptionDefault Value
Storage Area Geometry  
Initial water Depth (m)Depth of water in the wetland at the start of a model run0.0m
Bottom Elevation (m)Elevation at the lowest point in the wetland10.0m
Depth Area CurveDepth area curve for the entire wetland (Dry and wet areas), Starts at the bottom elevation of the wetland0.000 100
0.500 100
1.000 100
1.500 100
Storage Area – Soil  
Soil Thickness (m)Thickness of the soil layer constraining movement between surface and ground water1.25m
Hydraulic Conductivity (mm/day)Saturated hydraulic conductivity for soils in areas with ponded water, represent the ease at which moisture can move through a soil in which all easily drained pore spec is filled with liquid10.0 mm/day
Groundwater ID 1
Fringe Area  
Soil TextureDescription of soil base on relative content of sand, silt, clay particlesClay Loam
Total PorosityFraction of soil that is made up of spaces (pores) be-tween particles0.464
Field CapacitySoil moisture held in soil after excess water has drained away0.310
Wilting PointMoisture left in dry soil that is not accessible to plants, causing them to wilt0.187
Saturated K (mm/day)Hydraulic conductivity of the soil in dry areas when saturated, represent the ease at which moisture can move through a soil in which all easily drained pore spec is filled with liquid24.38 mm/day
CNCurve number used for SCS68
IA (mm)Pervious Area Depression Storage10 mm
Land CoverGeneral description of vegetationCrops to shoulder height
kK = GI /Pan Evaporation – Growth index of a crop / Pan Evaporation1.4
VEGK3ET opportunity coefficient, used to calculate ET from soil6.0
TypeChoice of method for defining outlet (Currently only Stage Discharge is available)Stage Dis-charge
Discharge CurveDepth discharge curve for the wetland, depth is defined from the bottom elevation of the wetlandRefer to Error! Reference source not found.


DiverHyd can be used to simulate diversion channels and multi-outlet structures. By entering a table of inflow-outflow relationships the hydrologic object can split a hydrograph into a maximum number of five hydrographs. The five hydrographs must add up to the original inflow hydrographs.

The inflow-outflow relationship is defined with FLOW TABLE. As shown in Table 3 9, maximum 20 values can be defined for the inflow and each outflow. All outflows should add up to the total inflow.


Total Inflow1st Outflow2nd Outflow3rd Outflow4th Outflow5th Outflow
QTOTAL (1)Q1(1)Q2(1)Q3(1)Q4(1)Q5(1)
QTOTAL (2)Q1(2)Q2(2)Q3(2)Q4(2)Q5(2)
QTOTAL (20)Q1(20)Q2(20)Q3(20)Q4(20)Q5(20)

Flow Merging Hydrologic Objects

AddHyd is used to add any number of hydrographs. There is no parameter associated with AddHyd.



ReadHyd is used to read a previously saved hydrograph from a file. The parameter FILEPN is the file name of the saved hydrograph. 


StoreHyd is used to enter ordinates of a hydrograph directly.


Parameter NameDescriptionDefault Value
AREAThe watershed area from which the hydrograph was derived (ha or acre)30
HYD POINTSA list of hydrograph ordinates entered at time steps equal to DT. Up to 2000 values can be entered (m³/s or ft³/s)Empty


Common Parameters

LID parameters have been set based on the 2010 Low Impact Development Stormwater Management Planning and Design Guide. Certain parameters are available for most LIDs. These parameters are given in Table 3-11.


Parameter NameDescriptionDefault Value
WIDTHWidth of LID layer (m)1
LENGTHLength of LID layer (m)1
HEIGHTDepth of LID layer (m)1
POROSITYFraction of void space (pores) in the storage layer0.4
Min Drawdown TimeDrawdown time (hour)24
SOIL TEXTUREDescription of soil base on relative content of sand, silt, clay particlesLoam
INFILTRATION RATERate with which water moves through porous materials (m/hour)0.051
Total PorosityThe volumetric water content of a soil (volume of water per total volume) when its pore spaces are at saturated, vol/volOnly available for continuous when infiltration type is set as Green ampt.
FIELD CAPACITYThe amount of water a well-drained soil holds after free water has drained off, or the maximum amount it can hold against gravity, vol/volOnly available for continuous when infiltration type is set as Green ampt.
WILTING POINTThe soil moisture content at which plants can no longer obtain enough moisture to meet transpiration requirements, vol/volOnly available for continuous when infiltration type is set as Green ampt.
SATURATED KSaturated Hydraulic Conductivity (mm/day)Only available for continuous when infiltration type is set as Green ampt.
SUCTION HEADWetting Front Suction HeadOnly available for continuous when infiltration type is set as Green ampt.
TYPEOrifice Equation
DISCHARGE CURVEDepth discharge curve for the underdrain, depth is defined from the bottom of the storage layer. Users can enter 0 discharge up to the invert of the subdrain, if perched.
DIAMETERDiameter of underdrain pipe (mm)250
MANNING COEFFICIENTManning’s Roughness Coefficient0.013
LENGTHLength of underdrain (m)4
MAXIMUM FLOWMaximum flow allowed through underdrain (m³/s)
SLOPESlope of the underdrain (m/m)0.001
PERFORATIONS/ROWNumber of perforations per row of perforations (#)10
ORIFICE COEFFICIENTOrifice coefficient used in orifice equation0.63
MINIMUM SPACINGMinimum spacing between perforations (mm)25
DIAMETERDiameter of perforations (mm)25
NUMBER OF ROWSNumber of rows of perforations (#)4
SAFETY FACTORSafety or Clogging factor applied to subdrain outflow (#)1


Soakaway pits are excavations in the ground filled with clean granular stone or other void forming material that receives “clean” runoff designed to infiltrate into the native soil. Soakaway pits are designed with an overflow and an optional subdrain feature. Key features for Soakaway pit designs are highlighted in the diagram below.


Parameter NameDescriptionDefault Value
 Storage Layer 
POROSITYFraction of void space (pores) in the gravel storage layer0.4
Initial Water LevelDepth of water in the storage layer at the start of a model run0


Underground storage chambers are typically designed to store larger volumes of water and are often used for quantity control. Underground storage chambers have large void spaces and typically have open bottoms allowing the system to infiltrate into the surrounding soils. Currently ADS Storm Tech Chambers (all sizes) can be designed in VO. Users can input their design parameters and the program will automatically calculate how much storage is required and provided. Underground storage chambers are designed with an overflow, an outflow, and an optional subdrain feature. Key features for the underground storage chamber designs are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Chamber Selection 
NUMBER OF CHAMBERSNumber of individual storage chambers used in design900
BASE OF STONE ELEVATIONBase of stone/storage elevation100
DEPTH OF STONE BELOW CHAMBERSAmount of stone between the base of the systems and the in-vert of the storage chambers (mm)152
DEPTH OF STONE ABOVE CHAMBERSAmount of stone between obvert of the chamber and the top of the storage layer (mm)152
MIN BOTTOM AREABottom footprint area of the underground storage layer100
MAX STORAGE VOLUMEMaximum volume provided with current system design. Max storage is automatically calculated with the input parameters (m³)978.51
DEPTHSTORAGE RATING CURVEVolume provided with current system design. Storage volume is automatically calculated with the input parameters (m³)
Initial Water LevelDepth of water in the storage layer at the start of a model run0


Permeable pavement allows stormwater to drain through the surface layer into a stone reservoir. This is where water can be stored and then infiltrated into surrounding soils. Permeable pavement is designed with an overflow and an optional subdrain feature. Key features for permeable pavement designs are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Surface Area Ponding 
PERMEABILITYRate with which water moves through porous materials (m/hour)1
DEPTHAREA CURVEDepth area curve for surface ponding area, depth is defined from the bottom of the ponding layer.


Rain gardens generally function as a stormwater filter and infiltration practice which can be used to temporarily store, treat and infiltrate runoff. The LID is designed with an engineered soil layer to store and treat runoff which then infiltrates into surrounding native soil. Rain gardens are designed with an overflow and a native soil infiltration rate. Key features for rain garden designs are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Surface Area Ponding 
DEPTHAREA CURVEDepth area curve for surface ponding area, depth is defined from the bottom of the ponding layer
INFILTRATION RATE (m/hr)The rate at which water on ponding surface enters the engi-neered soil layer0.5
 Engineered Soil Layer 
TOTAL POROSITYFraction of soil that is made up of spaces (pores) between particles0.463
SEEPAGESaturated hydraulic conductivity for soils in areas with ponded water, represent the ease at which moisture can move through a soil (m/hour)0.013
SOIL MOISTUREInitial soil moisture content of the engineered soil layer. (fraction)0.3


Bioretention areas filter, detain and infiltrate stormwater runoff. Bioretention areas are known for their water quality, quantity and water balance benefits. The LID is designed with an engineered soil layer to store and treat runoff, a storage layer to detain larger volumes of water which then can infiltrate to surrounding native soil. Bioretention areas are designed with an overflow, optional subdrain, an outflow and a native soil infiltration rate. Key features for bioretention designs are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Surface Area Ponding 
DEPTHAREA CURVEDepth area curve for surface ponding area, depth is defined from the bottom of the ponding layer
 Mulch Layer 
DEPTHDepth of mulch layer (mm)0.3
POROSITYFraction of soil that is made up of spaces (pores) between particles0.4
 Engineered Soil Layer 
SOIL MOISTUREInitial soil moisture content of the engineered soil layer (fraction)0.3
INFILTRATIONRate with which water moves through porous material (m/hour)0.5
POROSITYFraction of soil that is made up of spaces (pores) between particles0.467
 Storage Layer 
DEPTHDepth of the storage layer (m)1
POROSITYFraction of storage layer that is made up of spaces (pores) between particles0.4


Enhanced grass swales are vegetated open channels designed to convey, treat and store stormwater runoff. Enhanced swales will calculate the volume of surface ponding and infiltration throughout the swale. Users have options for designing enhanced dry or wet swales. Enhanced dry swale are designed with an additional storage layer, and water could be infiltrated from surface and then seepage to a native soil layer. Otherwise, in wet swale, water could penetrate from surface ponding layer to native layer without a storage layer. Key features for swale designs are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Surface Area Ponding 
DEPTHMaximum depth of bioswale surface ponding area (m)0.3
CHECK DAM DEPTHThe depth of check dam0.3
WIDTHWidth of bioswale surface ponding area (m)1
LENGTHLength of bioswale surface ponding area (m)1
LEFT SIDE SLOPEHorizontal: Vertical left-side slope of bioswale (3:1)3
RIGHT SIDE SLOPEHorizontal: Vertical right-side slope of bioswale (3:1)4
SURFACE SLOPESlope in the direction of the flow path (m/m)2
 Storage Layer 
DEPTHDepth of the storage layer (m)1
POROSITYFraction of storage layer that is made up of spaces (pores) between particles0.4


Green roofs are covered with growing media and vegetation that enable rainfall infiltration and evapotranspiration of stored water. They are particularly cost-effective in dense urban areas where land values are high and on large industrial or office buildings where stormwater management costs are likely to be high. In VO6, users could have two options for designing green roof, including subsurface ponding and surface ponding types. The difference between the two types is water storing in different layers. In surface ponding types, users could set a parameter for a depth of ponding layer, while in subsurface type, a depth would be set in the storage layer. Both types have roof drains to convey storm water to drainage sewer or retaining area. Key features for green roof are shown in the diagram below.

Parameter NameDescriptionDefault Value
 Surface Area Ponding 
DEPTHAREA CURVEDepth area curve for surface ponding area, depth is defined from the bottom of the ponding layerFor subsurface type, users could set 0 for depth
 Engineered Soil Layer SOIL 
MOISTUREInitial soil moisture content of the engineered soil layer (frac-tion)0.3
INFILTRATIONRate with which water moves through porous material (m/hour)0.5
POROSITYFraction of soil that is made up of spaces (pores) between particles0.467
 Storage Layer 
DEPTHDepth of the storage layer (m)1
POROSITYFraction of storage layer that is made up of spaces (pores) between particles0.4


Filter systems is designed to remove the primary pollutants of concern from runoff and allows runoff to be treated close to its source without additional collection. Users could set a targeted treated flow rate with removal efficiencies in VO6. Key features for filter are shown in the diagram below.

Parameter NameDescriptionDefault Value
TREATED FLOW RATE (m³/s)Inflow flow rate smaller than targeted flow rate would be treated0.02
 Water Quality Removal Efficiency 
TSSTotal Suspended Solids removal percentage80
Tp(%)Total phosphorus removal percentage60


The interface has been designed to provide plenty of working space for the schematic and map model while maintaining easy access to the hydrologic objects and their associated parameters. The layout consists of various sections as explained in the text below. Most of the windows can be docked to selected location. In the case that a window is closed, it can be re-opened through the Windows drop-down list  in Home tab.

1) Toolbox: Gives the user access to all the hydrologic objects (e.g. Hydrographs, Routing Routines) for the respective project type. Each object is categorized for ease of access and is represented by an icon in the Toolbox.

2) Toolbar: Provides easy access to common program features found in all Windows pro-grams (e.g. New, Open, Save, etc), as well as VO’s own program features (e.g. Resource Library, Hydrograph, etc.). There can be up to three ribbon tabs (Home, GIS and Simulation).

3) Project Manager: Shows the user the names of all the hydrologic scenarios within the open project. This window also provides a simple way of modifying those scenarios (e.g. Add, Delete) and it holds the climate data used in the simulation.

4) Properties Window: Provides the user with the main form for inputting hydrologic object parameters (e.g. catchment area, slope, length). This window is where the bulk of the data entry takes place.

5) Map View: The geospatial representation of the same model. Each hydrological object is assigned to a geometry either from manual drawing or from existing GIS data. Hydrological objects of different type are in different layers. For more information, please refer to Chapter 7.

6) Schematic View: Where the user builds their model schematic from the hydrologic objects in the Toolbox. Objects are dragged from the Toolbox and dropped on the Designer Canvas. Links are generated by dragging the centre of a glyph to the centre of another. For more information, please refer to Chapter 6.

7) Parameter Tables: Lists all parameter values in tables. It provides a spreadsheet-like environment for data editing.

8) Hydrograph Results / Water Balance Results: This is where the simulation results are summarized. For single-event simulation, it has peak flow and runoff volume. For continuous simulation, average annual water balance components are summarized.

9) Error List: Violations are categorized to warnings and errors which are categorized on the Error List. The model is constantly checked for compliance with the established rules. Models with errors can’t run a simulation.


The following tables list the hydrologic objects from the Toolbox and their name. Hydrologic objects with bold font can be used in both single-event and continuous simulation. Users of previous versions of Visual OTTHYMO and OTTHYMO will recognize these commands. For a more detailed description of each command, refer to Chapter 3.


Generate Hydrograph Objects 
Route Hydrograph Objects 
Flow Manipulation Hydrograph Objects 
Manual Input Hydrograph Objects 


The following tables list the icons from the Toolbar and their name. There are three (3) tabs in total: Home, GIS and Simulation. A brief description of the contents of these tabs are given in Table 4-3 Table 4-5. For a more detailed description of each Toolbar item, please refer to the Help System within the program.


New Project
Creates a new project
Open Project
Opens an existing project
Save Project
Saves the current project
Save Project As
Saves the current project under a different name
Import scenarios
Export the current scenario
Copy to Clipboard
Copy select objects to clipboard
Print the canvas


New Project
Creates a new project
Deletes selection
Open Project
Opens an existing project
Removes the latest action
Save Project
Saves the current project
Repeats the last action
Save Project As
Saves the current project under a different name
Edit History
Displays list of actions performed by the modeler(s)
Copies the selection to the clip-board
Locates a specific object by ID or name
Extracts the selection to the clipboard
Provides dropdown for selection of windows to be displayed
Pastes copied or cut hydrologic objects
Accesses window to define gen-eral settings details such as Unit and Precision, etc.


Add Layer
Provides Dropdown to add an imported GIS layer,
Group Map layer or Base Map Layer into the Map View.
Find Feature
Locates a specific object or specific text in the Map.
Attribute Table
Accesses Attributes Table of selected feature.
Allows to pan around the map.
Zoom In
Zooms into the selected area.
Zoom Out
Zooms out from selected area.
Fixed Zoom In
Zooms in on the map at a preset scale, usually into
the center of the Map.
Fixed Zoom Out
Zooms out on the map at a preset scale usually from the center of the Map.
Last Extent
Brings the user back to the last extent view.
Next Extent
Brings back to current extent if the Last Extent was used to view previous extent.
Zoom Pan
Zoom in/out depending on panning up or down.
Full Extent
Shows the full extent of the map.
Select Feature
Select features on the Map.
Identifies the selected features and displays all the attributes of it.
Add Vertex
When in editing mode (double click a focused feature)
allows to add a new vertex along an existing polyline or polygon shape.
Remove Vertex
When in editing mode deletes any existing vertex from a project object
**Edit Tool*
Select features on map to edit
Add Link
Create the link between two hydrologic objects on map
Allows to snap to a point.
Allows to snap to a vertex when required
Allows to snap to the vertex.
Allows to snap to the intersection
Allows to snap along the edge of polyline or polygon.
Allows to snap to the end point of polyline or polygon.
Assign Geometry
Assign geometry to hydrologic objects
Calculate CN
Calculate CN based on soil and land use layer and assign to NasHyd or StandHyd
Calculate Area Weighted
Calculate the parameter values with given layer using area-weighted method
Calculate Landuse Percentage
Calculate the Area and Percent-age values with given layer using are-weighted method


Runs simulation
Displays hydrograph window
Detail Output
Displays the detail text output for selected objects
Summary Output
Displays the summary text output for selected objects
Cross Scenario Plot
Plots hydrograph for objects from different scenarios
Hydrograph Result
Display the hydrograph summary in a table
Flow Data
Displays the hydrograph time-series in a table
Resource Library
Displays the Resource Library window
Convert to CN *
Converts the CN to CN *
Batch Assign
Assigns parameter value of selected parameter to given values
Calibrate Commands
Changes parameter values with given percentage
Plot Calibration
Plots calculated and observed hydrograph
Export Channel Roughness
Export the channel roughness to a file
Export Channel Cross Sections
Export the channel cross sections to a file

Project Manager

The Project Manager is to manage the scenarios and climate data in the project. By default, it’s located on the right-hand side besides the Properties window.


On the top of Project Manager is the tool bar. The buttons are described in Table 4-6.


Add Scenario or Open Resource Library
Duplicate Selected Item
Delete Selected Item
Resource Library
Open Resource Library

Below the toolbar there is a tree view of scenarios and climate data. Items are represented with icons described in Table 4-7. Some of the sections would open the detailed information window by double-click, which is also described in Table 4-7.


Scenario SectionEvaporation Data Section
Open Scenario
Groundwater Data Section
Rain Data SectionEvaporation / Groundwater Group
Rain Group
Open Rain Group Viewer for Single-event Simulation
Evaporation / Groundwater Data
Open Data Viewer
Rain Data
Open Storm Viewer
Scenario Comparison
Temperature Data SectionFlow Data Section
Temperature GroupFlow Group
Temperature Data
Open Temperature Data Viewer
Flow Data
Open Flow Data Viewer
Water Quality SectionWater Quality Data
Open Water Quality Data Viewer

For example, the Rain Group Viewer as shown below will be opened by double-clicking on Rain Group section.

Context menus are also available in the tree view as shown in the figure below. If one menu item is not applied to current item, it will be greyed out. For example, the Set as Default Scenario menu is only applied to scenario. All the menus are described in Table 4-8.


MenuCommandApplicable Items
Set as Default ScenarioSet selected scenario as default scenario
Add…Add Scenario or Open Resource Library
Edit…Open Scenario or Data Viewer
RenameRename the selected item
DeleteDelete Selected Item
DuplicateDuplicate Selected Item


Properties window shows all properties of selected hydrologic object(s) or current scenario. To use Properties window:

  • Select single hydrologic object in Map View or Schematic View to view and edit all properties of the object;
  • Select multiple hydrologic objects in Map View or Schematic View to view and edit common properties;
  • Deselect all hydrologic objects to view and edit scenario properties.

Properties are usually grouped in different categories to be easily located.

Tooltip is given for each property to give more detailed information. This is useful for users that are not familiar with OTTHYMO model.

The search bar is located at the top. It will filter properties to only display those with given string.

Editing Property Value
The property value can be edited using the text box, combo box, or button at the right-hand side of the property name.

Editing Simple Property

For simple property, a text box is given to enter the new value. The change will take effect by using the ENTER key or switching to other properties.
The combo box is available for properties with limited options, e.g. LGI Type of StandHyd as shown in the figure below. The property value can be changed by choosing a different option from the list. Other property values may also be affected by the change.

Editing Collection Data

Some properties are a collection of data values, e.g. DIST/ELEV of RouteChannel and Rating Curve of RouteReservoir as shown in the figure below. A text box and a button are usually given to this type of property. The text box shows the number of rows in the collection data and is read-only. To change the collection data, click the button on the right to open the corresponding editor window.

The collection editor window usually has two parts. For the first part, the left is the data table which lists all the data records. The second part is the data plot is the one on the right. Data can be edited directly in the table or pasted from a spreadsheet software. Context menus are provided to help basic edit operations. Data can be copied and pasted by using CTRL+C and CTRL+V key respectfully. The data plot window will update automatically once the data value is changed in the table.

Editing Less Routine of STANDHYD

The LOSS property of StandHyd is to change the loss routine for pervious area. As different parameters are used in these routines, it needs to be edited in a separate editor. To edit the LOSS routine, click on the button to open the LOSS Editor where the LOSS routine type and corresponding parameters can be changed. This editor is similar to the Collection Data editor as it has the same text box and button format.

CN* Flag

When SCS equation is used to calculate the rainfall excess in pervious area, the Curve Number (CN) is the most important parameter. From the early research of OTTHYMO, it has been rec-ommended to use the modified CN, i.e. CN*. VO has provided a tool (Convert to ) to convert the CNII to CN*. To indicate the conversion has been conducted, a check box is given on the right of the CN property as shown in the figure below. The sole purpose of this check box is to tell the CNII has been converted to CN* to avoid repeating the conversion. It’s not recommended to manually uncheck or check it although it doesn’t affect the CN value.

Parameter Tables

The Parameter Tables window provides a spreadsheet environment for parameter editing. By default, it’s located at the bottom of the main interface.

Parameters of each type of hydrologic object is displayed in the same table and sorted by NHYD. These tables are arranged in different tabs. The tab’s name consists of the hydrologic object name and the number of the hydrologic object in the current scenario. To view the data table of another type of hydrologic object, click on the corresponding tab.

The data record in the data table is connected to the hydrologic object. By double-clicking a data record, it will zoom to the hydrologic object in Canvas and cause this object to flash.

The value of simple parameters can be edited directly in the table. For parameters that are collection data, it’s greyed out and needs to be edited in the Properties window.

The data table can be sorted by any data column. It’s useful to find abnormal values for some parameters, e.g. slope and curve number.

The data in the data table can be copied using CTRL+C or Copy menu in context menu. Data from other sources can also be pasted using CTRL+V or Paste menu in context menu. When a data table is pasted, the first data value will be pasted to the current cell and other data values will be pasted to cells after the current cells in the horizontal and vertical directions. It’s important to make sure the parameter values in the source is same as the one in the data table before pasting.

The parameter values can also be changed by using the Field Calculator which is opened by choosing the Calculate Field … from the context menu. It’s used to change the parameter value by percentage or fixed value (replace). In Field Calculator window, all available parameters are given on the left and the formula for property change is on the right. By default, the parameter (da-ta column) highlighted in the data table is added to the formula. It can be changed to implement the changes as shown in the figure below. Click OK button to make the change.

Hydrograph Results / Water Balance Results

The Hydrograph Results and Water Balance Results window is the same window which appears differently for single-event simulation and continuous simulation. By default, this window is at the bottom of the main interface. It’s used to show the summary results of each hydrologic object.

For single-event simulation, the summary is the peak flow and runoff volume of the hydrograph. For continuous simulation, it’s the peak flow and average annual amount of each water balance component. The summaries are shown in a data table for each available hydrological object. Data can be sorted by any column.

In case there are multiple simulation runs, only summary results of select run (selected in scenario property) is shown in the table. To switch to another run, select the run from the drop-down list above the data table. It’s possible to show summary results of all runs by clicking on the Show All Runs button.

Same as Parameter Tables window, the hydrologic object can be zoomed in by double-clicking on the data record.

Water Quality Results

The Water Quality Results window is the same window appearing for single-event simulation and continuous simulation. By default, this window is at the bottom of the main interface. It’s used to show the water quality summary results of each hydrologic object.

In case there are multiple simulation runs, only summary results of select run (selected in scenario property) is shown in the table. To switch to another run, select the run from the drop-down list above the data table. It’s possible to show summary results of all runs by clicking the Show All Runs button.

Same as Parameter Tables window, the hydrologic object could be zoomed in by double-clicking on the data record.

Error List

The Error List window shows the errors and warnings in the model. By default, it’s located at the bottom of the main interface.

Certain rules apply to an OTTHYMO model. If these rules are not met, a warning or an error will show in the Error List window. A model with any error can’t be run. The error and warning information is shown in a data table with the error type, hydrologic object ID and name, hydrologic object type and error message. An error will be shown as  and a warning will be . Same as the Parameter Tables and Hydrograph Results windows, the hydrologic object can be zoomed in by double-clicking on the data record in the table. Note that, a red outline is added to the object icon if an error is found. For example, a RouteChannel without source link will be shown as  in canvas.

The data table can be filtered to only show errors or warnings. The three buttons on the top is to switch each message on and off. By default, all errors and warning are shown in the table.


This chapter discusses how to use projects and scenarios to manage multiple models, such as models for existing and post-development condition.

To the user, a project may represent a specific type of work which consists of multiple hydrologic models. Each model in a project is a scenario. In VO, a scenario is an independent drainage net-work. A common practice for a modelling project is to create a project with multiple scenarios. At the beginning of the project, a base scenario is usually created first for the base scenario (existing condition). After that, new scenarios (post-development) are created by modifying the base sce-nario.


This chapter introduces how to create, open and save a project.

Project Types

There are two project types available in VO6:

  • Classic Single-Event OTTHYMO – For single-event simulation with design storms. Typically used for quantifying control purpose.
  • Continuous OTTHYMO – For continuous simulation with long-term climate data. It can be used for water balance analysis and flow forecasting.

These two project types share same hydrologic objects and can be converted to each other. Users should use the proper project type based on the project requirements.

Creating a New Project

A new project is created automatically when VO is opened. The project type of the default project can be specified in the Options window. By default, it’s set as Single-event OTTHYMO.



Similar options are also available for Open Project, Save Project and Save Project As.

Opening an Existing Project

To open an existing project, click the Open Project button  or menu in Home tab, Quick Access Toolbar or File Menu. The Open window will appear to browse VO project files (*.voprj).

VO maintains a recent project file list. When a project is opened at the first time, it will be added to the list. To open it again, simply select it from the Recent Files list in the File Menu.

A project file can also be opened by double-clicking on the project file in File Explorer. The project file (.voprj) has been registered with VO and appears with  icon in *File Explorer.

Saving a Project

All changes made to a project are temporary until they are saved. To save changes to the project file, click the Save Project button  or menu in Home tab, Quick Access Toolbar or File Menu.

The working project can also be saved to another location with the Save Project As button  or menu in Home tab, Quick Access Toolbar or File Menu.

Note that the imported layers are not saved in the project file. You should make sure the file sources are valid.


In VO, a scenario is an independent drainage network. It includes all hydrological objects and the links between them. It can be displayed in Map View and Schematic View. All scenarios are listed in Project Manager.

Creating a New Scenario

To create an empty scenario, click the Add button  at the top of the Project Manager or select Add menu from the context menu. The Scenario Create window will appear. Enter the name and description and click OK button to create the scenario. The new scenario will be opened immediately in Schematic View and/or Map View.

Duplicating an Existing Scenario

In often cases, a new scenario is created by copying an existing scenario and then making changes. This is done by completing the following steps:

  1. Select an existing scenario in Project Manager and then click the Duplicate button  at the top of the Project Manager or select Duplicate menu from the context menu.
  2. The Duplicate Scenario window will appear.
  3. Enter the name of the new scenario.
  4. Options are available to duplicate both Commands and Runs. Always keep the Commands checked. If different climate data will be used in the new scenario, uncheck the Runs.
  5. Click the Duplicate button at the bottom to create the new scenario.

Opening an Existing Scenario

If a scenario is not displayed in Schematic View and/or Map View, double-click it in the Project Manager. A new tab is created with scenario name and the scenario is displayed in the Schematic View and/or Map View.

Setting Default Scenario

A default scenario is the scenario that will be opened when a project is opened. It’s not necessary to open all scenarios at the beginning. To set an existing scenario to the default scenario, select Set as Default Scenario menu from the context menu. The selected scenario will be opened when the project file is opened the next time.

Scenario Settings

Scenario settings are available in Properties window. To show the scenario properties, open the scenario and deselect all objects.

The settings are described in the table below.

NameThe name of the scenario
DescriptionThe description of the scenario
TypeThe type of the view. It can’t be changed.
Run #The active simulation run
Show BackgroundIf the background will be displayed in Schematic View
WidthThe width of the background in point
HeightThe height of the background in point


A scenario can be created by importing from model data files. This is useful when 1) the model is in another model platform (e.g. SWMM) or in older VO data files and 2) there is need to integrate a scenario to current project.

Model Data Files Supported

VO6 supports the following data input files:

  • -SWMM5 (*.inp)
  • -Visual OTTHYMO v2.4 and higher (*.voprj)
  • -Visual OTTHYMO v2.0 – v2.3 (*.sce)
  • -Visual OTTHYMO v1.0.x (*.ott)
  • -OTTHYMO-89/INTERHYMO (*.ott, *.dat)
  • -OTTHYMO-83 (*.ott, *.dat)

Please note that while every attempt has been made to verify that the import routine works in everyday cases, there may be some data files that will have errors upon import (or may not even import). As a precaution, users should verify that the data files actually run in the previous model version, prior to import.

Importing SWMM5

The catchment and channel systems in SWMM can be imported to VO. The subcatchments are converted to either NasHyd or StandHyd based on the imperviousness and the open channel is converted to RouteChannel. It is not recommended to import models with detailed sewer networks.

The SWMM5 import function is available for single-event simulation. To import from a SWMM5 input file (.inp), select *File -> Import -> Import SWMM to open the Import SWMM to Otthymo window. Browse the SWMM input file by clicking the button below the exit button on the top right corner. Click OK to start the import. The imported model will be added as a new scenario to the current project.

Importing Visual OTTHYMO V2.4 and Later

Importing a Visual OTTHYMO v2.4 and higher project file (*.voprj) is necessary when you want to bring an existing scenario into your current project. This may be necessary if you are combining projects or if you want to use an existing model scenario to build a new model.

Once you have your project open, select File -> Import -> Import VH Scenario (Current Project) menu to browse the VO project file and open the Import Scenarios window as follows. All scenarios in the project are listed in the window. You may want to only select the scenarios that you want to import. Click Import Selected button at the bottom and the selected scenarios are added to current project.

Before running your new scenario, you should check that the storm files and any external files (e.g. READ HYD) are referenced to the proper location on your system.

Importing Visual OTTHYMO V2.0 and Later

Scenarios in Visual OTTHYMO v2.0 – v2.3 are saved in separate files (.sce). They can be imported to current project with menu *File -> Import -> Import VO2 Scenario (Current Project). The scenario will be added to current project immediately after the import. Rain data will also be imported.

Importing Classic OTTHYMO Data files

The model file used in Visual OTTHYMO v1.0.x, OTTHYMO-89/INTERHYMO and OTTHYMO-83 is in similar text file format. It may have the extension of ottdat or txt. These files can be imported as a new scenario to current project with menu File -> Import -> Import Scenario (Otthymo 89). As the location of the hydrologic objects are not specified in the model file, the objects will be arranged from top to bottom. The most upstream objects will be located on the top. Their locations can be changed later to represent the shape of the drainage network.

Any commands that are not included in VO5 are ignored during the import. These commands may include COMPUTE VOLUMEPRINT HYDPLOT HYDSAVE HYD and ERROR ANALYSIS.


Canvas is where the hydrologic objects can be visually created and connected with icons. Initially introduced in version 1, it has transformed the way to create an OTTHYMO model, through text file editing to LEGO-like structure building. The time spent on model creation has been significantly reduced. As well, the need to remember format of various commands have been removed. Building a model with VO is now more enjoyable.

Adding Background

A background showing the study area helps the modelers to place the hydrologic objects and the reviewers to quickly understand the model structure.

To add the background:

  1. Select the Background -> Change Background … menu from the Canvas context menu.
  2. Select the background picture file in the Select an Image window. It supports JPEGPNG and BMP format.
  3. Click the Open button to add the selected picture file as the Canvas background.
  4. Use the navigation functions to PanZoom In or Zoom Out the Canvas until the background is in its desired location.
  5. Change the width and height of the background in the scenario properties window if necessary. Note that the aspect ratio is not locked.

To remove background, select the Background -> Remove Background menu in Canvas con-text menu.

To hide background, uncheck the Show Background in the scenario properties window. Note that this is different from removing background. The background is still in the scenario.

Adding Hydrologic Objects

Hydrologic objects are added to the Canvas by dragging the hydrologic object’s icon from the Toolbox and then dropping on the canvas.

New hydrologic objects can also be created by copying existing objects. To do this:

  1. Select the objects to be copied. Multiple objects can be copied at the same time.
  2. Copy the selected objects with either of the three options:
    1. Copy button  in Home Tab
    2. Copy menu in the context menu
    3. CTRL + C keys.
  3. Similarly, paste the copied objects with one of the three options:
    1. Paste button  in Home Table
    2. Paste menu in the context menu
    3. CTRL + V keys.

Selecting Hydrologic Objects

An individual hydrologic object is selected by pressing placing the mouse cursor over the object and clicking the left button. A blue outline will be added to the object icon.


Multiple objects are selected by holding the CTRL or SHIFT key and then selecting each object.


Adjacent objects can also be selected by dragging a rectangle covering all the objects. There is no difference on how the rectangle is created (e.g. from left top corner to right bottom corner).



All objects can be selected using CTRL+A keys. Various other selection functions are available from the context menu. These menus are described in Table 6-1.


Select AllSelect all objects
Invert SelectionSelect objects that are currently not selected and deselect the selected objects
Select UpstreamSelect objects located upstream of selected objects. The selection can be all objects or given types.
Select DownstreamSelect objects located downstream of selected objects. The selection can be all objects or given types.

Changing the Location of Hydrologic Objects

Once objects are on the canvas it can be moved by selecting it and holding the left mouse button to drag it to the desired location. Multiple objects can be moved at the same time.

The location of objects can be also adjusted with various Align and Distribute tools in the Canvas context menu. To use these tools:

  1. Select the objects whose locations need to be adjusted. More than one object should be selected.
  2. Select the desired Align or Distribute tools from context menu.

The available Align or Distribute tools are described in Table 6-2.


Align Left
Align to the far left of all select-ed objects
Align Top
Align to the top of all selected objects
Align Center
Align to the center (from far left to far right) of all selected objects
Align Middle
Align to the middle (from top to bottom) of all selected objects
Align Right
Align to the far right of all selected objects
Align Bottom
Align to the bottom of all selected objects
Distribute Horizontally
Distribute selected objects to have same horizontal distance
Distribute Vertically
Distribute selected objects to have same vertical distance

Note that the location change on canvas doesn’t have an impact on its geospatial location in Map View and vice versa.

Linking Hydrologic Objects

The hydrologic objects and its immediate downstream objects are presented with an arrow pointing to the downstream objects. The arrow is called link in VO. To create a link on canvas:

  1. Move the mouse cursor on top of the source object icon.
  2. Keep left button pressed and move the mouse cursor on top of the downstream object icon.
  3. Release left button and the link will be created.

Some rules and guidelines for linking objects are given in Table 6-3. The link will not be added if the number of input or output links reaches the limit.


Hydrologic Object
  1. of Input Links
  1. of Output Links
12Minor: Red
Major: Black

1Up to 3Over flow: Red
Out flow: Black
Underdrain: Orange

Navigating Canvas

The canvas can be easily navigated. Available navigation functions are described below.

  1. Zoom In – Scroll the mouse wheel up.
  2. Zoom Out – Scroll the mouse wheel down.
  3. Pan – Move the canvas any direction. Available as the first context menu.
  4. Zoom Extent – Zoom the canvas to show all objects on the canvas (context menu).
  5. Zoom to Selected – Zoom to selected objects (context menu).

Creating Labels

There are two types of labels on Canvas:

  1. System label – is the label below each object icon which can’t be turned off.
  2. Customized label – Certain properties and result summary can be labeled at the right side of the object icon. The label content and content can be customized. It can also be turned off.

To setup the customized label:

  1. Select the objects you wish to create a label for.
  2. Right-click and select the Set Labels from the context menu to open the Label Editor window, which lists all available label items. Depending on the project and object type, the available label items can be different.
  3. Drag and drop the label items from Available Properties list on the left to Selected Properties list on the right.
  4. Change the label background color with the color picker at the bottom.
  5. Click OK to create the label.
  6. To show the labels on canvas, select Show Labels from the context menu.

Note that the result summary value is empty if the output is not available.

Printing Model Schematic

The model schematic (including the background and labels) can be copied to clipboard or printed directly. This is useful for project report.

To copy the schematic to clipboard, click the Copy to Clipboard button  in File menu. Then it can be pasted to a third-party software application. Note that only the selected objects will be copied if the selection is not empty.

To print the model schematic directly, click the Print button  in File menu. The print preview window will appear where it can be viewed and printed. The schematic was zoomed to have all objects displayed and the selection doesn’t have any impact.


With VO 6.0, we are introducing TatukGIS that will be built into the program. No additional GIS license is required to use Tatuk Map. Users can now choose an ArcGIS or TatukGIS map. Note that an ArcGIS license is required to use the ArcGIS map. Map is another way to create, parameterize and present a VO model. It adds geospatial location information to each hydrological object and shows the model drainage network on the map. This section helps you work with the map in VO (ArcGIS map is used as default in this section). Several benefits from using the map are:

  1. Easier model creation with existing GIS data. For watershed and sub-watershed studies, various GIS data are usually available, which may include catchment, stream and out-put layers. With Map View, the model drainage network can be created directly from these layers without the need to create hydrologic objects one by one from scratch.
  2. Reduced time for model parametrization. The geometry-based parameters (e.g. Area, Length, etc.) are usually obtained from a third-party software and then entered in the model. With geo-spatial location information, these parameters can be automatically calculated. Tools are provided to calculate area weighted CN and Imperviousness.
  3. Easier to understand. The model schematic may be difficult to understand without knowing the study area. This is improved with map by providing viewers the spatial context.

In VO, a scenario can be viewed in both the Schematic View (Canvas) and the Map View.

By default, only one view is visible. To have them side-by-side (split view), drag Map or Schematic tab to the desired location. Note that the Map View and GIS tab will not be available if an ArcGIS license is not found.

Map View Layout

There are three components in the map view:

  1. Table of Content – A tree view of all GIS layers. It controls the appearance of the layers.
  2. Map – The main working space to display and edit hydrologic objects. The features can be created, edited and selected.
  3. Status Bar – Showing the coordinates.

Default Coordinate System

The default coordinate system of the map and layers is NAD_1983_UTM_Zone_17N. To change it:

  1. Click the Options button  in Home tab. The Options window will appear.
  2. Switch to GIS tab and select the desired coordinate system from the Default coordinate system list. Note that the GIS tab is not available without a valid ArcGIS license.
  3. Click the OK button to save the option. The new default coordinate system would take affect for the next new project. It won’t change the coordinate system of the current project.


A map usually has multiple layers. The content shown on the map depends on the data in each layer and the order in Table of Content. Layers on top will cover the one on the bottom. VO utilizes various layers to represent the model and calculate model parameters. Users have the options to add, edit and remove layers.

Layer Types

Layers used in VO can be grouped to four (4) different types:

  1. Hydrologic Object Layers – Layers that are used to store and display hydrologic objects.
  2. Support Layers – Layers that provide support functions.
  3. Base Map Layers – Layers that provide background map data (Available only in ArcGIS).
  4. Imported Layers – Layers added by users.

Both the Hydrologic Object Layers and Support Layers are system layers and are saved in the project file. They can’t be removed from Table of Content.

Hydrologic Object Layers

Hydrologic object layers are the geospatial representation of hydrologic objects. Each type of hydrologic object has one corresponding layer as shown in the figure below. These layers have the same name of hydrologic objects and are grouped in the same way as they are grouped in the toolbox. The geometry type (polygon, polyline or point) of each layer is given in Table 7-1.


CategoryHydrologic object
& Geometry Type
CategoryHydrologic object
& Geometry Type
Hydrograph Polygon
Operation Point
Route Polyline
Utility Point
LID Point
LID Point

Support Layers

Support Layers are added to help define rain gauges (Raingauge), display the type of hydrologic objects (Command) and display connections between hydrologic objects (Connector and CatchmentLine).

  1. The Raingauge layer displays rain gauge on the map. It is used in DRMT (7.6.3) to define the location of rain gauges.
  2. The Command layer shows hydrologic object icons on top of GIS features as shown in the figure below. Users may find it helpful when differentiating hydrologic objects. For polygon-hydrologic objects, the icon is shown at the centroid of the polygon. For polyline-hydrologic objects, the icon is shown at the middle point of the polyline.
  3. The Connector and CatchmentLine layer are virtual connection lines representing the connection between certain hydrologic objects, which is similar to the link in Schematic View. The CatchmentLine layer represents the connection starting from a catchment. All other connections are represented by the Connector layer. The connections are represented by a straight line as shown in the figure below.

Layer Context Menu

Context menus are available for each layer in the Table of Content. These menus are described in Table 7-2. The availability of the menus is dependent on type and status of the layer.


Attribute TableOpen the attribute table of the layer.
Remove LayerRemove the layer from map. Not available for hydrologic object layers and support layers.
Move Layer UpMove the layer up one level.
Move Layer DownMove the layer down one level.
Layer SelectableMake the features selectable. Not available if it’s already selectable.
Layer UnselectableMake the feature un-selectable. Not available if it’s already un-selectable.
Zoom To LayerChange the map extent to show all features in the layer.
Deselect all but thisTurn off all other layers but turn on current layer.
Select all but thisTurn on all other layers but turn off current layer.
Show/Hide ArrowsShow or hide arrows for polyline layer.
Export as Shape FileExport current layer to shapefile.
Export as CSVExport the attribute table to CSV file.
File SourceDisplay the file source.
Layer PropertiesOpen the Layer Properties window.

Adding a Layer

To add a layer, use the following button from Add Layer  sub-menus in GIS tab:

  • Feature Layer button  to add any supported GIS data files.
  • Group Layer button  to add a new group layer.
  • Basemap Layer button  to add the base map layer (Available only in ArcGIS).

The new layer will be added to the bottom of Table of Content. For feature layers, multiple layers can be added at the same time.

Moving Layers

The order of layers in the Table of Content affect how the features show up on the map. Features in top layers, would cover features in bottom layers. It’s generally recommended to put point and polyline layers on top of polygon layers to avoid any cover-up.

To move the layers in Table of Content, select the layer first and drag it to the new location. Moving a group layer will also move all children layers.

Layers can all be moved by using the Move Layer Up and Move Layer Down context menu. Layers in a group layer can only be moved inside that group.

Removing Layers

Base map layer and imported layers can be removed from the Table of Content by selecting Remove Layer menu from the context menu. Note that the hydrologic object layers and support layers can’t be removed.

Defining Layer Visibility

The visibility of a layer can be defined using the check box in Table of Content and the Scale Range setting in Layer Properties window.

Unchecking the check box would hide all features in that layer. For a group layer, it also hides features belonging to its children layers.

A scale range is the upper and lower scale limit when the features can be displayed on map. It is necessary to define the range when a certain layer is not useful when the map is zoomed in or out. To set up the scale range:

  1. Select the Layer Properties menu from layer context menu. The Layer Properties window will appear.
  2. Click on Scale Range tab. By default, the Show layer at all scales is checked, which indicates that features in this layer would display at any scale.
  3. Select Don’t show layer when zoomed and give a value for the Minimum Scale and Maximum Scale.
  4. The current map scale can be used by selecting the Use current scale as the minimum scale and Use current scale as the maximum scale button.
  5. Click the OK button to save the changes.

In the example shown in the figure below, the features in the layer would only display when the scale is between 1:50,000 and 1:250,000. If the scale goes beyond this range, all features would be hid-den and the checkbox beside the layer in Table of Content becomes grey to indicate the status.

Defining Layer Symbol

The appearance of features on the map are defined using symbols. For an empty project, a default symbol is generated for each layer. To change the symbol:

  1. Select the Layer Properties menu from layer context menu. The Layer Properties window will appear.
  2. Click on Symbology tab. Depending on type of data and existing symbol, the look of the window may be different. Two types of symbols can be defined for a vector layer:
    1. Simple Single Symbol –Defines only one symbol for all features in the layers. With this symbol, all features in the layers would look the same. It is typically used to differentiate the layers with other.
    2. Categories symbol – Defines multiple symbols with different shapes, colours, or sizes for features with different attribute values. This can effectively describe the spatial distribution of features against selected attribute.

  1. For Simple Single Symbol, select Features in the list on the left and click the button in Symbol group to change the symbol type, color, and size.
  2. For Categories Symbol, select Categories in the list on the left. The right side will be changed in the categories symbol view. In this view, select the field name from the Field List and click the Add Values button to generate the symbol for each unique value of the selected field.

  1. For Raster data, simply select the color ramp from the list.
  2. Click OK button to save the changes.

The symbols will be saved in project file. It will be restored when the project is opened.

Labelling Layer

Attribute values can be displayed on top of features on the map. To create the label:

  1. Select the Layer Properties menu from layer context menu. The Layer Properties window will appear.
  2. Click on Label tab.
  3. Check the Display Label.
  4. Select a field from the attribute list to add the field into the expression string.
  5. Click the OK button.

The value of selected field will be labeled on top of the feature as shown below.


The model components and other helper information are displayed on the map. With the geospatial information, the model is easier to understand. Furthermore, it enables to utilize existing GIS layers to create the model structure and determine the parameter values.

Navigating the Map

To zoom in and out of the map, wheel the scroll mouse up and down. More navigation tools are available in GIS tab and they are shown below. These tools are described in Table 4-4. The Pan tool is also available from the context menu. Note that in TatukGIS, the Zoom Out button is not available. Instead, To Zoom In, the user can click on the Zoom In button followed by clicking and moving the mouse to the right side to form a rectangle. To Zoom Out, follow the same step above but forming the rectangle towards the left after clicking on the map. The user can always use the scroll mouse to zoom in and zoom out. Identify button is only available in ArcGIS. Multi-selection of Snapping tools is available only on ArcGIS.

The scale of the Map View and Schematic View is synchronized by default to zoom to same hydrologic objects, which works best for the split view. The synchronization can be turned on/off in Options window. Turning off the synchronization would improve performance for a model with many hydrologic objects.

The two views can also be synchronized manually by choosing the Show Same Hydrologic objects as in Schematic View in map context menu or the Show Same Hydrologic objects as in Map View in schematic view context menu.

Note that the views are not synchronized when it is not in split view model even when the synchronization is turned on. Please use the manual synchronization context menu when switching to another view.

Selecting Features on Map

Features can be selected on map. To select features:

  1. Click the Select button  in GIS tab.
  2. Click the feature on the map. The outline of the selected feature will be changed.
  3. To add other features to the selection, hold the CTRL key and click on the feature.
  4. To select features in a rectangle, drag to draw the rectangle on the map.

Similar as in Schematic View, certain hydrologic objects can be selected by using the Selection context menu. These menus are described in 6.3.

For hydrological object layers, the hydrologic objects are also selected if the corresponding features are selected.

Creating Hydrologic Objects Manually

VO provides tools to create hydrologic objects on map. Different from the Schematic View, the geometry needs to be digitized on the map before the objects can be created. To create a hydro-logical object on map:

  1. Turn on the base map layer for reference.
  2. Configure the snapping environment in the GIS tab to snap certain point on existing shapes. Their usage is described in Table 7-3. It can be changed anytime during the digitization process. Notice that only Vertex is selected by default.


Icon CommandIcon Command
Allows to snap to a point.
Allows to snap to the end point of polyline or polygon. (Available only in ArcGIS)
Allows to snap to the vertex.
Allows to snap to the middle point of a polyline
Allows to snap along the edge of polyline or polygon.
Allows to snap to intersection of two geometries
  1. Specify the type of new hydrologic object by:
    a. Dragging and dropping the hydrologic object icon from the toolbox to Map View.

b. Selecting the type of hydrologic object from map context menu.

Notice that the cursor changes to a pencil shape indicating it is ready to draw on map.

  1. Start to draw the desired shape on the map by clicking on the map. For point-hydrologic objects (e.g. AddHyd), the hydrologic object will be created immediately when a point is created. For polygon-hydrologic objects and polyline-hydrologic objects, multiple points are required. During digitalization, the context menu is shown below allowing users to delete or finish the sketch. The sketch can also be deleted by pressing the ESC key. To finish the sketch, users must double-click.
  1. With the newly created shape, a new feature is created in the corresponding layer. A hydrologic object is also created and selected in both views.

Creating Hydrologic Objects with GIS Data

Existing GIS data can be utilized to define hydrologic objects. The data may be created from DEM data using other hydrological analysis toolsets. These datasets usually have a catchment layer, a stream layer, and an outlet layer. They can be imported into VO and used to create NasHyds/StandHydsRouteChannels and AddHyds.

To create hydrologic objects from existing GIS layers:

  1. Add existing GIS data using Add Feature Layer button  in GIS tab. Move the layer on top of base map layer and zoom to it using the Zoom to Layer context menu.
  2. Select feature(s) in imported layers using the Select tool  in GIS tab or the Map Context Menu.
  3. In Map Context Menu, select the desired type of hydrologic object in Generate Selected Geometry As menu. The new hydrologic object will be this type. Depending on the type of selected features (point, polyline or polygon), some menus may be disabled as there is no proper geometry for those hydrologic objects. In this case, no polyline feature is selected, so RouteChannelMuskingumCunge and RoutePipe are disabled.

  1. A new hydrologic object will be created for each selected feature. The shape-based parameters (e.g. area for catchments and length for channels) are automatically calculated from the shape and all other parameters will use the default values.

Linking Hydrologic Objects

Links between hydrologic objects can also be created in Map View. To create a link:

  1. Click the Add Link button in GIS tab or choose the Add -> Link menu from the map context menu. Notice the cursor changes to cross hair shape.
  2. Move the cursor to the hydrologic object that will be the source of the link. Notice the hydrologic object will flash in red, which indicates the hydrologic object is eligible for the link source. If the hydrologic object is not eligible, the hydrologic object won’t flash. For example, a NasHyd which already has been linked to an AddHyd is not eligible as the link source as it can only have one output link. The connection rules are given Table 6-3.
  3. Click the desired hydrologic object to set it as the source of the link and move the cursor to the hydrologic object it will be connected to. Notice that a red line will appear to connect from the hydrologic object to the location of the cursor as shown below. This line represents the link that will be added. 

The starting point of the line is determined based on the shape of the hydrologic object. For polygon-hydrologic object, the starting point will be the centroid of the polygon. For polyline-hydrologic object, the starting point will be the middle point of the polyline. The starting point of a point-hydrologic object will be where the point is located. In TatukGIS, the connecting line appears only after the source and destination points are selected.

  1. To select the destination object feature, move the cursor on top of that feature. If that feature flashes, click it to add the link. If the number of the input link of that hydrologic object reaches the limitation, that hydrologic object won’t flash, and the link can’t be added.
    Same as the starting point, the ending point of the link is also determined by the shape of that hydrologic object. The Middle point is used for polylines.

A new feature is added to CatchmentLine layer for the new link as shown below. The symbol may be different based on your settings.

Once the link is added, the output property of the starting hydrologic object is updated automatically to the output hydrologic object and a link is also added in the schematic view as shown below.

  1. To restart the process in the middle, press the ESC key and the starting point will be cleared.
  2. To remove the link, click the edit tool, select the link on map, and press the DELETE key. A confirmation window would appear as shown below. Click Yes to confirm the deletion. The link in the schematic view will also be removed and the outlet property will be set to empty.

Assigning Geometry to Existing Hydrological Objects

Hydrologic objects do not have geospatial location information when 1) they are created on canvas or 2) they are imported from an older model. To take advantage of GIS functions, the geospatial location information can be assigned to hydrological objects. To do this:

  1. Click the Assign Geometry button in the GIS tab. The Assign Geometry window will appear. By default, the window lists all hydrologic objects that have no associated geometry. It can also show all hydrologic objects by deselecting Filter Commands with Geometries.

  1. To assign a manually created geometry, click the Assign New Geometry button at bottom and the cursor changes to a pencil. Follow the same procedure as in 4.2.1 to create the ge-ometry. Once the geometry is successfully created, it will be assigned to the selected hydro-logic object.
  2. To assign exiting geometry to a selected hydrologic object, add the data layer first and then select the geometry on the map. Next, select the correct hydrologic object in the list and click Assign Selected Geometry button at the bottom.

Moving, Editing, and Deleting Hydrologic Objects

The shape of the hydrologic object features can be edited and deleted on the map. To do this:

  1. Select Edit Tool  in GIS tab or map context menu.
  2. Select the hydrologic object feature on map.
  3. To move a feature, move the cursor on top of the selected feature and then hold the left button to drag it to the new location. When the moving is underway, a new shape is shown to represent what it will look like after the move. Release the left button to confirm the move. Note that moving is not allowed when more than one hydrologic object is selected. The moving option is available only on ArcGIS.

For polygon-hydrologic objects, the new shape may cover, cut or clip other hydrologic objects. This is not allowed. If that happens, a warning message shows up and the new shape will be rejected.

If the new shape clips other hydrologic objects like the one shown below, the behaviour is defined in the Options window. The default behaviour is Reject, which would reject the change. If it is set to Clip, the affected hydrologic objects would be clipped and the area is updated. This setting also applies when a new polygon hydrologic object feature is created and when an existing polygon hydrologic object feature is changed by moving, adding or removing a vertex.

Due to the possible changes to other hydrologic objects when moving polygon-hydrologic objects, it is not generally recommended to do so.

  1. To change the shape of the hydrologic object, double-click it on the map to show all the vertices. Move the cursor on top of one of them and drag it to the new desired location.

  1. Vertices can be added or removed by using Add Vertex  and Remove Vertex  in GIS tab. For Add Vertex, set the snapping properly to make the new vertex more accurate. Click on the map, a new vertex is added at that location and the shape is changed accordingly. For Remove Vertex, the vertex will be removed by clicking on it. Note that these two tools are only available when a polygon or a polyline is selected using the Edit tool.

  1. To delete selected hydrologic object(s), press the DELETE key or choose Delete on map context menu. A warning window appears to provide the opportunity to confirm the deletion. Click Yes to confirm the deletion.

Cutting Polygon - Hydrologic Objects

In some cases, a polygon-hydrologic object needs to be split into several smaller ones. For example, if part of NasHyd is developed and the imperviousness surpass 20%, users will have to:

  1. Select the polygon-hydrologic object first and select Cut from context menu. Notice the cursor change to a pencil.
  2. Setup the snapping environment if necessary, to snap to desired points.
  3. Draw the polyline that will be the boundary between new hydrologic objects and double click to finish the sketch. If the polygon can be cut into pieces, new hydrologic objects will be created.

If the polyline can’t cut the polygon into pieces, a warning window will appear.

Updating Hydrologic Object Location in Schematic View

The location of hydrologic objects in schematic view can be determined based on their geospatial location on map. As a result, the hydrologic objects will be in a similar location in schematic view, making the comparison easier.

To update the location, choose the Update Schematic Position context menu and click Yes on the confirmation window. The location of all hydrologic objects would be updated.

For some models with large number of hydrologic objects, the location of few hydrologic objects may not be completely accurate. They need to be adjusted manually.


GIS tools take advantage of GIS data to help with model parameterization and calibration. Three tools are provided in this version.

  • Calculate CN  – Assigns CNII to catchments based on land use and soil layer. It also has a built-in lookup table.
  • Calculate Area Weighted  – Assigns catchment parameters from exiting GIS layer.
  • DRMT  – Generates unique rainfall data for each catchment by interpolating the rainfall data from rain gauges.

Calculating CN

Curve Number (CN) is the most important parameter to determine surface runoff when SCS equation is used. Its value varies for different soil types, land use, and Antecedent Soil Moisture Condition (AMC). The CN for the average antecedent soil condition (CNII) is usually used. Lookup tables have been established for land use, soil hydrological group, and CNII (Table 7-4).


In most cases, CNII of each catchment is estimated with the land use and soil information and applicable lookup tables. If land use and soil layers are available, GIS software is usually used to conduct the calculation. For catchments covering more than one land use or soil type, an area weighted CNII are usually calculated.

The Calculate CN tool makes the process easier. To use this tool:

  1. Add soil and land use layers. Make sure the soil layer has the soil hydrologic group data and the land use layer has the land use data.
  2. Select catchments (StanHyd, NasHyd, WilHyd or ScsHyd) to be updated in Map View or Schematic View. Note that the selected catchments must be assigned to a geometry.
  3. Click the Calculate CN button  in the GIS tab. The Calculate CN window will appear. Notice that the number of selected hydrologic objects available for CN calculation is shown on the top of the window. The window has the left part for soil mapping and the right part for the land use mapping.
  4. On the left part, choose the soil layer from the Soil Layer list and then choose the Soil Group Field. The soil hydrologic group lookup table will become available, allowing the user to define which hydrological group (A, B, C and D) will be mapped for each value of the selected field. Define the soil group lookup table by selecting a proper hydrological group from the drop-down list.
  5. On the right-hand side, choose the Land Use Layer from the layer list and then choose the Land Use Field. Define the land use lookup table by selecting from the dropdown list for each land use type.
  6. Click the Update CN button at the bottom to update the CN based on soil and land use data. A message window will appear indicating that CN of selected hydrologic objects have been updated. In TatukGIS, the users will now have a drop-down menu for Mapping with TRCACVC, and TR-55 as options.

  1. To export the soil and land use layer mapping information, click the Export button at the left bottom corner to save the mapping information to a file. The saved file can be imported using the Import button.
  2. The Clear button at the bottom can be used to clear all field mapping information and start all over again.
  3. To see the default CN lookup table, click the CN Lookup Table button. Note that the lookup table is not editable. To use a different lookup table, please contact us.

Calculating Area Weighted

n some cases, model parameters are available in GIS layers. The Calculate Area Weighted tool reads the parameter values, calculate the area weighted value if necessary and then assign to catchments. The imperviousness (TIMP and XIMP) and initial abstraction are two parameters that can utilize this tool. To use this tool:

  1. Add the GIS layer that has the parameter information.
  2. Select catchments whose parameter will be updated.
  3. Click the Calculate Area Weighted button  located in the GIS tab. The Calculate Area Weighted Parameter window will appear. Notice that the number of selected catchments is shown at the top of the window.
  4. Choose Parameter to Calculate from the drop-down menu. In this case, only StandHyd parameters are shown here as only StandHyds are selected. We want to use Timp this time.
  5. Choose the Source Layer and the Source Field, where the parameter values will be retrieved. The source layer should be a polygon layer and the source field can be any type. In this case, we choose Percentimp, which is a numerical field indicating the percentage of impervious area in the polygon.
  6. Define the field mapping to map all the field values to the desired parameter value.
  7. Click the Update button at the bottom of the window. A message window will appear, indicating that the parameter of selected hydrologic objects has been updated.

  1. Similar to the Calculate CN tool, the value mapping can be exported to a file and then imported later using the Export and Import button at the bottom.
  2. The Clear button at the bottom of the window can be used to clear the lookup table and start all over again.

Calculating Landuse Percentage

In some cases, model parameters are available in GIS layers. The Calculate LandUse Percentage tool reads the parameter values, calculate the area weighted value if necessary and then as-sign to catchments LandUse. To use this tool:

  1. Add the GIS layer that has the parameter information.
  2. Select catchments whose landuse will be updated.
  3. Click the Calculate LandUse Percentage button  located in the GIS Tab . The Calculate LandUse Percentage Parameter window will appear. Notice that the number of selected catchments is shown at the top of the window.
  4. Choose the Source Layer and the Source Field, where the parameter values will be retrieved. The source layer should be a polygon layer and the source field could be any type. In this case, we choose Percentimp, which is a numerical field indicating the percentage of impervious area in the polygon.
  5. Click the Update button at the bottom of the window. A message window will appear, indicating that the landuse of selected hydrologic objects has been added to water quality data in project manager.
  6. The Close button at the bottom of the window can be used to close the window.


Rainfall data, an essential element of storm water management analyses, is recorded at and collected from rain gauges. Therefore, the location of the rain gauge is important. The closer the rain gauge is to the flow meter the better. However, this may not always be possible. Also, rainfall data obtained and used for modeling from adjacent/closest rain gauge does not, in most cases, best represent the sub-catchments. Often modellers must use rain gauge data that are not truly representative of the area where the flow is being recorded. The best rainfall data are recorded at the center of each sub-catchment, as they capture the true influence of rain on that particular sub-catchment. In order to overcome this, the Distributed Rain Modeling Technique or DRMT was introduced. DRMT uses math interpolation on each actual live rain gauge and interpolate the values of intensity at each time step to create a virtual rain gauge. The DRMT takes data from multiple rain gauges, surrounding a site of interest or focus, and interpolates to create rainfall ‘surfaces’ for each modeling time step at the centroid of each catchment’s tributary area. This approach is helpful to account for temporal and spatial variability of storm events over a relatively large drainage area and to interpret the observed flow and level data. The interpolation technique currently used is Spline Interpolation.

The DRMT tool is useful to calibrate models covering large areas with multiple rain gauges. The simulated hydrographs are more reasonable compared to the one without using DRMT.

Adding Rain Gauges

To have a proper rainfall surface, minimum three rain gauges are required. There are two ways to add rain gauges on the Map:

  • -Add rain gauges manually
  • -Add rain gauges by importing a shapefile


Add Rain Gauge Manually

Adding a rain gauge is similar to adding other point hydrologic objects (e.g. AddHyd). To add rain gauges on map:

  1. Right click on the empty place on the Map to open context menu
  2. Select Add->Rainguage in map context menu.
  3. Click on the map to add the rain gauge. Once finishing the adding, click the Selection button in the GIS toolbar or press ESC key to exit adding rain gauges.
  4. Set the storm index pointing to the rainfall data in a rain group. Navigate to the Parameter Tables of the Rain Gauges. To edit the Storm Index column in the Parameter Table, click on the cell under the Storm Index column and then type the storm index.

It is recommended to use same storm index in different rain groups to avoid changing the storm index after switching between rain groups. In this context, the rain group is a collection of rainfalls happening at the same time but at different locations. It is recommended to name the rain group with the rainfall time range and to label the rainfall data with the rain gauge name.

Add Rain Gauge by Importing Shapefile

  1. Navigate to GIS toolbar and click the Import Layer button.
  2. In the Import Required Layers window, choose Raingauge for the Layer field. Click the button  next to the Input Source field to navigate to and open the rain gauge shapefile. Set up the mapping table to map Mapped Properties with Layer Properties. Click Import. If a window popping up and asking whether to overwrite existing layer, select Overwrite. Close the Import Required Layers window.
  3. Navigate to the imported Raingauge layer in the tree of the Map. Right click on the layer to open the context menu and select Zoom to Layer
  4. If the rain gauges are not displayed at the right location on the map, the coordinate system needs to be updated for the rain gauge layer. To do that, right click on the Raingauge layer to open the context menu and select Layer Properties.Click on the Coordinate System tab and click Preset button. In the pop up window, navigate to and open the shapefile that has the correct coordinate system (usually it will be the same rain gauge shapefile). Click Apply and Exit on the Layer Properties window.

Using DRMT Tool in Continuous Project

To use the DRMT tool in continuous project, interpolate the rainfall for each catchment:

  1. Click the DRMT tool  in GIS tab. The Generate DRMT window will appear. The number of selected catchments is given at top of the window. If no catchment is selected, it will apply to all catchments.
  2. Choose the rain group from the Input Rain drop-down list and change other parameters if necessary. Rain gauges can be removed from or added to the Selected Rain Gauges list by drag-and-drop.
  3. Click OK button at the bottom. A message window appears once the process is done.
  4. After the process finishes, a new rain subgroup is created and added to the Rain Data group in Project Manager. The number of rain datasets in this new rain subgroup matches with the number of hyetographs.
  5. The DRMT-generated rain groups can be used to run the simulation as other rain groups. In Batch Run window through the Run button in the Simulation tab, select the rain group from the Rain drop down list as shown below. The corresponding hyetographs will be automatically assigned to hydrograph commands.

Using DRMT Tool in Single-Event Project

To use the DRMT tool in single-event project, interpolate the rainfall for each catchment:

  1. Click the DRMT tool  in GIS tab. The Generate DRMT window will appear. The number of selected catchments is given at top of the window. If no catchment is selected, it will apply to all catchments.
  2. DRMT tool uses a two-dimensional minimum curvature spline technique to generate the rainfall intensity surface. The parameters in Generate DRMT window are described in the following table. Most of the parameters have a default value.
Input RainSelect the rain group as the hyetograph data source for rain gauges.
Output cell sizeThe number of cells at which the output raster will be created. By default, it is 400.
Spline TypeThe type of spline to be used. It can be REGULARIZED or TENSIONREGULARIZED yields a smooth surface and smooth first derivatives. TENSION tunes the stiffness of the interpolant according to the character of the modeled phenomenon.
WeightParameter influencing the smoothness of the surface interpolation. When the REGULARIZED option is used, it defines the weight of the third derivatives of the surface in the curvature minimization expression. If the TENSION option is used, it defines the weight of tension. The default value is 0.1.
Number of pointsThe number of points per region used for local approximation. The default value is 12. (Available only on ArcGIS)
Intensity over catchmentThe method to calculate the rain fall intensity over catchment using the rainfall surface. It can be centroid or catchment averageCentroid uses the rainfall intensity at the catchment polygon centroid. Catchment Average calculates the average rainfall intensity over the catchment. The centroid is the default value.
Rainfall raster output pathFolder for generated rainfall surface raster data
Selected rain gaugesRain gauges whose location will be used for the interpolation. All rain gauges are selected by default.
  1. Choose the rain group from the Input Rain drop-down list and change other parameters if necessary. If there are more than three rain gauges, some rain gauges can be removed from the selected list by drag-and-drop.
  1. Click OK button at the bottom. The GeoProcessing window appears to show the progress and a message window appears once the process is done (GeoProcessing is available only on ArcGIS). 
  1. After the process finishes, a new rain group is created. It has 5 hyetographs, which are corresponding to the 5 selected sub-catchments. As they are generated from the interpolated surface, each hyetograph is unique. Notice that the icon for DRMT-generated rain group is different with those created manually . A capital letter “D” appears on the top right corner to represent DRMT. These rain groups are not editable.
  1. The DRMT-generated rain groups can be used to run the simulation as other rain groups. In Batch Run window through the Run button in the Simulation tab, select the rain group from the Rain drop down list as shown below. The corresponding hyetographs will be automatically assigned to hydrograph commands.

Turning off Map Functions

By default, VO 6.0 will load the TatukGIS map functions. When an ArcGIS license is available, the user can choose ArcGIS from the GIS Type selection drop-down menu.. This can be changed in Options window as shown in the figure below. Uncheck the Use GIS option in General tab. Note that this option only appears when ArcGIS license is available, and the change will take effect after the program is restarted.


Resource Library is a local climate data library with the ability to share with others. Shipped with standard design and regional storm, it is widely used in Toronto and Region Conservation Authority (TRCA). It can be further expanded with any project related climate data.

The climate data in Resource Library includes Intensity-Duration-Frequency (IDF), design storm, regional storm, rain gauge, temperature gauge, evaporation gauge, precipitation time series, temperature time series and evaporation time series. More climate data will be added in later versions.

The climate data is organized in a folder-like structure, supporting unlimited levels. The Library Explorer in the Resource Library provides similar functionality as the File Explorer in Windows operating system.

With the current version, the climate data is either entered or imported from a file. The database connection will be added in the later version to grab data from an existing database.

Review agencies may find that the Resource Library is a good way to distribute the design storms required in the development submissions. Modellers and engineers may find themselves save time by avoiding using file-based climate files.

Opening Resource Library

To open Resource Library, use the Resource Library button  in the Simulation tab or the Project Manager tool bar.

The Resource Library has three (3) components: Toolbar, Library Explorer and Main View. The Toolbar is on the top where buttons are placed in different groups. The Library Explorer, a tree view of all items in the library, is located on the left. Each item has a unique icon and context menu. The Main View displays the information of selected item and it changes depending on the type of the items.


The buttons in the toolbar are described in Table 8-1. Most of them are also available through the context menu in Library Explorer.


The Library Explorer shows all items in a tree view structure.

Order of Items

Items of different types in one group are organized in a certain order as described below. Items with higher order will appear before items with lower order. Items of same type are arranged alphabetically by name.

  1. Sub-group
  2. Rain Gauge
  3. IDF Group
  4. Manual Input Design Storm
  5. Read-in Design Storm
  6. Chicago Design Storm
  7. Mass Design Storm


Save changes
Save As
Save the library to another location
Export selected item
Import from a previously exported database to merge new data into library
Top Group
Create a new top group
Sub Group
Create a new sub-group
IDF Group
Create a new IDF Group
IDF Curve
Create a new IDF Curve
Manual Input
Create a new Manual Input design storm
Create a new Read-in design storm
Create a new Chicago design storm
Create a new Mass design storm
Rain Gauge
Create a new rain gauge
Read-in precipitation time series
Temperature Gauge
Create a new temperature gauge
Read-in temperature time series
Evaporation Gauge
Create a new evaporation gauge
Read-in temperature time series
Groundwater Gauge
Create a new groundwater gauge
Read-in groundwater time series
Flow Gauge
Create a new flow gauge
Read-in flow time series
Remove selected item
Add to Model
Add selected item to current project
Open help system


In Library Explorer, icons are used to identify different items in the library as shown in Table 8-2.


IconData TypeIconData Type
IDF GroupIDF Curve
Manual Input Design StormRead-in Design Storm
Chicago Design StormMass Design Storm
SCS Type II Design StormAES Design Storm
Rain GaugeRead-in Measured Storm
Temperature GaugeTemperature Data
Evaporation GaugeEvaporation Data
Flow GaugeFlow Data

Context Menu

Context menus in Library Explorer is content-sensitive. An example is given below.

Drag and Drop

The items in Library Explorer supports drag-and-drop for various functions based on the type of source and destination item. Examples are:

  • -Move item from one group to another
  • -Apply IDF to Chicago design storm
  • -Add design storm and temperature data to Project Manager.

Main View

The items in Resource Library is displayed and edited in the Main View. The Main View changes depending on the type of the item. The view for IDF group, Chicago design storm, MASS design storm, and temperature data is given below.


Items can be added to the library by using the buttons in the Toolbar or the context menu in Library Explorer.

Adding Group

There are two group types. The top group is the group located at the top level and the sub-group is the group located in another group.

To add a top group, click the Top Group button  in Toolbar.

To add a sub-group in an existing group, select the parent group first in the Library Explorer and then click the Sub Group button  in the toolbar or choose Add New SubGroup in the context menu.

Adding IDF Group

An IDF group is a group of IDF curves of various return periods (2-100 year) for the same location or area. To add an IDF group, first select the parent group and then click the IDF Group button  in the Toolbar or choose Add New IDF Group in the context menu, which will open the Add A New IDF Group window as shown below.

The IDF curves in IDF group can be defined with either fitting parameter A, B, C or the duration-intensity ordinates. For latter case, the A, B, C parameter will be automatically calculated. In most cases, the A, B, C values will be provided. Please note that the rainfall intensity equation used is i = A/(t+B)C. If a different format is used, the correct values should be used.

The new IDF group will be created by clicking the OK button. Note that the new IDF group is not empty. 6 IDF curves are created for return period 2 year, 5 year, 10 year, 25 year, 50 year and 100 year respectively with data from the City of Toronto (published by Environment Canada on December 21, 2014).

Utilizing IDF Files from Environment Canada

IDF files are publicly available from website for various locations across Canada. There are four (4) tables in the downloaded file and the Table 2a and 2b can be used to create the IDF group.

To create the IDF group, follow the steps given below.

  1. Copy Table 2a or 2b to spreadsheet editor and convert it to a data table;
  2. If Table 2a is used, convert the rainfall amount to rainfall intensity (mm/hr);
  3. If Table 2b is used, remove the accuracy lines;
  4. Transpose the data table to be in the format used in Resource Library;
  5. Create a new IDF Group with the IDF Definition which is set as Duration-Intensity Data Values.
  6. Copy the data table and paste into IDF Table. To paste the data correctly, select the cell for 2-year return period and 5 min duration.

Adding IDF Curve

It is necessary to add a new IDF curve if the desired return period doesn’t present in the IDF group. To add a IDF Curve, first select the IDF Group that you want to add a IDF Curve to in the Library Explorer, and then click the IDF curve button  in the Toolbar or choose the Add New IDF Curve menu in the IDF Group context menu. The return period of the new IDF curve is 0 and needs to be edited later.


There are four ways to create a design storm in Resource Library which are corresponding to different scenarios.

  1. Manual Input is used for the case where the design storm will be created from scratch. It gives the flexibility to add, insert, modify, and delete the time series data directly. Although it also supports copy-and-paste from other sources, it is not recommended to use manual in-put for that case. Read-in should be used instead.
  2. Read-in is designed for an existing design storm, which may be exported from another design storm tool or modelling software. Two file formats are supported which are stm and csv.
  3. Chicago is used to create a Chicago design storm based on given parameters including IDF curve. It is often used for cases where model needs to run on Chicago design storms from IDF curves with different return period. It is recommended to add the IDF information into the library first and then create the design storm.
  4. Mass is used when a mass curve file and total precipitation is available. Examples are SCS Type II and AES design storms.
  5. SCS Type II is used to create a 6-hr, 12-hr or 24-hr SCS Type II design storm.
  6. AES is used to create a 1-hr AES design storm.

To create these design storms, click the corresponding button in toolbar or choose the corresponding menu in the context menu as shown in the figure below.

New manual input and Chicago design storms will be added immediately and set as the active item. A window will appear for Read-in and MASS design storm for user to preview the data be-fore its added into the library.

Chicago, SCS Type II and AES design storms can also be created based-on IDF Group as shown in the figure below. The design storms are created for each return period in the IDF Group.

New Read-In Design Storm

The window for new read-in storm is shown in the figure below. It allows users to create a name and browse the data file for preview.

Click the Browse … button in the window to browse the data file to display it in the Data Preview area at the bottom as shown below. It supports the STM file and CSV file. The format of climate data files can be found in 10.2 Climate Data Files.

If Use File Name is checked, the file name will be set as the name of the new read-in design storm. If you are satisfied with the data, click OK button to create the design storm in the library.

Multiple read-in design storms can be created from multiple storm files. It is available from the Group context menu as shown in section 7.5.4. The new design storms will be added to the select-ed Group without preview. It is convenient when there are a few storm files.

New MASS Design Storm

The window for new MASS design storm is shown in the figure below. The window is similar as the one for read-in design storm. The mass curve file SMT file is supported and the total precipitation can be entered. The format of climate data files can be found in 10.2 Climate Data Files.

New Design Storms based on IDF Group

Chicago, SCS Type II and AES design storms can be created using the IDF data. To create the design storms,

  1. Select the IDF Group and choose the corresponding context menu based on the type of design storms to be created.
  2. The parameter window will appear as shown in the figure below. The available parameters are different depending on the type of the design storms.
  3. Once the parameter is entered, click the OK button to create the design storms. A new folder is created with all new design storms as shown in the figure below. The folder is named with the name of the IDF Group and the type of the design storm. And the new design storms are named with return period, duration, time step and type of design storm. 

Adding Rain Gauge, Temperature Gauge, Evaporation Gauge, Groundwater Gauge, and Flow Gauge

For continuous simulation, the time series data usually comes from monitoring gauges. It is necessary to have the gauge information (e.g. ID and location) included in the Resource Library to enable connect to monitoring database and apply distributed rainfall models. Each gauge may have more than one time-series data which may have different time step or cover different time range.

For evaporation, it may be a pan evaporation gauge or a general climate monitoring station where the potential evapotranspiration can be calculated using various equations.

To add a new gauge, first select the parent group in the Library Explorer and then click the Rain Gauge button  , Temperature Gauge button Evaporation Gauge button Groundwater Gauge button , or Flow Gauge button  in the Toolbar. For evaporation gauge, it is necessary to specify the type of the data type. It can be lake evaporation, pan evaporation or potential evapotranspiration.

Adding Precipitation, Temperature, Evaporation, Groundwater, and Flow Data

The monitoring precipitation, temperature and evaporation data is added to corresponding gauges from a data file. To do this:

  1. Select the gauge first and then click the proper Read-in button  in the Toolbar.
  2. In the New Monitored Data window, click the Browse … button in the window to browse the data file to display it in the Data Preview area at the bottom. If Use File Name is checked, the file name will be set as the name of the data item.
  3. If you are satisfied with the data, click OK button to add the data item to the library.
  4. For the measured Rain Data, Choose proper Unit and Rain Format. The Unit has two options: Metric and Imperial. For the Rain Format of VO project, only VOLUME is supported, but for VO-SWMM project, all the three types of Rain Format from EPA SWMM5 are supported: INTENSITYVOLUME and CUMULATIVE

The data file should be in a simple CSV file with first column as the time and the second column as the data value. The first row is treated as the column name and will be ignored.

Although data from any time step can be added to the library, the continuous simulation only sup-ports certain time steps. For precipitation, it is 5min, 10min, 15min, 20min, 1hr and 1day. For temperature and evaporation, it is 1day. The format of climate data files can be found in 10.2 Climate Data Files.

Adding Water Quality

In the Library Explore, select the group where the new Water Quality will be located. Click the Water Quality button  in the Water Quality toolbar, the LandUse table appears with default LandUse types, User can make changes to the Basic Information section and to LandUse section.


The Chicago design storm can use the information of any IDF curve in the library. There are two ways to assign the IDF curve information to a Chicago design storm:

  1. Copy and paste from the IDF curve;
  2. Drag and drop the IDF curve to the Chicago design storm in Library Explorer.

Copying and Pasting A, B, C

To copy and paste A, B, C from an IDF curve to a Chicago design storm:

  1. Select the desired IDF curve in Library Explorer.
  2. In the Main View, click the Copy A,B,C button to copy the A, B, C values. Note that if the fitted A, B, C is invalid, the values can’t be copied.
  3. Select the desired Chicago design storm in Library Explorer and click Paste A,B,C but-ton in the A,B,C section. The A, B, C values are overwritten by values from the copied IDF curve. Upon the change, the time series data table and graph are automatically updated to use the new values. Note that the Paste A,B,C button is not available if no IDF curve is copied.

Dragging and Dropping IDF Curve to Chicago Design Storm

To use drag-and-drop to assign the IDF curve information to a Chicago design storm:

  1. Select the desired IDF curve in Library Explorer.
  2. Keep the left mouse button pressed and drag it over the Chicago design storm in Library Explorer. Notice that the mouse cursor changes to the one shown below, which indicates the IDF curve information will be copied to the Chicago design storm.
  3. Release the mouse button to assign A, B, C values to the Chicago design storm. Notice the A, B, C values change, and the time series are updated automatically.


Resource Library sharing is necessary to:

  • -Make sure the required IDF, design storm and time series data are used;
  • -Save time to prepare and import the data file.

Resource Library can share any part of the library. An item and its child items will be exported to a separated database, which can be merged to other database with the structure unchanged. Single items (e.g. design storm) can also be exported to data files.


To export an item (and its children):

  1. Select it in Library Explorer and click the Export button  in the toolbar or choose the Export menu in the context menu.
  2. In the Save As window, change the default name and file format and then click Save. The default output format is SQLite database.

Export function is used to export a branch of the library. To export the whole database, click the Save As button  in the toolbar and follow the same procedure.


To import data from an exported database, click the Import button  in the Toolbar. Browse the right file in the Open window and click OK.

Importing merges the two databases together by only adding different items. Not all the data will be imported into the local library. If the same structure with the same name already exists in the same level, the existing item will be kept. For example, if a top group named Toronto exists in both local library and the imported database, no new group is created and the existing one will be used. These processes will be applied to all items in the imported library.

The process is demonstrated in the figure below. A merged library is generated based on the local library and the imported library. In this case, Group 1 and IDF Group 1 exists in both libraries. The one in the local library will be kept without any change even the one in imported database is different. Sub-group 2 and Design Storm 3 doesn’t exist in local library and is added into the final merged library.

Adding Climate Data to Model

Resource Library acts as the sole source of climate data for model. The climate data (rain, temperature, evaporation, groundwater and flow) can be added to the working model by 1) using the Add to model button/menu, 2) using Add All Design Storms to Project menu or 2) drag-and-drop.

To use the Add to model button/menu:

  1. Select the climate data in the Library Explorer;
  2. Click the Add to Model button  in the toolbar or choose the Add to model … menu in the context menu;
  3. The Add Storm(s) to Rain Group window will appear. The first one allows to choose one of the existing rain groups to add the design storm. The second one would create a new rain group with the given name.
  4. Click OK to add the design storm to the model. The new rain group will appear in Project Manager.

To use Add All Design Storms to Project Menu

Design storms in a group can be added to a model at once. To do this, select the group in the Library Explorer and choose the Add All Design Storm to Project from the context menu as shown below. All the design storms will be added to the model and the run group is also created for each design storm.

To use the drag-and-drop:

  1. Make sure the Project Manager is visible and not covered by Resource Library window.
  2. Select the desired climate data in the Library Explorer
  3. To create a new climate data group, drag it to the corresponding section in Project Manager:
  •  for design storm and long-term precipitation
  •  for temperature
  •  for evaporation
  •  for groundwater
  •  for flow.
  • -To add the climate data to an existing climate data group, drag it to existing rain group  , temperature group , evaporation/groundwater group  or flow group  in Project Manager;
  • -The cursor would change to indicate climate data can be added to current location.
  • -Release mouse to finish the operation. The climate data will be added to the current location and appears in Project Manager.

Certain rules apply when adding climate data to model:

  1. The climate data to be added should have the same time step and total duration as cli-mate data in the destination climate data group. If they don’t match, the operation will be rejected as it would cause error in simulation. It also makes sense from a practical point of view.
  2. The number of data points in the storm to be added can’t exceed 2000 for single-event model.
  3. It is not recommended to mix design storm and measured storm in one rain group.
  4. For rain groups with measured storms, it is recommended to use storm data with same starting date and time.
    Note that the last two rules are not enforced in VO. It is the modeler’s responsibility to make sure they are followed.

Adding Water Quality Data to Model

Water quality calculations have been added in VO 5.2. Total suspended solids (TSS) and total phosphorus (TP) loading and removal rates can be specified and calculated in the model. The user can input loading rates based on existing or proposed land use. To add water quality inputs to the model, the user must complete the following:

Open the Resource Library and select the Water Quality Button.

This will open the LandCover inputs tab. From here the user can specify TSS and TP loading rates for individual land covers. Once rates have been specified, the user must click “Add to Model” to add the water quality loading rates to the model scenario. Tables can also be saved for future use and so the user can save various municipality or conservation authority loading ratings.

The user can then assign land use parameters to each sub-catchment which the model uses in loading calculations. Typical loading rates can be assigned based on the existing or proposed land use. Removal efficiencies can be specified in the LID and Route Reservoir commands.


This chapters discusses the steps to create and run a simulation after the drainage network has been created and climate data has been added.


A simulation run is a combination of drainage network and climate data. For single-event simulation, the climate data may be design storm, regional storm or observed rainfall event. For continuous simulation the climate data is a combination of long-term precipitation, temperature, and evaporation time series data. Essentially, creating a simulation run is to combine the drainage network (scenario) with the right climate data. For single-event simulation, it is also possible to combine same drainage network with various design storms.

In VO, the simulation run is always created for current working scenario. Therefore, there is no need to specify the scenario for a simulation run. Setting up a simulation run is just to specify the climate data.

Note that, to create the simulation run, the scenario must be free of error and the climate data is already added to the Project Manager.

Single-Event Simulation

To create a single-event simulation, click the Run button  located at the Simulation tab to open the Batch Run window, where all simulation runs are shown in a data table with three columns.

  • -The first column is a check box to indicate if the simulation run will be run.
  • -The second column is the name of the simulation run which will be used in various place to switch results. It is recommended to use a recognizable name. To change the name, double click it.
  • -The last column is the rain group used for the simulation run. It features a drop-down list to have all available rain groups created in Project Manager.

A new project usually has a default simulation run using the default rain group (a Chicago design storm). To add a new simulation, click the Add button  in the toolbar. A simulation run can be deleted using the Delete button  in the toolbar.

To run simulations, make sure the simulations are checked in the first column and then click the Run button at the bottom. A window will appear to show the simulation run progress. The Batch Run window will be closed after the simulation run is finished.

The simulation results are saved in files. To delete these results file, click the Clear All Results button at the left bottom corner.


This chapter introduces how to set and run a continuous model.

Setting Simulation Engine

Before creating and running continuous simulations, it may be necessary to change the global parameters. To change these parameters, click the Engine Options button  in Simulation tab. The Simulation Engine window will appear.

In the Simulation Engine window, the parameters are grouped to four (4) categories, generally based on the hydrological process. Each group has a corresponding tab in the window. There is only one parameter in the Time Step group, which is the simulation time step in minutes. The default time step is 5 minutes, which may need to be changed for long-term simulations. All other parameters are described in Table 9-1, Table 9-2 and Table 9-3 for snow, initial abstraction, and soil.


ParameterUnitDescriptionDefault Value
Snowfall Temperature°CThe dividing temperature for snowfall and rainfall0
*Rel. Densityvol/volRelative density of new snow which varies from 0.02 to 0.150.1
Fraction of Liquid WaterN/AThe fraction of liquid water in new snow0
Compaction Coef. AN/ACompaction coefficient A used in
KC = B * EXP (-A * TAIR)
Compaction Coef. BN/ACompaction coefficient B used in
KC = B * EXP (-A * TAIR)
Max. Rel. Dry Densityvol/volThe snow pack maximum relative dry density0.35
Snowmelt Temperature°CThe snowmelt base temperature-0.2
Snowmelt Factormm/(day.°C)The monthly snowmelt factorN/A
Fraction Free Water CapacityN/AIrreducible water saturation fraction of total pore volume0.05
Rel. Density of Icevol/volThe relative density of ice0.92


ParameterUnitDescriptionDefault Value
Infiltration Ration in Per.N/AThe infiltration ration in pervious area0
Fraction Runoff Indirectly Imp. to Per.N/AThe fraction of runoff from indirectly con-nected area flow to pervious area1


ParameterUnitDescriptionDefault Value
Lake Evap. to Pan Eva. RatioN/ALake evaporation to pan evaporation ratio, 0.6 – 0.80.8
PET Reduction Ratio Due to RainN/APET reduction ratio to account for rain0.1
Soil Storage Capacity Reduction Ratio Due to Frozen SoilN/ASoil storage capacity reduction ratio due to frozen soil conditions0.12
Monthly Evaporationmm/monthMonthly evaporationN/A
Monthly Growth IndexN/AMonthly growth indexN/A

The snow parameters, Snowmelt TemperatureSnowmelt Factor and Max. Rel. Dry Density, are assumed to change in a year between a given minimum value and maximum value. The minimum value appears in each month. The change is described using a sinusoidal curve as shown in the figure below for a snowmelt factor. To edit the minimum, maximum value and the month for the minimum value, click the  button on the right side of each parameter. The preview curve will be updated automatically once the values are changed. By default, a fixed value is used for Snowmelt Temperature and Max. Rel. Dry Density but Snowmelt Factor changes.

To edit the monthly evaporation and growth index, click the Edit button after the parameter name as shown in the figure below.

The monthly data editor will appear as shown in the figure below. New values can be entered in the table on the left and the plot will be automatically updated.

Creating and Running Simulations

To create a continuous simulation, click the Run button  located at the Simulation tab to open the Batch Run window. This layout is similar to the single-event simulation and has more columns which are described below.

  • -The first and second columns are the same as the one for single-event simulation.
  • -The third to fifth columns are to choose the rain group, temperature group and evaporation group for the simulation, of which the temperature and evaporation is optional. If temperature is left as empty, a fixed temperature (10 °C) will be used in the simulation and the snow process will be ignored. If evaporation is left empty, the monthly evaporation in Simulation Engine window will be used. To appear in the drop-down list, the precipitation, temperature and evaporation should be added to Project Manager first.
  • -The last two columns are to set the simulation starting and ending date. It will be set to the minimum common time period based on the selection of the three climate data items and can be changed by clicking on the calendar icon.

Different from single-event simulation, no default simulation is created (as there is no default precipitation data). To add a new simulation, click Add button  in the toolbar. All other operations are the same as single-event simulation.

The Engine Options button  is also available in the toolbar, which can be used to change the global parameter before running the simulations.

Note that the continuous simulation run usually takes more time than single-event simulation. It is recommended to adjust the simulation time step if it takes too long.


This chapter discusses how to utilize the output features of VO. By becoming familiar with this chapter, users will find that their modeling time decreases rapidly, as they will find their answers more quickly and efficiently.

Overview of Output Features

Different outputs are available for single-event simulation and continuous simulation.

Hydrograph is the main output from single-event simulation. The hydrograph is for only one event covering a few hours or days. VO provides five (5) different ways to view the hydrographs:

  • -Summary Data (table and label)
  • -Hydrograph Data
  • -Hydrograph Plot (single, cross scenario and plot calibration)
  • -Detailed Output
  • -Summary Output

-Water balance is considered in the continuous simulation. The outputs are the long-term water balance and flow. VO provides two (2) output features to view continuous simulation outputs.

  • Summary Data (table, pie chart and label)
  • Time Series Plot (Single and Plot Calibration)

These output features can be found on the Simulation tab in the Output section. Access to these output features will be enabled after a successful simulation is run. Users will find that each feature has its use and at least one output feature will meet their needs for viewing output. A short description of each output feature is given as follows. More detailed information on each feature is contained within subsequent sections.


Output FeatureDescription
Summary DataThis gives a summary table of key output data, based on objects selected by user.
Hydrograph DataThis gives a summary table of actual hydrograph points (time and flow) for the objects selected by the user.
Hydrograph PlotThis shows a graphical plot of the hydrograph(s) and run(s) as selected by the user.
Detailed OutputThis shows the user the detailed text output file, which is familiar to previous users of OTTHYMO.
Summary OutputThis shows the summary text output file, which is familiar to previous users of OTTHYMO.
Summary DataThis gives a summary of all water balance components.
Time Series PlotThis shows a graphical plot of the time series data and run(s) as selected by the user.


This chapter introduces how to view and plot the simulation results of single-event model.


Users can view key output data (Table 10-2) at any location within their model. Summary data can be viewed once a model simulation has been performed. The summary data can be viewed in a table or with labels.


DThrSimulation Time Step
AREAhaContribution Area
PKFWm³/sPeak Flow
TPhrTime to Peak
RVmmRunoff Volume
DWFm³/sDry Weather Flow


Volume Reduction Rate(%)(Runoff Volume In – Runoff Volume Out) / Run off Volume In
Volume of Water for Drawdown(m³)Time taken for maximum volume stored in the LID to empty.
Time of Max Ponding(Hr)Time at which the maximum ponding volume occurs.
Seepage Rate(m/Hr)Seepage rate into native soil.
Calculated Draw-down Time(Hr)Time taken for LID to empty after the duration of the storm event.
Maximum Volume of Storage Used(m³)Maximum volume of water stored in the LID throughout the event.

Viewing Summary Data in Table

To view the summary data in a table, use the Hydrograph Result window at the bottom besides the Parameter Tables window. It shows the summary data of all objects in the model. On the top of the window, there are two options to specify the run and switch to show all runs. By default, only the summary data of the active run is displayed.

When there are large number of objects, it is necessary to show only the summary data of a few objects. To do this, first select the object on map view or schematic view and then click the Hydrograph Result button  in Simulation tab. A new window is created besides the Map View and Schematic View.

Viewing Summary Data in Label

VO can provide labels for each object on Schematic View, on an individual basis or all command basis. The label content includes the NHYD, name, and hydrograph summary data. To create labels:

  1. Select the objects you wish to create a label for.
  2. Right-click and select the Set Labels from the context menu to open the Label Editor window, which lists all available label items.
  3. Drag and drop the label items from Available Properties list on the left to Selected Properties list on the right.
  4. Change the label background color with the color picker at the bottom.
  5. Click OK to create the label.
  6. To show the labels on canvas, select Show Labels from the context menu.

Note that the summary data value is empty if the output is not available.

Hydrograph Data

This output feature allows the user to view the actual hydrograph numerical data, in terms of time step and flow. Users can view an individual hydrograph, at a selected location within the model, or multiple hydrographs. Users can also copy the desired values and paste them in Notepad or Excel.

To view the hydrograph flow data, first select the objects in Map View or Schematic View and then click the Flow Data button  in Simulation tab. The Hydrograph Flow Data window will appear where the hydrograph data is shown in a table. In case where more than one simulation is available, the simulation run can be switched through the combo box at the top. To export the data to a file, use the Export button on the top right corner.


The Hydrograph Plot feature can be used to view hydrographs graphically without the need for a third-party application. This feature allows users to quickly view their hydrographs, compare them, and make qualitative assessments about their model.

VO provides three (3) options to plot hydrographs for single-event simulation:

  • -Hydrograph tool  – Plot hydrograph with rainfall data. It can be used to check the hydrograph responses and compare the hydrographs from different objects in the same scenario;
  • -Cross Scenario Plot tool  – Plot hydrographs from different scenarios (e.g. existing and proposed). It can be used to compare hydrographs of the same command from different scenarios;
  • -Plot Calibration tool  – Plot simulated and observed hydrograph. The comparison is helpful for model calibration.


To view the hydrograph, first select the objects in Map View or Schematic View, then click the Hydrograph button  in Simulation tab. The Hydrograph window will appear.

The hydrographs of selected objects are plotted at the bottom of the plotting area. The figure above is the rainfall plot. These two plots have same time scale. To zoom in, simply scroll the mouse scroll wheel up or draw a rectangle to the area of interest on the plot. To zoom out, scroll the mouse wheel down or select the Un-Zoom or Undo All Zoom/Pan from the context menu. All graph options available from the context menu are described in Table 10-4.


CopyCopies the graph
Save Image As …Saves the graphical image as the user-defined name
Page Setup …Allows user to modify the setup for the page
Print …Prints the hydrograph plot displayed
Show Point ValuesGraph will display the value that the cursor hovers
Un-ZoomGraph un-zooms the most recent zoom
Undo All Zoom/PanGraph will return to its original scale
PanningUse the cursor to drag and pan the graph image
ZoomingUse the mouse scroll to zoom into and zoom out of the graph. Enclose the specified area using the mouse to zoom into that area

The data source, visibility and color of the hydrographs can be changed with the controls on the left side of the window. At the top is the simulation run, selection box to switch to other simulation runs. Below that are the controls for rainfall and hydrograph plots. Individual plots can be turned off and changed to another color.

Cross Scenario Plot

The Cross Scenario Plot feature can be used to compare hydrographs from two different scenarios (existing and proposed) on the same graph. This is a good tool for quickly comparing hydro-graphs at known locations, based on proposed modifications within the upstream catchments.

To open the Cross Scenario Plot, click the Cross Scenario Plot button  in the Simulation tab. The Cross Scenario-Run Hydrograph Output window will appear. This window has two parts with a tree view of all scenarios, simulations and objects on the left and the plot area on the right.

Hydrographs are not plotted when they are first opened. To show the hydrographs, check the checkbox on the left of the desired objects in the tree view. Although it is possible to plot hydro-graphs of all objects, it is recommended to compare the hydrographs from the same objects.

Plot Calibration

It is necessary to compare the observed and simulated hydrograph in model calibration. Plotting the two hydrographs and the corresponding rainfall in the same plot is very helpful to guide the calibration.

Identifying the Gauge Objects

To compare to an observed hydrograph, the location of the flow monitoring needs to be defined first. This is done by selecting the object and then right-clicking to open the context menu and choosing Has Gauge Here from the context menu.

Importing Observed Data

To import the observed data and setup the comparison, click the Plot Calibration button . The Compared to Observed Data window will appear. In this window, on the left is where the observed data will be imported, and the comparison can be configured on the right.

To add observed data, click the Add … button at the left bottom corner and select the observed hydrograph data file. For file format, please refer to 10.3 Calibration Files. The observed data will be added to the Observed Data list. To remove it, select it in the list and click the Remove button at the bottom.

Adding Result Comparison

There are three components in a result comparison: 1) observed data, 2) simulated data and 3) corresponding rainfall. The observed data is added by drag-and-drop from the Observed Data list. The Simulated Data is specified by selecting the simulation Run and the corresponding Gauge Location. The hydrograph from the specified object will be used to compare with the observed data. The Corresponding Rainfall is the rainfall data that is responsible for the hydrograph response. It will be plotted on the top of the hydrographs as a reference. It is important to have this when analyzing the observed and simulated hydrograph. If the observed hydrograph is not a clear response from the rainfall, it may be necessary to check the rainfall data.

To create a result comparison, double-click an observed data in the Observed Data list or drag it to the Comparison list on the right. A new result comparison will be added in the Comparison list and the observed data is set to selected observed data. Once added, all the three components can be changed by selecting the desired items in the drop-down list. Note that Gauge Location only lists the objects that is set as a gauge using the Has Gauge Here context menu.

More than one comparison can be added using the same methods. This is useful for large watersheds with multiple flow monitoring stations.

Viewing Observed and Simulated Plot

After result comparisons are created, it is time to plot the observed and simulated hydrograph. To view the plot, click the View button on the far-right side of each result comparison. The Ob-served/Simulated Plot will appear.

The plot area is at the top where the rainfall is plotted above the observed and plotted hydro-graph. The observed hydrograph is in green and the simulated hydrograph is in red.
Below the plot area are the statistics of two hydrographs including the minimum flow (m³/s), maximum flow (m³/s), volume (m³) and relative difference of peak flow and volume. Often the latter should be in an acceptable range for a calibrated model. For example, TRCA usually requires the simulated volume should be +20% to -10% of the observed one and the simulated peak flow should be +25% to -15% of the observed peak flow. The given relative difference in this window would help determine if the simulated hydrograph is acceptable or not.

At the bottom is the control bar to navigate through multiple comparisons in cases where multiple flow monitoring locations are available. Click  or  to move to the previous or next comparison in the list. A summary information of current comparison is also shown between the two navigation buttons.

Water Quality Data

This output feature allows the user to view the actual water quality results in Water Quality Results window found at the bottom of VO.

Traditional Detailed and Summary Output

When a simulation is run successfully, VO generates traditional Summary Output and Detailed Output and stores them in the project folder. For those new to VO, the Summary Output file is a text-based file that contains the key output data for each object during each storm simulation. The Detailed Output file is a text-based file that contains all the output data for each object during each storm simulation. Review agencies often require these text output files when reviewing a model.

The detail output and summary output are available through the Detail Output button  and Summary Output button  and their sub-menus in the Simulation tab. The process to view and export the detailed and summary output are similar. In this guide, only the detailed output is described. You can easily follow the same steps for summary output.

Viewing Detailed Output

To view the detailed output of selected objects, first select the objects in Map View or Schematic View, and then click the Detail Output button  in Simulation tab. A new window will appear besides the Schematic View tab. Note that the output of all simulation runs is listed. The infor-mation in this view can be exported or copied to a file through the context menu.

To view the detailed output of all objects in the current simulation run, deselect all objects and click the Detail Output button  in Simulation tab. Similar to the previous case, a new tab is created besides the Schematic View to have all the information. To switch to another simulation run, select it from the drop-down list above the Output button  in Simulation tab.

To view the detailed output of selected simulation runs, select View Selected runs detailed output sub-menu by clicking the triangle sign on the right of the Detail Output button  in Simulation tab. The detailed output from selected simulation runs of all objects will be listed in a new tab besides the Schematic View tab.

Exporting Detailed Output

The detailed output can be directly exported to files. To do this, use Save detailed output as … or Save selected runs detailed output as … sub-menus by clicking the triangle sign on the right of the Detail Output button  in Simulation tab. They export the detailed output of all object in Current simulation run or Selected simulation runs (the second one). Give a file name in the Save As window and click the Save button. The information is saved as a simple text file.

Reviewing Output

Prior to printing and submitting your model for review, it is a good idea to review the Detailed Out-put file and check for any Warning or Error messages. As with most computer programs, VO will often run with improper input and yield some suspect results. It is therefore recommended that all users become accustomed to reviewing output files.

Warning messages such as “Warning: Incoming Hydrograph is dry” may not necessarily be a problem, provided that the user intended it this way.

A common warning message for the STANDHYD commands is “Warning: Storage coeff. is small-er than time step”. This message says that storage coefficient is smaller than the time step, DT. This means that the time to peak of the unit hydrograph is greater than calculated time of concentration which can result in an underestimation of the peak flow. Therefore, the user should reduce the DT to an integer value less than the storage coefficient. Do not reduce DT less than 1.0. This warning message can be safely ignored when DT is set to 1.0.

All routing commands should be checked to ensure that the input rating curves have not been exceeded, which will result in erroneous results.


There are two main differences between the output from continuous simulation and single-event simulation.

  1. The summary data is for the long-term time-series data including the water balance. Most of the summary data is on average annual basis.
  2. One hydrological object has multiple time series outputs.

Summary Data

The summary data in continuous simulation is listed in Table 10-5. Note that the average annual summaries may not be the average annual values if the simulation time period doesn’t cover the whole year.


PmmAverage Annual Precipitation
RainmmAverage Annual Rainfall
SnowmmAverage Annual Snowfall
*Snowmelt mmAverage Annual Snowmelt
ETmmAverage Annual Evapotranspiration
INFILmmAverage Annual Infiltration
GWImmAverage Annual Groundwater Infiltration
RunoffmmAverage Annual Runoff
Runoff CoefAverage Annual Runoff Coefficient
Peak Flowm³/sPeak Flow
Peak Flow TimeThe Time when Peak Flow Happens

Similar to single-event simulation, the summary data can be viewed in a table or with label as shown in the figure below. For continuous simulation, the summary data window is changed to Water Balance Result. Note that the water balance summary data is only available for catchments (i.e. NasHyd and StandHyd).

The water balance can also be summarized on monthly and yearly basis. To view the monthly and yearly water balance summary, choose the Water Balance menu from Schematic View context menu. The Water Balance window will appear as below.

On the left is the yearly and monthly water balance summary table shown in two tabs. Besides the summary data of each year and month, the average is also given at the bottom. For a month summary, a total summary is also available which basically is the average annual summary.

On the right side is a pie chart to highlight the three important water balance components: Infiltration, ET and Runoff. The chart is corresponding to the active summary data row in the summary table. To change to another summary data, click on the desired row or use the arrow key to move up and down. In this pie chart, default colors are used: green for ET, red for runoff and blue for infiltration. Note that the percentage in the pie chart is the percentage of the total of the three components. For runoff, it is not the runoff coefficient.

The water balance summary table can be for individual catchment or for all catchments. To show the summary table for individual catchment, select it before choosing the Water Balance context menu. For all catchments, deselect all objects before choosing the Water Balance context menu.

Wetland Water Balance

The Wetland Water Balance output is best represented using the scenario comparison tool . This tool provides graphical results for the Wetland Water Balance command and allows users to plot results for multiple scenarios as a time series or water balance graph. It should be noted that if you run multiple years of data, the graphs completed for a season or month provides the average values for all years. In order to show seasonal or monthly results for individual years, use a user defined time period. This can be set up in your model so that each year is run separately.


Multiple time series data are available for each hydrologic object. The available time series is different depending on the types of the objects. The time series can be viewed using three tools:

  1. Hydrograph  – Plot the simulated flow data with precipitation data.
  2. Plot Results  – Plot selected time series data with given time interval.
  3. Plot Calibration  – Plot observed and simulated flow or water level.


Flow data is available from all hydrologic objects. To view the flow data, select the objects in Map View or Schematic View, and then click the Hydrograph button  in Simulation tab. The Hydrograph window will appear as below. This window is same as the single-event hydrograph window. For more information, please see

Plot Results

To view other time series data, select the objects in Map View or Schematic View, and then click the Plot Results button  in Simulation tab. The Plot Results window will appear.

On the left side is the control panel to control the data source and appearance of the plots. From the top to bottom, the options are:

  1. Run – The simulation run.
  2. Type – The type of objects. Objects of same type have similar time series.
  3. Variables – The available time series.
  4. Interval – The time interval, for the time series data, will be summarized and displayed. The options are Original, Day, Week, Month and Year. The original will use the simulation time step. The line graph is used for Original, Day and Week. Month and Year will use the bar chart.

Below the control panel is the list of selected objects which is where the plot visibility and color can be changed. As continuous simulations tend to have large amounts of data in each time series data, it is not recommended to display multiple objects at the same time.

An example is shown in the figure below. In the first screenshot, the flow is plotted in 5-min time intervals. The same time series is summarized on weekly basis and plotted in the second screen-shot.

The time series data can be exported to a simple CSV file by clicking the Export button  on the top left corner.

Plot Calibration

Similar to the single-event simulation, it is important to compare the observed and simulated time-series data for model calibration. For file format, please refer to 10.3 Calibration Files. As there are more than one time-series data available, it is necessary to specify which time series to use for the comparison. This is done by selecting the Observed Data Type in the Calibration window as shown in the figure below. The two options are Flow and Water Level in the current version.

As the time series data may have multiple events, besides the same normal statistical values that are used to evaluate the agreement of the observed and simulated data in the event-based simulation, two other statistical values are provided:  and Nash-Sutcliffe Coefficient (NSE). A value close to 1 indicates a good match. These two statistical values update automatically when the plot area is zoomed in or out.

For wetlands, this plot calibration window provides the modeled vs observed water levels in the storage portion of the wetland and includes a blue line which shows the groundwater elevation. The statistics at the bottom of the screen includes a % difference as well as the  and Nash-Sutcliffe Coefficient. These values are provided at daily, weekly and monthly intervals for the time range shown on the graph.


This chapter introduces the type and format of files that can be used by VO.

Project Files

The project files are used to save all project data including:

  • -All hydrologic objects in each scenario
  • -All climate data

The locations (on map and on canvas), labels, links and symbols are all saved in the project file.

There are two project files in the project folder. The main project file has the extension of voprj and the secondary one is vdata file. Both files use the same project name. To submit a VO5 model, it is necessary to have both files.

Climate Data Files

VO supports various climate files in Resource Library.

READ Storm File (*.stm)

This is the storm file used by OTTHYMO-89/INTERHYMO READ STORM command. In VO5, it is used to create a read-in design storm in Resource Library. The file format is described below.
1st line: 2 (1 indicates in/hr, 2 indicates mm/hr)
2nd line: comment line (up to 60 characters in length)
3rd line: 10 (storm time step, min)
4th line: 24 (number of rainfall increments)
5th line: 2.071 (1st rainfall intensity)
6th line: 2.266 (2nd rainfall intensity)
7th line: 2.524 (3rd rainfall intensity)
…th line: …
xth line: 2.135 (xth and last rainfall intensity, x corresponds to the number in the 4 th line)
last line: -1 (indicates the end of the file).

An example of a Storm data file (25mm4hr.stm) is as follows:

MASS STORM File (*.mst)

This is the storm file used by OTTHYMO-89/INTERHYMO MASS STORM command. In VO5, it is used to create a MASS storm in Resource Library. The file format is described below. Note that the ordinate is the fraction of the accumulated rainfall volume to total volume. So it starts from 0 and increases to 1.

1st line: comment line (up to 60 characters in length)
2nd line: 20 (the time increment between each ordinate, min)
3rd line: 13 (the # of ordinates used to describe the mass curve, max.=400)
4th line: 0.00 (the first ordinate of the mass curve)
5th line: 0.01 (the second ordinate of the mass curve)
6th line: 0.04 (the third ordinate of the mass curve)
…th line: …
xth line: 1.00 (xth and last ordinate of the mass curve)
last line: -1 (indicates the end of the file).

An example of a MASS Storm data file (aesmass.mst) is as follows:


Simple CSV

For single-event simulation, CSV file is used to import the precipitation. For continuous simulation, CSV file is used to import the precipitation, temperature, and evaporation data. The continuous simulation only supports certain time steps. For precipitation, it is 5min, 10min, 15min, 20min, 1hr and 1day. For temperature and evaporation, it is 1day.

Two columns are required for precipitation and evaporation. The daily temperature file has three columns with date time, minimum daily temperature, and maximum daily temperature. The first row of the CSV file is the column header and will not be read. For single-event design storm, the first column is in minutes and it is suggested to have the data starting from 0 minute. For continuous climate data, the first column is in a format of “year-month-day”.

An example of single-event rain data in CSV file is shown as below. Please download the CSV file template of single-event rainfall data.

An example of continuous rain data in CSV file is shown as below. Please download the CSV file template of continuous rainfall data.

An example of continuous temperature data in CSV file is shown as below. Please download the CSV file template of continuous temperature data.

Calibration Files

The calibration files are used to import the observed time series data and compare to the simulation outputs. Two file formats are supported: 1) SWMM format and 2) simple CSV format.

SWMM Format

The SWMM calibration file format is supported. From SWMM user’s manual, the format of the file is as follows:

  1. The name of the first object with calibration data is entered on a single line. This name will be the observation data name in Plot Calibration tool.
  1. Subsequent lines contain the following recorded measurements for the object:
  • Measurement date (month/day/year, e.g. 6/21/2004). Note that the elapsed time format is not supported.
  • Measurement time (hours:minutes) on the measurement date or relative to the number of elapsed days
  • Measurement values
  1. Follow the same sequence for any additional objects.

An excerpt from an example file is shown below. It contains flow at two locations FG02HC023 and FG02HC009. Note that a semicolon can be used to begin a comment.

Simple CSV Format

The simple CSV format has two columns: 1) measurement date and time column and 2) measurement value. The data and time column is in a format of “year-month-day hour:minute:second”. It is assumed that the first row is the header and will be ignored.

An example of calibration data in CSV file is shown as below. Please download the CSV file template of calibration data.

Hydrograph Files

If the READ HYD object is used to read an external hydrograph file into the model, then the file must be coded in the correct format. This format is in the same format as the hydrograph files used in OTTHYMO-89/INTERHYMO. Therefore, previously made hydrograph files (from SAVE HYD command) may be used. The file format is described below.

1st line: 2 (1 indicates in/hr, 2 indicates mm/hr)
2nd line: comment line (up to 60 characters in length)
3rd line: 5 (storm time step, min)
4th line: 50 (the catchment area from which the hydrograph was obtained, ha or acre)
5th line: 0.000 (1st hydrograph ordinate, m³/s or cfs)
6th line: 0.100 (2nd hydrograph ordinate, m³/s or cfs)
7th line: 0.200 (3rd hydrograph ordinate, m³/s or cfs)
…th line: …
xth line: 0.100 (xth and last hydrograph ordinate)
last line: -1 (indicates the end of the file).

An example of a Hydrograph data file (test.hyd) is as follows:


This Chapter is written to aid users in resolving some of the simpler problems, warning messages, and error messages that might arise when using the model. These problems or messages may be attributed to invalid input, using the model for situations where it was not designed to be used, or not following a procedure properly as set out in the User’s Manual.


Error and warning messages can appear within the interface and within the Detailed Output Files. The messages stem from an input error or computational error, which is likely caused by the input variables. This section outlines the most common messages and gives a brief description of their meaning.

Interface File Messages

Unable to save project: Project files can only be saved on a drive with a capacity of 10MB or more (e.g. local hard-drive or network drive). It is recommended that users only run models from local or network hard drives.

Multi-Instance is not allowed: Users can only run one copy of VO on their machine at a time.

You have exceeded the maximum number of objects for this license: The version of VO you are running has a specific number of objects allowed, per project. This number has been exceed-ed. Either decrease the number of objects or upgrade your version.

Output File Messages

ERRORCHECK NUMBER OF RAINFALL INCREMENTS: This occurs in the old URBHYD command where the number of rainfall increments was an input variable. It therefore has to match with the storm being simulated.

ERRORCHECK THE STORAGEDISCHARGE TABLE.: There is an error in the input variables in the storage-discharge table.

ERRORUNITS OF INPUT DATA NOT SPECIFIED: Either the scenario or project settings have a conflict in the input units. Make sure each unit type is consistent (i.e. metric or imperial).

ERRORSHIFT TO LARGERAIN ELIMINATED: This error occurs with the SHIFT HYD command. If the shift is too large, the rainfall may be eliminated past the 800 point time steps.

ERRORTIME STEP = 0, COMMAND ABORTED.: A DT of 0 has been inputted and is unacceptable.

ERRORRAINFALL INCREMENT = 0, COMMAND ABORTED.: The number of rainfall increments is zero, therefore runoff will not be calculated.

WARNING: A MINIMUM SURCHARGE OF (X) CAN BE EXPECTED WITH THE GIVEN PIPE.: This is from the ROUTE PIPE command. The message indicates that surcharge may occur due to lack of conveyance capacity.


WARNINGCANNOT COMPUTE 1a WHEN USING PROPORTIONAL LOSS METHOD: The initial abstraction cannot be calculated (i.e. cannot be -1) when using the proportional loss method. It must be specified by the user.

WARNINGCOMPUTATIONS FAILED TO CONVERGE.: The routing time step is too large, try reducing it.

WARNINGFIRST OUTFLOW IS NOT ZERO.: The first point in a ROUTE RESERVOIR command must be 0,0.

WARNINGFOR AREAS WITH IMPERVIOUS RATIOS BELOW 20%, YOU SHOULD CONSID-ER SPLITTING THE AREA.: This is a warning for STANDHYD. The STANDHYD command was designed for areas with impervious ratios greater than or equal to 20%. When the ratio is less than this value, the runoff may not be correct. The catchment should therefore be split into impervious and pervious components.

WARNINGHYDROGRAPH PEAK WAS NOT REDUCEDCHECK OUTFLOWSTORAGE TABLE OR REDUCE DT.: The routing command did not reduce the peak flow. Reduce the DT, check the input variables, or review the assumptions made with routing object.

WARNINGHYDROGRAPH WAS CUTCHECK VOLUME.: This happens in routing commands and may be caused by a high DT or too many points in the hydrograph (i.e. approaching 800). If the volume out is only marginally less than the volume in, then this may not be of concern.

WARNINGLENGTH OF STORM HAD TO BE CUT FROM (X) POINTS TO 800.: The maximum number of hyetograph points is 800. The rest have been truncated. To include all points increase the DT of the storm file and hydrographs computations as well.

WARNINGMINIMUM PIPE SIZE REQUIRED = (X) FOR FREE FLOWTHIS SIZE WAS USED IN THE ROUTING.: Too small of a pipe size was inputted. The program automatically calculated a new size and this was used in the routing calculations.

WARNING: N DID NOT CONVERGE AFTER 50 ITERATIONS.: This is from the WILHYD command. There was a problem calculating an internal number for the number of linear reservoirs. Check the input parameters and re-run.

WARNINGSELECTED ROUTING TIME STEP DENIED.: The DT of the routing command is not less than or equal to the DT of the incoming hydrograph. Change the input parameter for this command.

WARNINGSLOPE <=0.0 (HYD WAS ONLY TRANSFERRED): This is from the ROUTE PIPE Command. The pipe slope must be positive or there is no routing calculated.

WARNINGSTORAGE COEFF. IS SMALLER THAN TIME STEP!: The peak flow may be underestimated since the storage coefficient relates to the time-to-peak of the unit hydrograph. Users should manually decrease the DT of the hydrograph until the warning disappears or the DT is as small as 1.0 minute.

WARNINGTHE PERVIOUS AREA HAS NO FLOW.: This may happen for small storms where the runoff volume is low, or in cases where the catchment imperviousness is high. Check the LGP (Pervious Area Flow Length) parameter as well. It can be too high.

WARNINGTHE TABLE WAS EXTRAPOLATED FROM (X) TO THE END.: The rating curve table has been extrapolated. Add more points to the rating curve.
WARNINGTRAVEL TIME TABLE EXCEEDED.: For a ROUTE CHANNEL command, the rating curve table has been exceeded. Add more points to the channel geometry. Also this can result from too large of a computation time step.

WARNINGVALIDATE_UNDERDRAIN_PIPEDIAMETER.: For underdrain calculations. Check (Diameter of Perforations+Minimum Spacing)*Number of Rows cannot be larger than Pipe Circumference

WARNINGVALIDATE_UNDERDRAIN_WARNING.: For underdrain calculations. Check that (Circumference – Number of Rows * Diameter of Perforations)/Number of Rows is less than (Min-imum Spacing)

WARNINGSOAKAWAYPIT_VALIDATE_AREA.: For underdrain calculations. Minimum bottom Area doesn’t meet MOECC guideline. Area=(W(m) * L(m) * H(m))/(InfiltrationRate(m/hr) * Draw-downTime(hr))

WARNINGSOAKAWAYPIT_VALIDATE_HEIGHT.: For underdrain calculations. Height of Soakaway Pit should be less than (InfiltrationRate(m/hr) * Drawdown(hr)).

Program Quits During Run Simulation

The most common cause of the program quitting during a run simulation is due to incorrect input. To isolate the suspect input, try the following:

  1. After the program quits, open the Detailed Output file using a text editor. Make note of the command (by NHYD) where the simulation appears to have stopped.
  1. If there are Warning or Error messages, refer to the previous section in this Manual for an explanation on the message.
  1. Re-open the Project and suspect the scenario and view the parameters of the object in question. Make any adjustments as required. If the scenario will not open then retrieve a backup copy from the BACKUP folder located in the project folder.
  1. If the object parameters are acceptable then the error can be from the rainfall information. If using an external rainfall file, confirm that it is present and that the format is correct (refer to Reference Manual for format). To see whether the rainfall file is where the error occurs, try running the scenario with the default Chicago Storm.
  1. If the above fails, try importing the scenario into a new project with only one scenario contained in the project.


As stated in Chapter 4, hydrologic object parameters can be edited directly in Properties window and Parameter Tables window. In some cases (e.g. model calibration), the parameter values may need to be changed by a certain percentage. Tools have been provided to make these changes.
There are three parameter-edit tools available for single-event model:

  1. Convert to CN *  – to convert the CNII to CN*
  2. Batch Assign  – to assign parameter with given values
  3. Calibrate Commands  – to change parameter values by given percentage

All these tools are in the Simulation tab.

Convert to CN*

In the SCS runoff equation, it is assumed that the initial abstraction equals to 0.2×S, where S is the potential maximum retention. However, it has been found that it may underestimate the runoff volume. To fix this problem, OTTHYMO allows user to assign the initial abstraction explicitly and modify the CN accordingly. The modified CN is called CN* in OTTHYMO. For more information, see Chapter 2 in Reference Manual.

The conversion can be done using the Convert to  tool. To covert CN to CN*:

  1. Select hydrologic objects to be changed. It can be StandHydNasHyd or WilHyd.
  2. Click the Covert to  button  in Simulation tab.
  3. The Calculation window will appear.
  1. Enter the precipitation volume in the text box on the top. The total volume of the 100-year design storm is usually used.
  2. Select the hydrologic object in the Commands list on the left. Change the IA (initial abstraction) on the right if necessary. Note that this would change the IA property.
  3. Notice that values in the calculation will appear on the right and the  will be the CN* assigned to object. To see how each value is calculated, move cursor on top of the value and the equation will appear in the Description box on the bottom.
  1. Click OK button to assign the calculated  to CN property. The CN* flag in properties window will be checked. Note that the CN* flag can’t be used to convert CN back to its original value.

Batch Assign

Parameter values may be available in another source. If these parameters are in the appropriate order, they can be pasted in Parameter Tables window. If not, they can be assigned to hydro-logic objects using the Batch Assign tool.

The Batch Assign tool uses an NHYD-Value lookup table to find the hydrologic objects and as-sign values. If the given NHYD doesn’t exist, the corresponding will not be used.

To use the Batch Assign tool:

  1. Click the Batch Assign button  in Simulation tab.
  2. The Batch Assign Values window will appear.
  1. Check the property to be changed on the left panel. The properties are listed based on object type.
  2. Copy the data from the source and paste in the table on the right. The data should have two columns. The first column can be NHYD or NAME, which will be used to search hy-drologic object by NHYD and NAME. To use NAME, check the Select command by name instead of NHYD above the table. The second column is the parameter value to assigned. In the example given below, the CN of NasHyd will be updated. And the NasHyd with NHYD of 1 will be searched. If successful, its CN value will be assigned to 80.
  1. Click Assign button on the right bottom corner to assign the values.

Calibrate Commands

For model calibration, a few sensitive parameters may need to be adjusted several times before a good result can be achieved. The adjustment usually comes with a percentage change. To assist on this process, the Calibrate Commands tool can be used. It allows users to change parameters of different hydrologic objects at once.

The Calibrate Commands tool can select hydrologic objects in a sub-area. The sub-area can be all hydrologic objects 1) upstream of a given object or 2) between several objects. This is very useful in multi-site calibration when the watershed needs to be separated to sub-areas by flow monitoring stations.

To use the Calibrate Commands tool:

1. Select appropriate hydrologic objects.
a. To only update selected hydrologic objects, select all of them;
b. To update hydrologic objects upstream of a certain point, select the hydrologic object corresponding to that point, which is usually the AddHyd corresponding to the flow monitoring station.
c. To update hydrologic objects between one point to another, select the hydrologic object corresponding to the downstream point.

2. Click the Calibrate Commands button  in Simulation tab.

3. The Command Calibration window will appear.

4. Set up the selection in the Selection portion on the top.
a. To only update selected hydrologic objects, choose Selected;
b. To update hydrologic objects upstream of a certain point, choose Upstream;
c. To update hydrologic objects between one point to another, choose Upstream and enter the NHYD of the most upstream hydrologic objects in Stop at Commands (Exclusive) text box. Multiple NHYD should be separated by comma.

For the model given below, the AddHyd 5 is selected. The updated hydrologic objects with the three options are given below.
a. Only AddHyd 5 will be updated if Selected is used;
b. NasHyd 1 & 2, RouteChannel 3 & 4 and AddHyd 5 will be updated if Upstream is used.
c. RouteChannel 3&4 and AddHyd 5 will be updated if Upstream is used and Stop at Commands (Exclusive) is set to 3, 4.

The number of updated hydrologic objects will be given at Selected Commands in Selection section. Note it is not always the number of selected hydrologic objects.

5. Specify the percentage change for RouteChannelStandHyd and NasHyd in the three sections below the Selection section. All common parameters are listed and the default value is 100. To decrease the parameter values, enter a value smaller than 100 (e.g. 80). Otherwise, enter a value larger than 100 (e.g. 120).

6. Click OK button to apply the changes.

Scenario Comparison

Scenario Comparison tools allows to compare model results of different scenarios at a certain location. For example, you could compare the peak flow at your site outlet from pre-development condition and post-development condition with a pond. Engineers could setup the comparison in the submitted model for reviewers to open directly for verification. This tool is available for both single-event and continuous simulation.

To use the Scenario Comparison tool:

1. Navigate to Simulation toolbar and click the Scenario Comparison button 

2. In the popped-up window choose the Design Storm from the drop-down list, check off the Scenarios that you want to include the Graph and Statistics table, and select the Command that you are interested to make a comparison.

For RouteWetland command, there are two extra tabs called Hydroperiod and Inflow Mass Curve for Continuous project. In those two tabs, VO uses the results of one pre-development scenario to calculate difference from the other scenarios to the pre-development scenario. Therefore, before doing the scenario comparison, users should select one scenario from all the scenarios in the project as the Pre-Development Scenario. By default, the first scenario under the Drainage Network Scenarios in the Project Manager is the pre-development scenario. To assign any scenario as the pre-development scenario, select and right-click on the desired scenario under the Drainage Network Scenarios group in the Project Manager. In the opened context menu, click on Set as Pre-Development Scenario

Open Scenario Comparison and choose RouteWetland command. Click on Hydroperiod or Inflow Mass Curve to see the comparison. The grey lines in the graph of Hydroperiod are the the upper and lower boundaries of the confidence interval. You can change the Year to be compared and the desired Confidence Interval (%) from the drop-down list. Below the graph is the Statistics table summarizing the days when the Storage depth of the RouteWetland is out of the confidence interval for each scenario in the selected year.

For RouteChannel command, there is an extra tabs called Erosion Indices for Continuous project. The Erosion Indices compares RouteChannel results in three erosion indices: Time of ExceedanceCumulative Erosion Index and Cumulative Effective Work Index. Before doing the comparison, users should also assign the pre-development scenario as described above. Open the Scenario Comparison and choose RouteChannel command. Click on the Erosion Indices tab. In the drop-down window of Type, choose one index from the three erosion indices to be compared. Type the parameters and click Update. The Graph and the Statistics table will be updated.

Please note that it may take a while to show the results of Cumulative Effective Work Index.

User input parameters required for each index:
Time of Exceedance: Critical velocity
Cumulative Erosion Index: Critical velocity
Cumulative Effective Work Index: Critical Shear Stress, Coefficient

3. Export the Statistics table as a CSV file by clicking the Export button

4. Click the Save button. In the popped-up window, type a name for the comparison chart.

5. The current Scenario Comparison will be saved in the Project Manager under the Scenario Comparison group  . To open the same scenario comparison, simply double click on the saved item.


Smart City Water is dedicated to continuously improving Visual OTTHYMO to design the best user interface and hydrologic model simulation software for our users. For example, we designed the LID toolbox for clients to more easily meet the requirements set by MOECP and GTA Conservation Authorities/Municipalities. We hope that Visual OTTHYMO has successfully run simulation analyses for Watershed Studies, Sub-watershed Studies, Master Drainage Plans, Functional Storm-water Management Plans, Site Plans, and Stormwater Management Pond Designs.

We are fully committed to our users and we want to make the best experience for you. Please contact us if there is something you want us to add to the software to make your experience easier. Should you have any question not answered in the User’s Manual or Reference Manual, contact Smart City Water’s VO Technical Support or +1 (905) 417-9792.

We also offer software customization services to provide users specific functions in VO. Contact our VO Technical Support team at for any customization inquiries.

Thank you for using our software and we look forward to seeing your amazing results completed with the help of Visual OTTHYMO.


This Reference Guide contains all of the hydrologic theory behind the program and gives guidance for users to select or measure object parameters.


This section outlines different methodologies for modelling ungauged rural catchments. While it is preferable to use a calibrated hydrologic model for water resources studies, especially for rural catchments, this is not always possible. Satisfactory results may still be obtained for macro level studies provided that the modeller chooses the appropriate parameters for each catchment.
The focus of this section is on the Initial Abstraction parameter, IA, and the Time to Peak parameter, TP, parameter. While the CN parameter plays a large role in determining the runoff character-istics of a particular catchment, this parameter can be readily determined and is rarely in dispute by watershed regulating authorities. Guidance is provided in this section on determining the Modified CN parameters, called .


This section introduces the calculation and selection of curve number based on initial abstraction parameter.

Modified Curve Number Method (CN*)

When using the Modified Curve Number Method, the IA parameter should be set to a value in the range of 1.0 mm and 5.0 mm, depending on the circumstances. The IA value must then be used to calculate  (see below).

SCS Method (CN)

When using the SCS Curve Number Method, IA should be set to 0.2S where S is the soil storage (a function of CN). Bear in mind that this method may underestimate the peak flow for small storms because the initial abstraction is higher than the total rainfall, which is not accurate. A literature review of this method has found that for lower CN values, a lower IA should be used. Suggests guidelines are as follows:

CN ≤ 70 IA = 0.075S
CN > 70 ≤ 80 IA = 0.10S
CN > 80 ≤ 90 IA = 0.15S
CN > 90 IA = 0.2S

Please note that the above guidelines are for the SCS Method only, where the SCS Curve Num-ber is used to define the soil type.


Unlike the urban catchment hydrographs, rural catchment unit hydrographs do not calculate the time to peak TP as a function of the other variables. The TP parameter must therefore be deter-mined by the modeller. It should be noted that most methods of estimate TP, start by calculating the time of concentration, . Time of concentration is the time at which the centroid of the flow reaches the bottom of a catchment. TP is usually a fixed ratio of , depending on the unit hydro-graph chosen.

Over the past 40 years there have been numerous studies in both the United States and Canada in which empirical, semi-empirical, and mathematical relationships for  have been derived. Most of the relationships state that  is a function of catchment slope, catchment area, and ground cover. While no single method can be used for every situation, we have included the most common methods in this manual so that the modeller can choose what is appropriate for their situation.

Listed below are five methods for calculating TP. We have included both the source of the method as well as the context in which it was derived. This way the modeller should be able to choose a method that was derived for a similar situation as their own.

Upland's Method

With Upland’s Method the average overland flow velocity is determined for a catchment based on the catchment slope and ground type, as shown in the figure below. Once the velocity has been determined then the time of concentration is determined by dividing the catchment length by the overland flow velocity.


Bransby - William's Formula

In catchments where the runoff coefficient, C, is greater than 0.40, the Bransby Williams formula is a popular choice. The method calculates time of concentration as a function of catchment area, length, and slope as follows:


 = time of concentration (min)
L = catchment length, (m)
 = catchment slope (%)
A = catchment area (ha)

Airport Method

For catchments where the runoff coefficient, C, is less than 0.40, the Airport formula may provide a better estimate of the time of concentration. This method was developed for airfields and calculates time of concentration as a function of runoff coefficient, length, and slope as follows:


 = time of concentration (min)
C= runoff coefficient
L = catchment length, (m)
 = catchment slope (%)

William's Equation (1977)

Williams, who co-developed the William’s Unit Hydrograph (WILHYD in Visual OTTHYMO) with Hann in 1973 later derived empirical relationships for both the K and TP variables in WILHYD. These relationships are:



The above relationships were derived for watersheds in the southern United States. Refer to the Theory Reference section of this manual for more information on the derivation of the WILHYD unit hydrograph.


This section provides direction for modellers who are modelling ungauged urban catchments. In most cases, urban catchments are not gauged since the response to rainfall can be accurately simulated. However, like any model the user should be aware that the inappropriate selection of parameters can lead to erroneous output. This section will guide the modeller in selecting parameters that have been successfully used in the water resources industry.


There are two impervious ratios required, the amount of directly connected imperviousness, XIMP, and the total imperviousness, TIMPXIMP must be less than or equal to TIMP.

TIMP is a function of the land use of the catchment. Land use is a planning term that describes the approved, or proposed, use for the catchment (e.g. residential, commercial, industrial). Water resources studies are generally tied to planning applications and depending on the level of planning application, (i.e. Secondary Plan, Official Plan Amendment, Draft Plan), the modeller will have a little or a lot of information about the land use. Therefore, it is important to select a conservative value for the imperviousness when performing more macro level studies so that when the subsequent more detailed studies are completed, the more refined land use calculations will still be valid in the overall model.

The following table gives examples of suggested TIMP and XIMP values, based on land use, for the macro-level studies. These values can be used with the information supplied by the planner to determine area weighted values for the catchment of interest.

Estate Residential2040
Low Density Residential (e.g. Single Units)2550
Medium Density Residential (e.g. Semi-detached Units)3555
High Density Residential (e.g. Townhouse Units)5060

For more detailed level studies (i.e. Site Plan), there should be more information available so that the XIMP and TIMP can be calculated.

Loss Routine

In both the United States and Canada, either the Horton’s Method (LOSS = 1) or the CN Method (LOSS = 2) are commonly used for urban catchments. The Proportional Loss Method (LOSS = 3) has been successfully used in France for urban catchments. While the selection of Loss Routine can be somewhat arbitrary and at the discretion of the user, there are a few things to keep in mind when choosing a loss routine.

Horton’s Method is what is used in the SWMM model, therefore if the user is comparing results with a SWMM based model, or working in a watershed where the overall model used was SWMM, then this method may be the most appropriate. However, the user should bear in mind that for longer duration storms (greater than or equal to 12 hours) the Horton’s Method may not accurately predict the runoff from pervious areas. We have seen cases where the model simulates no runoff from a previous area during a 12-hour 100-year storm. This is clearly erroneous. The CN Method does not have any limitations with respect to storm length and often yields more conservative results as compared to Horton’s Method.

If the user selects the CN Method, then the IA parameter should be set somewhere between 1.5 mm and 5 mm. Note that this is a different value than what would be used for a rural catchment with the same CN value. An urban catchment generally has less pervious depression storage than the same catchment in its rural state.

Parameters for the Pervious Component

The pervious slope, SLPP, is the average slope of the pervious areas. This is not the catchment slope from highest point to lowest point, but an average when considering only the pervious are-as. For example, if the catchment consists of a residential subdivision, this value would represent the average slope of the pervious lot surface. In this example the slope would not be less than 2% or whatever the municipal minimum is.

The overland flow length, LGP, should be set to the representative value for the pervious areas. It is not the length of the catchment from high point to low point. This value represents the average length over which flows from pervious areas would travel before being intercepted by channels, sewers, or roads. For example, in a residential subdivision this value might be the representative lot length which is typically 40 m.

The Manning’s roughness coefficient for pervious surfaces, MNP, should be selected based on sheet flow and not channel flow. This is a common mistake for modellers. Most listed values of Manning’s values are for channel flow, whereas the pervious runoff simulated is sheet flow. Therefore, if we assumed a grassed surface then the sheet flow Manning’s roughness coefficient would be approximately 0.25, whereas the channel roughness coefficient for the same material might be 0.025.

For an ungauged urban catchment, the pervious storage coefficient, SCP, should be set to 0, which will let the program determine the storage coefficient.

Parameters for the Impervious Component

The impervious depression storage, DPSI, should be set to an appropriate value for the representative impervious surface. For roads, driveways, and roofs, this value is typically between 0.8 mm and 1.5 mm.

The impervious slope, SLPI, is the average slope of impervious areas. This is not the catchment slope from highest point to lowest point, but an average when considering only the impervious areas. For example, if the catchment consists of a residential subdivision, this value would represent the average slope of the impervious road surfaces. In this example the slope would not be less than whatever the municipal minimum is. Typically, SLPI ranges between 0.5 to 2.0.

The impervious length, LGI, is one of the most important parameters for modelling urban catchments. A common mistake when modelling ungauged urban catchments is to set LGI equal to the measured catchment length. Previous studies by Paul Wisner Associates Inc. have determined that LGI is related to the catchment area based on the following equation:


A = catchment area ()
LGI = impervious length (m)

This relationship will yield runoff characteristics similar to those which would be measured. The LGI parameter should only be adjusted from this relationship if the model is being calibrated.

The Manning’s roughness coefficient for impervious surfaces, MNI, should be selected based on channel flow, not sheet flow as in MNP. For example, if the representative impervious surface were a road, then the MNI should be set around 0.013.

For an ungauged urban catchment, the impervious storage coefficient, SCI, should be set to 0, which will let the program determine the storage coefficient.


Probably the single biggest use for Visual OTTHYMO is to help create water resources strategies whereby stormwater management ponds are implemented to address issues of water quality control, erosion control, and water quantity (i.e. flooding) control. Visual OTTHYMO can be utilized to examine many scenarios that help water resources planners and engineers determine the most effective strategy, on a watershed or sub-watershed basis.

How to Build a Rating Curve Using Route Reservoir

A rating curve for any stormwater management pond describes how the pond operates. In Visual OTTHYMO the command ROUTE RESERVOIR is used to enter a pond rating curve and simulate routing. The rating curve is described by the Discharge (i.e. outflow) and Storage relationship. Note that the Stage or water depth variable is taken out of the input, since both Discharge and Storage are a function of Stage. The Stage-Storage and Stage-Discharge rating curves are essentially combined into one Discharge-Storage curve. An example of a Discharge-Storage Curve is as follows:

Discharge (m³/s)Storage (ha-m)

Designing a Discharge-Storage curve, at the watershed or sub-watershed planning level, involves determining each storage ordinate for every given discharge ordinate. Discharge ordinates are usually known or can readily be determined. They may represent allowable flows or release rates that when combined with other flows are the allowable flows at key locations. Storage ordinates are what the modeller is trying to calculate in order to meet the discharge targets.

For single event analysis the Discharge-Storage curve is built from the smallest to largest values, which corresponds to the smallest to largest rainfall events. For example, the above Discharge-Storage curve was based on the following design storm events.

Discharge (m³/s)Storage (ha-m)Design Storm
0.060.3425 mm
0.210.482 year
0.370.605 year
0.660.8325 year
0.941.00100 year

When building a curve, the storms must be run from smallest to largest and the storage iterated until the pond outflow matches that of the target value in the Discharge-Storage curve. Only then can the modeller move onto the next largest storm. The proper pond sizing methodology is therefore:

  1. The modeller enters the first 2 sets of points on the curve, (0,0) and the first target flows (e.g. 0.06). The modeller guesses a storage value and then runs the model with the storm that corresponds to the target flows.
  2. The modeller checks the outflow and compares it with the target. If the outflow is too high then the modeller must increase the storage. If the outflow is too low then the modeller must decrease the storage. Note that if the storage curve has been exceeded then the outflow may be erroneous. It is better to iterate from a large storage value to the correct storage than from a small storage value.
  3. The modeller iterates step 2 until the calculates outflow matches (or is slightly less) than the target outflow. At this point the calculated storage should also match the storage in the input table.
  4. The modeller then enters the next discharge ordinate for the next largest storm, guesses a new storage and runs the model.
  5. Steps 2 and 3 are repeated until the outflow and storage are matched.
  6. Step 4 is repeated with the next largest storm until the final storm is reached.
  7. Once the last storm is iterated then the Discharge-Storage curve is complete. (e.g. when the (0.94,1.00) point is determined in the above example curve).

If the modeller is designing a SWM pond based on a real storm, or is analyzing an existing pond with design storms, then the actual discharge storage curve must be used. This can be obtained by combining the pond’s Stage-Storage curve (i.e. geometric relationship) and the Stage-Discharge curve (i.e. hydraulic relationship).

Also, a SWM pond’s actual Discharge-Storage curve must be used when creating a detail pond design, to ensure that the outflows match the targets from the design curve that was determined in the watershed or sub-watershed analysis.

When working with ponds users can choose to have an overflow hydrograph generated if the pond volume is exceeded. If the overflow option is not selected the discharge-storage curve will be extended automatically by VO.


This section introduces how to calculate rainfall losses by curve number and Horton’s equation.


This following section will introduce how to obtain SCS curve number and review its procedure.

The SCS CN procedure is based on the equation


It is assumed in the procedure that the initial abstraction  = 0.2 S. This results in the equation


The curve numbers CN are functionally related to S by


CN can be obtained from tables based on land use, soil type and soil moisture conditions. However, the soil moisture is determined only for three antecedent moisture conditions (AMC), classified on the basis of precipitation in the previous 5 days. CN has no intrinsic meaning but is only a non-linear transformation of S, which is a storage parameter. CN varies from 0 (Q=0 for all P) to 100 (Q=P for all P). In Eqn. 8, the 10 and 1000 have inch dimensions. Conversion can be made to the metric system.

Background information on the derivation of the procedure can be found in a paper by Rallison and Cronshey (1979). In the mid-50s when the SCS CN procedure was developed, the only data available were daily precipitation and runoff records from agricultural watersheds and infiltration curves from infiltration studies. Rainfall versus Runoff (P vs Q) data were plotted. A grid of plotted CN for  = 0.2S was then overlaid and the median CN selected. The values in the SCS NEH-4 manual (1971) represent the averages of median site values for hydrologic soil groups, land cover and hydrologic conditions. The SCS work involved considerable interpolation and extrapolation for different soil types and land cover. The rainfall versus runoff plots were also used to define enveloping CN for each site.

The SCS CN procedure is in widespread use and there has been criticism of the procedure (Hawkins 1978, Altman et al. 1980, Golding 1979) because it is often applied beyond the original conditions and intended use.

Some of the concerns about the procedure are over:

  1. why the antecedent moisture range of values is a discrete rather than a continuous relationship,
  2. the lack of a clear definition of AMC II, the standard reference moisture condition,
  3. use of a 5-day time interval as a basis for classifying antecedent moisture conditions,
  4. why is  = 0.2S,
  5. what probability levels are associated with the envelopes in defining AMC I and AMC III.

The SCS CN procedure may severely underestimate the runoff volume, especially for small rainfalls. It was found that the runoff volumes obtained from real measurements on two residential watersheds were greater than those computed using CN = 90 (corresponding to a high degree of imperviousness).


A study in Texas (Altman et al. 1980) involving four watersheds found that the optimized CN were greater than the weighted CN for four of the six watershed conditions studied (Table 1).


WatershedDate (calc.)CN (calc.)CN (opt.)
Austin, Texas Region   
Waller Creek (urban)1957-1959
Wilbarger Creek1964-19758385
Dallas, Texas Region   
Turtle Creek (urban)1967-19768693
Spanky Branch1973-19758496

For areas with low CN, the SCS procedure may give significant errors. Golding (1979) utilized the SCS CN procedure to simulate runoff from a gauged urban basin in South Florida (58.3 ac., Group A soil, 36% imperviousness, 18% directly connected imperviousness). He found that the computed  amounted to 0.86 inches (CN=70), which was greater than the total recorded rainfall on the basin, which had peak flows of up to 40 cfs in many cases. Reduction of the initial abstraction may give a more realistic runoff volume. The figure below compares the runoff volumes obtained for different  and  = 0.2S for storms of 3 return periods (Rowney, 1982). The SCS CN procedure is still a popular and simple tool, which will be around for some time to come. It is felt that with some improvements in the procedure and proper application, the method is still useful.


The methodology used in OTTHYMO involves determining the initial abstraction  from the runoff threshold curve obtained from rainfall and runoff records (Jobin, 1982). A program called SECSER has been written in order to do this. The runoff volumes for the storms are then used to calibrate the CN with this . The resulting CN are called CN*. Instead of using 3 discrete AMC classes, the antecedent moisture condition is classified by the API (antecedent precipitation index) which is calculated from the hourly rainfall records. The API for each storm is then plotted against the CN*. The CN* for other storms can then be determined from this CN*-API relationship once the API for these storms are determined. This relationship would be a continuous one as compared to the 3 discrete classes used in SCS. It also would not require the definition of a standard reference moisture condition.

A small program has been written to calculate the runoff volumes Q for different rainfalls P using the specified .


The results can then also be plotted on the Q-P chart. These charts are useful for a quick comparison of CN and CN*. Since CN* are a function of the , different charts will result for different .

Hydrology of Runoff Equation, Q = (P-0.2S)/(P+0.8S)

Calibration of the Modified SCS CN Procedure

The modified SCS CN procedure was tested first on the Seymaz watershed in a joint study by the University of Ottawa and the Ecole Polytechnique Federale de Lausanne who had previously done extensive monitoring. This watershed is composed of 30.2  of rural areas and 8  of urban areas and is located in the suburbs of Geneva, Switzerland.

Using the SECSER program and the rainfall and runoff records, a runoff threshold curve can be plotted. From the curve, the initial abstraction Ia was found to be 1.5 mm. The figure below also shows a comparison between the simulations using both  = 1.5 mm and  = 0.2S. In the latter case, the first peak cannot be simulated accurately because of the large initial abstraction.


The variation of CN* with API for some storms on the Seymaz watershed is shown in the figure below.


CN* is the calibrated CN obtained by using  = 1.5 mm as obtained from the runoff threshold curve. (It was not possible to find a similar correlation of this type with  = 0.2S). With the  = 0.2S assumption, peak fitting for small rainfalls is possible if the CN values are increased, without consideration of antecedent conditions, to unrealistic values, e.g., CN = 90 or higher. Two of the simulated storms obtained with the new  are compared with the observed storms in the following two figures.The CN* values used in the simulations are determined from the curve as shown in the figure above, once the API for the storms are obtained.



A similar CN*-API relationship has been determined for the Etobicoke Creek watershed in Metro Toronto as shown in the figure below.


One of the typical comparisons between simulated and observed storms is shown in the figure below.



In VO STANDHYD is usually used to model the catchment of urban area. This section introduces the calculation procedure of STANDHYD infiltration loss.

Horton's Equation

For pervious areas, there are two options for calculating the infiltration losses. The first option is Horton’s equation where the infiltration capacity rate is an exponential function of time, which decays to a constant rate. It is written as follows:


 = the infiltration capacity rate (in/hr or mm/hr) at time t;
 = the initial infiltration capacity rate (in/hr or mm/hr);
 = the final infiltration capacity rate (in/hr or mm/hr);
 = the decay rate (1/hr).

The equation is only satisfactory for the condition that the rainfall intensity is higher than the infiltration capacity rate. To overcome this problem, the cumulative form of the equation can be used. It has the advantage that the infiltration rate becomes a function of the amount of water accumulated into the soil.


where F is the cumulative infiltration volume, at time t.

The average infiltration capacity rate during the next time step is


In order to determine the actual infiltration rate f, the average infiltration capacity rate is then compared with the average rainfall intensity i during the time period ∆t.


then the calculation proceeds to the next time step with the cumulative infiltration volume at . If f = i, then the actual cumulative infiltration would be



The new time , which would correspond to the cumulative infiltration , is determined by means of an iterative process. The calculation then continues from this point for the next time step.

The antecedent moisture condition can be represented by the water, F, accumulated into the soil before the start of the storm. F can be directly specified as input. The other infiltration parameters also need to be specified.

For a decay rate of 4.0  the infiltration capacity rate declines 98% towards the limiting value  after 1 hour (if the rainfall intensity is always higher than the infiltration capacity rate). For α = 2.0 , the decline is 76% after 1 hour. This should be considered when selecting the time increment ∆t for computation.

Modified CN Procedure

The second option for infiltration losses in the previous area is the modified CN procedure, which is used in NASHYD.

Considerations in Using the Rainfall Losses

For flood control purposes and master drainage planning, there are both rural and urban areas in the watershed. In Visual OTTHYMO, the rainfall losses in the rural areas are computed by means of the CN* procedure. The critical storms for rural conditions are long-duration storms such as the Southern Ontario Regional Storm with a peak intensity of 2.08 in/hr. The modified SCS method (CN*) is used in such conditions. The Horton model may result in underestimating the runoff mainly for low intensity storms, since it generates runoff only if the rainfall intensity is higher than the infiltration capacity rate. In such cases, the rainfall losses in the previous portion of the urbanized areas should also be computed with the CN* procedure. The ratio of the peak rainfall excess intensity to the peak rainfall intensity is an indicator of the effect of rainfall loss model. This ratio is called  and the figure below shows  against CN* and the maximum infiltration capacity rate for (Horton) for the Regional Storm.  for this storm would be sensitive to the  (minimum infiltration capacity rate) selected.


If the same storm is used in studying the effects of urbanization (e.g. comparing pre- and post-development flows), the CN* procedure can continue to be used for post- development conditions with STANDHYD.

For design purposes under urban conditions, however, the critical storms are the short- duration, high intensity storms such as the Chicago-type storms. Here Horton’s procedure is preferred because it is more sensitive to the storm intensity and in general results in higher peak flows than the CN* procedure. This is shown in the figure below for a residential watershed (30% imperviousness) for three storms, the 5-year, 100-year Chicago and the Regional storms.

VERSUS AND REGIONAL STORM ( =0.30 in/hr, α=2.0/hr)

A series of numerical experiments have been done to find a range of values in which the Horton and CN* procedures would give the same runoff volumes. The runoff volumetric coefficient Cv was calculated for different combinations of  and values (Horton) and CN* values (with  = 0.10 in). The range of values tested were 1.0 to 5.0 in/hr for , 0.10 to 0.50 in/hr for  and 2.0/hr and 4.14/hr for α (decay constant). The results are shown in the following two figures. It is observed that equivalent Cv does not mean that the corresponding peak flows are equivalent.


Cv VERSUS AND WITH =3 in/hr, α=2, =0.10 in

It is also found that total runoff for the Regional storm is more sensitive to  while for the Chicago storms they are more sensitive to . There is no range of values for which the Cv are matched for all three storms. The results show that the Cv for the Regional storm can be matched by varying  and that the Cv for the Chicago storms can be matched by varying .

These results also show that for consistency the selection of infiltration parameters should consider the characteristics of the soil and also those of the storm. Tables given in literature in which infiltration parameters like  and CN are given in terms of soil groups A, B, C, D alone may not give consistent results.

If data is available and the CN*-API relationship has already been derived during the planning stage, the CN* procedure can also be used for design purposes. The use of the CN* procedure with design storms is discussed in the section on design storms. This will result in compatibility between the planning and the design stages for the watershed.


In Visual OTTHYMO, the response of a watershed to the effective rainfall is obtained by convolution of a short duration unit hydrograph (UH) derived from the theory of conceptual “instantaneous unit hydrographs” or IUH. The characteristics of these unit hydrographs are not dependent on rainfall duration. However, depending on the size of the area being simulated, their use usually requires short computational time steps (1 to 15 minutes).

Visual OTTHYMO has three types of IUH’s which have a common parameter, the time to peak, Tp. Another parameter, K, is related to the hydrographs’ recession limb. K is also called a ‘storage coefficient’ and has different values in each IUH.

Another option in Visual OTTHYMO is the SCS non-dimensional UH, which is a specific NASH IUH defined only by Tp (see IUH Relations).

For Tp equal to the computational time step, the STANDARD IUH is identical to the single linear reservoir IUH from the URBHYD command in OTTHYMO 83.

IUH Relations


STANDARDFor Tp = DT (the computational time step) the STANDARD IUH becomes the URBHYD IUH from OTTHYMO 83.NASHN = Tp / k + 1
N is also the “number of reservoirs”WILLIAMSCalibration recommended.
Where t_o is the inflection point after the peak; K_n is the storage coefficient of each reservoir; N is the number of reservoirs, and Γ(N) is the gamma function.SCSIs the NASH IUH with N = 5 

Standard IUH

The standard IUH is used mainly for urban areas with pervious and impervious contributions calculated separately.

The standard IUH was developed and tested in Germany by Verworn and Harms in 1978. It is used in the model HYSTEM. In Visual OTTHYMO, Tp > DT and therefore, for a given storm, Tp varies with the size of the watershed. (The URBHYD command in OTTHYMO 83 is equivalent to a STANDARD IUH with the time to peak equal to the time step, DT).

A relation derived from overland routing by the kinematic wave method (Peterson and Altera) gives the storage coefficient, K. This relation is close to the relation by Neumann used in HYSTEM.

L_= an equivalent flow length which requires calibration. A default value for impervious areas obtained from  where _A is the watershed area, was frequently tested with measurements. For pervious areas, the default value is L = 40 m, representing an average travel length on inter-spaced green areas.
n = the roughness coefficient. Testing shows that adequate results are obtained it n = 0.013 for impervious areas and n = 0.25 for pervious areas.
i = the dominant rainfall intensity (maximum average intensity during K).
s = the characteristic slope in m/m.
C =a constant (0.00775 for L in feet, i in inches/hour).

The STANDHYD command is based on analysis of comparisons with measurements and practical applications. In STANDHYD, the dominant rainfall intensity is averaged over the duration of K. Since K varies with rainfall intensity this IUH varies from one rainfall to the other, the STANDARD IUH is a quasi-linear model.

For the impervious area, the time to peak, Tp, in the STANDARD IUH of Visual OTTHYMO is equal to the storage coefficient, K. For the pervious areas, fragmented in backyards and connected to storm sewers, Tp is equal to K pervious + K impervious, at time of convolution, Tp is rounded to the nearest multiple of the time step, DT.

In the STANDHYD command, the pervious hydrograph and the impervious hydrograph have, in general, different Tp values. There is also a lag between the peak discharge of the total hydrograph and the end of the peak rainfall intensity.

For watersheds with large estate lots and semi-urban areas with relatively large pervious components, it is recommended to simulate two component hydrographs:

  1. The first, an equivalent smaller urban area can be simulated with STANDHYD.
  2. The remaining area which is only (or mostly) pervious, can be simulated with NASHYD.

The equivalent urban area and the imperviousness of this area (e.g., say 30 %) satisfy the following rule of thumb:

Fore very large urban areas (> 200 hectares), STANDHYD requires calibration.


This linear IUH is used mainly for rural areas. With Nash, the peak discharge increases with N and decreases with Tp. Measurements in Ontario and in Switzerland indicate that an average of 3 number of linear reservoirs may be appropriate.

The time to peak, Tp, is obtained from the time of concentration, Tc:

Tp = (N-1)/N Tc where, N, is the number of linear reservoirs
Tp = 0.67 Tc

In general, the time of concentration, Tc, can be determined using one of three methods:

  1. Empirical formulas (only recommended if they are based on regional verifications).
  2. Velocity methods, Tc = 3 (Li/Vi). The overland velocities are determined with an SCS graph, and channel velocities can be determined from Manning’s equation.
  3. Kinematic wave method (which accounts for the rainfall intensity).

The NASHYD command is used for non-homogeneous areas, and in the case of SCS abstraction methods, uses a weighted average of CN. Comparisons with measurements show a better performance if the response from the pervious and impervious areas are simulated separately.

Fore very large urban areas (> 200 hectares), NASHYD requires calibration.

Furthermore, if the response time of an urban watershed is increased by significant channel storage, this effect must be simulated by channel routing (unless Tp is calibrated).


The shape of the SCS UH is obtained from the NASH relation, with N=5. This value is greater than the one determined from studies in Ontario, Switzerland, and the United Kingdom. It is, however, conservative if the time to peak is correct.

The SCS non-dimensional unit hydrograph is used by SCS abstraction methods for both rural and urban areas. Comparisons with measurements show that even if Tp, Ia, and CN* are calibrated, the proper shape of the hydrograph is not always generated.

The 1986 SCS TR-55 publication indicates the following limitations:

  1. Hydrographs obtained by this method are not developed for comparisons with measure-ments.
  2. The method should only be used in cases where runoff is greater than 12.5 mm.
  3. The lag formula given in previous SCS publications is no longer recommended (it may un-derestimate the peak flow).

The SCS methods apply the non-dimensional UH in conjunction the SCS CN method with the as-sumption that Ia = 0.2 x S. Although this may overestimate the rainfall losses, it was maintained in the SCS command for special agency requests.

It is recommended the to determine Tc with the velocity method:

Williams IUH

The method is recommended for rural watersheds where observations indicate a long hydrograph recession limb. The Williams formula for Tp is not recommended in Ontario as it has been shown to give significant errors.


As is predecessor (INTERHYMO / OTTHYMO.89), Visual OTTHYMO does not recommend de-fault values for K and Tp in the Williams command, since it is considered that this IUH requires calibration.


Use of IUH's for I/I Simulation and Baseflow (DWF)

Visual OTTHYMO can be used to simulate the Infiltration/Inflow into sanitary sewers or combined sewers. The four types or rainfall-induced infiltration/inflow are:

  1. Fast responses from directly connected impervious areas.
  2. Rapid responses from grassed areas in combined sewers systems.
  3. Semi-rapid responses from weeping tiles.
  4. Slow responses from cracked pipes and leaking joints in the sewers.

Visual OTTHYMO can simulate these responses during a single event by adding individual response hydrographs from each type of contributions within the same area. The first three responses can be simulated with the quasi-linear instantaneous Unit Hydrograph (STANDHYD) while the fourth, slow response, can be simulated with the NASH unit hydrograph.

Baseflow can be super-imposed to account for the domestic sewage contributions during wet conditions.


For computation of flows from rural watersheds, the subroutines NASHYDWILHYD or SCSHYD (NASHYD with N=5) can be used. The rainfall excess distribution is obtained by means of a modified CN procedure, which is then convoluted with the unit hydrograph obtained by means of the Nash model (NASHYD) or the Williams and Hann unit hydrograph (WILHYD).

Instantaneous Unit Hydrograph

Many ways of deriving synthetic unit hydrographs or IUH have been proposed since the early studies of Snyder in 1938. One frequently used way is by means of a conceptual model made up of a cascade of equal, linear reservoirs, first proposed by Nash in 1957. The IUH for Nash’s model can be written as:


Γ(n) = the gamma function;
n = the number of reservoirs;
 = the storage coefficient of each reservoir.

By differentiating Equation 15 with respect to t/ and equating to zero, the time to peak  in terms of n and  is obtained.


The peak flow then becomes

By substituting Equations 16 and 17 in Equation 15, the 2-parameter gamma equation is obtained

Williams and Hann (1973) use this equation from the time of rise to the inflection point for the IUH in WILHYD. Figure 19 shows the variation of the outflow hydrograph from NASHYD, with the number of reservoirs, n, for a fixed time to peak.

As shown in the figure below, for the same time to peak, the peak flow is sensitive to n in the range 2 to 6. The parameter n, can be a non-integer. The calibration of watersheds with areas of less than 15  on the Seymaz and Etobicoke studies presented in the previous section has shown that a first estimate for N = 3 can be used if data is unavailable. For consistency, the various subwatersheds should use the same n unless data is available for each subwatershed.


Estimation of Time to Peak (Tp) in NASHYD

It is, of course, best to obtain Tp by calibration with measurements. If data is available, the following procedure may be utilized to estimate Tp.


The first step involves determining the time lag  which is defined as the time difference between the centroids of the rainfall excess hyetograph and the direct runoff hydrograph (after subtracting baseflow).  is related to n and  in the Nash conceptual model by

Once  is determined and n is estimated by 3 for example, then  can be obtained by equation 16.


Since measurements are usually available only at the outlet of a watershed, the  values would still have to be determined for each subwatershed after discretization. The main parameters that affect  are the slope and the area. Since in small watersheds the slope does not vary too much, an approximate relation  can be utilized. With the calibrated  at the outlet, constants m and n can be obtained by trial and error.

In the Seymaz and Etobicoke studies, the Williams and Hann equation for t_p was found adequate. For smaller watersheds, the  values obtained can be checked by using the velocity charts in the SCS TR-55 tables (1975) for overland flow and swale flow.

Several relations for  or  can be found in the literature such as Chow (1962), Kibler et al (1982), Boyd (1978) and Nash (1960).

William's Unit Hydrograph

WILHYD is the subroutine that uses the unit hydrograph proposed by Williams and Hann (1973). The unit hydrograph is divided into three parts for computation. The first part, from the beginning of rise to the inflection point, , is computed by the 2-parameter gamma distribution equation (Equation 18). The second part from the inflection point,  to  where  =  + 2_K_, is computed by

The third part from  onwards is computed by

n is computed as a function of K/ and  is a function of n and . Therefore only 2 parameters, K and  are necessary to compute the entire unit hydrograph. Empirical relations have been derived for K and  (Williams 1977) based on Southern U.S. watersheds. These relations may not be applicable in other areas.



K = the recession constant (hr);
 = the time to peak (hr);
A = the watershed area (sq. miles); and
S = the difference in elevation in feet, divided by flood plain distance in miles, between watershed outlet and most distant point on the watershed.

The unit hydrograph in WILHYD has a longer recession tail than that in NASHYD and a smaller peak. It can therefore be used in those watersheds where the recession limb is longer.
A comparison of the two-unit hydrographs is shown in the figure below.



SHIFT HYDROGRAPHA simple translation of the hydrograph. Does not attenuate the peak discharge.LOW
ROUTE CHANNELCombines the three routing commands of HYMO into a single command, based on the hydro-logic method VSC (Variable Storage Coefficient).MEDIUM
ROUTE MUSKCUNGEApplies the Muskingum-Cunge method of routing, which is based on the continuity equation and the storage- discharge relation.HIGH
ROUTE PIPEApplies the VSC method for conduits, and gives the minimum size to avoid surcharge.MEDIUM

Simulation Time Steps

The storm time step is determined by the format of meteorological data. For synthetic storms it is usually five to ten minutes. The hydrograph computational time step, DT, is determined from the watershed characteristics. For example:

Convolution with NASHYD requires DT < Tp (time to peak – preferably DT about 1/5 Tp)

Visual OTTHYMO will transform automatically for each sub-watershed, new storm input with the time step DT.

In routing with the VSC method, it is recommended to maintain a small-time step. Although this is not required for mathematical stability, Ponce and others recommend short time steps and the use of the Courant criterion for hydrologic routing.

The celerity is given by:

Celerity ranges from 1.1 to 1.6 times the average velocity. Using the above criterion, it is found that, for time steps used in convolution (hydrograph commands) the length cannot be very short.

For short reaches, the hydrograph should be simply ‘shifted’ in time. In comparison, routing with EXTRAN is usually conducted with time steps of 2 to 10 seconds, and gives an error message if the courant criterion is not met.

Time Shift Routing

For discharges close to critical or supercritical flow, and for very short reaches (with time step constraints), SHIFT HYD can be used. Comparisons with the kinematic wave method show that, for a circular conduit, the time lag can be selected with the relation:

Time lag = reach length / (alpha – full pipe velocity)

where alpha is given by the following table:


Variable Storage Coefficient Routing in Visual OTTHYMO

Like other hydrologic routing methods, the variable storage coefficient (VSC) is based on the continuity relation. It does not apply empirical or calibrated parameters. It calculates channel storage based on average channel characteristics, and travel time based on Manning’s relation. It can be used for artificial and natural channels with three roughness coefficients in the same cross-section.

In Visual OTTHYMO, the three routing commands of the original HYMO model are lumped in a single command ‘Route Channel’. The VSC routing cannot be used when backwater effects are significant. In such cases, a fully dynamic model (e.g. EXTRAN should be used).

The updated RouteChannel command in VO 5.1 is used to route hydrographs through channel cross-sections with a defined channel and floodplain using the variable storage coefficient (VSC) method. The low flow channel section and a separate floodplain section are defined with X and Y co-ordinates.

The RouteChannel command in VO5.1 has been split to calculate channel routing in both a “low flow channel” and “floodplain” portions of the cross section with the routing effects then being combined for the final output. The user can define channel and floodplain lengths and slopes.

For circular or rectangular pipes, ROUTE PIPE command should be used. The command sizes the pipe to the minimum diameter necessary to avoid surcharging. For design, the user should increase the size to the next standard diameter.


The Muskingum method is based on the continuity equation and the storage-discharge relation. Cunge (1969) extended the method into a finite-difference scheme. The Muskingum-Cunge channel routing technique is a non-linear coefficient method that accounts for hydrograph diffusion based on physical channel properties and the inflow hydrograph. The advantages of this method over other hydrologic techniques are:

  • it is very simple conceptually, and can be readily applied by desk calculation, and is much cheaper than the other methods when applied by computer,
  • it would be advantageous to use the Muskingum-Cunge method for rivers that have major tributaries and are not well gauged,
  • this method can include a tributary as a discrete lateral inflow, which the other methods cannot do in a simple way,
  • the hydrologic approach greatly improves computational efficiency and speed, and reduces the amount and detail of field data traditionally needed for hydraulic routing,
  • the parameters of the model are physical based, the scheme is stable with properly selected coefficients,
  • the method has been shown to compare well against the full unsteady flow equations over a wide range of flow situations,
  • it produces consistent results in that the results are reproducible with varying grid solution,
  • it is comparable to the diffusion wave routing,
  • it is largely independent of the time and space intervals when these are selected within the spatial and temporal resolution criteria.

The major limitations are:

  • it cannot account for backwater effects,
  • the method begins to diverge from the full unsteady flow solution when very rapidly rising hydrographs are routed through flat channel sections,
  • a disadvantage with the Muskingum-Cunge method arises when there is a disturbance such as a tide affecting the flow in the river upstream of the downstream boundary,
  • it does not accurately predict the shape of the discharge hydrograph at the downstream boundary when there are large variations in the kinematic wave speed, such as due to the inundation of a large flood plain.

Basic Flow Equations

The outflow hydrograph at the downstream end is calculated using the following formula.


Q = discharge
K = travel time in seconds
x = weighting factor, 0 <= x <= 0.5
∆x = subreach length
∆t = time interval
q = lateral flow
c = wave celerity

The parameters of K and x are expressed as follows (Cunge, 1969 and Ponce, 1978):


B = top width
S = the channel slope.

Solutions of Flow Equations

The outflow hydrograph is iterative and is calculated based on equation 24, the routing coefficients ( ) are re-calculated for every distance step ∆x and calculation time step ∆t.

Numerical Stability

∆t and ∆x are chosen internally by the model for accuracy and stability.

∆t is selected as the smallest of the following 3 rules:

  1. the user defined computation interval, DT,
  2. the time of rise of the hydrograph divided by 20,
  3. the travel time of the channel reach.

The model checks the difference between the computational time interval (DT) and the time increment of the inflow hydrograph (SDT). If DT is less than SDT, the inflow hydrograph will be interpolated. The calculation time step must be equal or less than the inflow hydrograph SDT.

A computational space increment ∆x can be equal to the length of the entire routing reach or to a fraction of that length. It is initially selected as the entire reach length. If the size of this space increment does not meet the accuracy criteria for flow routing given by Ponce and Theurer (1982), it is re-evaluated by subdividing the length of the routing reach into even subreaches that produce ∆x’s that satisfy the accuracy criteria.



 = baseflow from the inflow hydrograph
 = peak flow from the inflow hydrograph

The Courant (C) number can be defined as:


Main and overbank channel portions are separated and modelled as two independent channels. Right and left overbanks are combined into a single overbank channel.
Momentum at the flow interface between the two channel portions is neglected, and the hydraulic flow characteristics are determined separately, for each channel portion. At the upstream end of a space increment, the total inflow discharge is divided into main channel and overbank flow components. Each are then routed independently, using the previously described routing scheme. The flow redistribution between the main and overbank channels is based on Manning’s equation.

Data Requirements

Data required for the Muskingum-Cunge method are as followings:

  • -channel length
  • -main channel bed slope
  • -floodplain bed slope
  • -channel cross section data
  • -number of cross section segments
  • -Manning roughness coefficient

Simulation Results

The channel routing in Visual OTTHYMO was tested using a natural channel, 5200 m long, main channel bed slope is 0.001, Manning’s n is 0.03, floodplain bed slope is 0.001, Manning’s n is 0.05, no lateral flow, the cross-section parameters are shown in the figure below.


The simulation results from Visual OTTHYMO-MC are compared with the complete unsteady flow equation (SWMMEXTRAN) and Visual OTTHYMOVSC and are shown below in the figure below.


The results show that the Muskingum-Cunge (MC) routing method compares very well with the complete unsteady flow equations of EXTRAN. The peak discharge is attenuated slightly more from EXTRAN than that from the MC method; however, the time to peak for both methods is the same. The difference in peak discharges could be due to the fact that the inertial terms in the complete unsteady flow equations are becoming more dominant when rapidly rising hydrographs are routed through the flat channel, compared to the bed slope, as the channel slope is decreased. The Muskingum-Cunge routing method does not account for the inertial effects, and consequently the method tends to show more diffusion than what may actually occur.


Flow simulation for urban drainage studies is mostly done with one-event simulation models. The single event models determine flows produced by a single storm event. Continuous simulation models require rainfall data over a continuous period for the desired length of analysis. A frequency analysis is then conducted on the peak flows so that a flow of a desired return period may be found.

The flow with a single event model may be found by using a series of selected historical events or by using a ‘design storm’. The historical storm series may be selected using a continuous simulation program or by analyzing a rainfall record using a selection criteria. Each event in the selected series is then run through the event simulation model. The generated peak flows are then analyzed to determine their return period.

Design storms or model storms are single event rainfalls that are assumed to produce flows of a desired return period. They are of two types; synthetic design storms and historic design storms. Synthetic design storms are storms developed from intensity- duration-frequency (IDF) curves. Historic design storms are large single storm events; usually containing the maximum precipita-tion on record. In southern Ontario, hurricane Hazel is used as an historic design storm. In this text only, synthetic design storms are examined.

Each design storm has a unique temporal variation of intensity. Two general methods are used to determine the hyetograph shape. The first method derives the storm pattern based on an IDF curve. The design storms using only an IDF curve are the Uniform design storm, the Composite design storm and the Chicago design storm. The second method obtains the temporal structure of the design storm from an analysis of historic storm events. These are the U.S. Soil Conservation Service (SCS) 24-hour design storm, the SCS 6-hour design storm, the Illinois State Water Sur-vey (ISWS) design storm, the Atmospheric Environment Service (AES) design storm, the Flood Studies Report (FSR) design storm, the Pilgrim and Cordery design storm and the Yen and Chow design storm. Design storms that are not discussed are the Sifalda design storm, the Hamburg design storm and the Desordes (French) design storm. A more detailed description of each design storm is contained in the Design Storm Profiles section of this document. Table 2 summarizes the main characteristics of these design storms.


*Design Storms*Design Return Period*Storm Duration*Total Rainfall Depth*Temporal Distribution*Antecedent Moisture Conditions*Intended Application
UniformUser spec.i x No variation in intensityNoSewer sizing
CompositeUser spec.i x User selectedNoSewer sizing
ChicagoUser spec.Usually be-tween 2-6 hrs. Ti x TBased on an IDF curveNoSewer sizing
SCS 24-hr.User spec.Long duration usually 12-24 hrs. Ti x TTabulated type 1 & 2 distributionsYesRural watersheds
SCS 6-hr.User spec.6 hrs.Given in mapsTabulatedYesDesign of small dams
ISWSUser spec.1 hr.I x 1Huff 1st quartile 50% distributionNoSewer sizing
AESUser spec.1 or 12 hrs. Ti x TRegional charts for the 1 and 12 hr. durationsNoNot specified
FSRUser spec.12, 30, 60,120 min. i x td50% summer profileYesNon-urban studies
Pilgrim & CorderyUser spec.User spec. Ti x TLocal analysis of storm eventsYesUrban and rural areas
Yen & ChowUser spec.i x TriangularNoDrainage facilities in small areas

 – time of concentration
T – user selected storm duration
 – storm duration selected using an iterative procedure. The design storm is tested using different durations. The one with the largest peak flow is selected.

Each of the design storms has a different hyetograph shape. Storm hyetographs were constructed and compared for some of the design storms. A five-year return period was selected and the storm volumes were obtained from the Bloor Street station (Toronto) IDF curve. The duration of the storms are not all the same, for this reason the storm volumes are different. The storm hyetographs for the Uniform, Composite, Chicago, SCS 24-hr., ISWSAESFSR, and Yen and Chow design storms are shown in the figures below.



All of the design storms are different. The peak intensities, storm profiles, durations and volumes vary even though they all have the same return period. The Uniform design storm has the lowest intensity. It has a constant intensity and is not recommended for use with an event simulation model. The Chicago design storm has a high peak intensity. The peak intensity of this storm de-pends on the time step one selects. In Figure 25 the time step was increased from 5 to 10 minutes this reduced the peak intensity by 29% from 168 mm/hr to 120 mm/hr. The FSR and Composite design storms also have high peak intensities, but their shapes are not similar. The ISWSSCS 24-hr., AES and Yen and Chow design storms have peak intensities that are in the same range. The peak rainfall for the SCS 24-hour and the Yen and Chow storms that were computed are the same.

The wide variety of hyetograph profiles is why design storms of the same return period will not produce the same peak flows. Studies are therefore required to determine if design storms can be used to predict flows of a desired return period.

A review of previous studies showed that there is contradictory opinion regarding the use of design storms. Marsalek (1978) does not recommend the use of design storms while the results of Arnell (1982) and Watson (1981) suggest that design storms should be used. Other researchers have concluded that further studies are required.

Those who recommend the use of design storms consider that their advantages outweigh the shortcomings. The advantages of using design storms are that:

  1. They are an inexpensive procedure for obtaining flows of a desired return period.
  2. If properly selected they give conservative results for peak flows and volumes.
  3. They are widely used in current engineering practice.

Some of the disadvantages of design storms are that:

  1. The runoff frequency is assumed to be the same as the rainfall frequency. This equivalence of return period has not been shown to be true.
  2. The rainfall volume is not the rainfall volume of real storm events.
  3. Using IDF relationships to obtain a design storm hyetograph may be incorrect.

A study was conducted using IMPSWM procedures to test two design storms commonly used by Canadian engineers. The uniform design storm was not tested because of its low, unrealistic in-tensity. The Chicago and the SCS 24-hr design storms were selected for the study. The AES de-sign storm could have been used but the 30% profile with a 1-hour duration gives peak flow results close the Chicago storm and historical storm flows (Wisner and Gupta, 1980). With a 12-hour duration the AES 50% profile gives results similar to the SCS 24-hr. design storm. The third chapter contains the results that compare the Chicago and the SCS 24-hr design storms. The methodology can be used for any other design storm by a municipality.


The researchers comparing peak flows from design storms and historical storms used different catchments and different simulation programs. A comparison of the peak flow frequency results for the Chicago design storm is presented first in this chapter. The results for this storm are summarized in Table 3.


StudyCatchmentChicago Design Storm
Arnell (1980)Bergsjon
Linkoping 1
Linkoping 2
Marsalek (1979)Burlington
(area 26 ha, imp. 30%)
Watson (1980)Pinetown

A study conducted in the IMPSWM program is also presented here. The methodology used to compare the Chicago and SCS 24-hr design storms with the historical storms is given in the second section of the chapter.

Results for the Chicago Design Storm

J.F. McLarens Ltd. (1978) has conducted studies on catchments in Edmonton and Winnipeg. They found that the ratio of the Chicago storm peak flow to the flows from an historic storm series ranged between 1.0 and 1.2. It was recommended that the Chicago design storm be used for urban drainage design.

Marsalek (1979) developed a Chicago design storm for the Burlington area. He found that the peak flows produced from the Chicago design storm are 80% larger than those produced from historical storm events. He also found that the peak flow was attenuated as the catchment size increased. The peak flow increased as the catchment imperviousness increased but the peak flow overestimation remains at approximately 80%. These results were analyzed in the IMPSWM pro-gram by Wisner and Gupta (1980). They concluded that discrepancies can be reduced if the peak intensity of the design storms are reduced to values in agreement with measured peak intensities.

Watson (1980) compared the peak flows obtained from the Chicago design storm and historical storm events. A 2-hr. duration and a non-dimensional time to peak of 0.28 is used to develop the Chicago storm. The rainfall data is discretized at 5 min. intervals.

Watson found that on the Pinetown catchment the peak flows from the Chicago storm agreed closely with those from the historical storms. The agreement for the Kew catchment was not quite as good. The peak flow is slightly underestimated. It is within 95% confidence interval bands of the historical storms, though. The Kew catchment is less impervious than the Pinetown catchment; therefore, it is more sensitive to antecedent moisture conditions.

Arnell (1982) used a Chicago design storm with a 4-hr duration. The non-dimensional time to peak, r, is 0.43 if the return period is less than 1 year. If the return period is greater than 1 year, r is 0.35. The Chicago storm is developed with a step size of one minute.

Arnell found that the Chicago design storm overestimated the peak flow by approximately 5%. On the Bergsjon catchment, the peak flow is underestimated by 2.2%. On the Linkoping 1 and Linkoping 2 catchments the flow is overestimated by 10.3% and 6% respectively. The Bergsjon catchment was the smallest of the three catchments. The Chicago storm produces peak flows al-most identical to the historical storms on this catchment.

With the exception of Marsalek (1979), the estimation of peak flow produced by the Chicago storm gave acceptable results compared with that produced by historical storms. Differences range from a 2% underestimation to an 10% overestimation of peak flow.

Watson recommended that the Chicago design storm be used for peak flow design. Arnell also found that the deviation of the Chicago Storm peak flow values from the historical storm peak flow values are not large. He concludes that the Chicago Storm should overestimate peak flows be-cause of the way it is developed. He does not recommend the use of the Chicago design storm because of the large overestimation of peak flow Marsalek found.


This section compares the design storms and real measurement data.

Rainfall Input

The rainfall inputs used with the event simulation models were a historical storm series and two design storms. The historical storm series was selected from the Bloor Street station rainfall record. A criteria was selected based on the storm volume and intensity so that approximately one storm event for each year in the record was chosen. This results in some years having more than one event and other years having no events. The selected events were then discretized to ten-minute time intervals. A summary of the storm events and their characteristics is given in Table 4.


DateDuration (hrs.)Volume (mm.)Time to Peak (hrs.)Peak In-tensity (in./hr)Average Intensity (in./hr)API (mm)
Sept. 15/576.6747.844.16759.187.1919.837.5
July 9/605.5062.335.00082.3711.3312.324.0
June 19/616.1737.122.86749.286.0224.545.0
Sept. 13/622.0042.620.167159.2630.2813.225.0
Aug. 11/646.5040.614.16739.626.2512.724.5
Aug. 5/684.6742.384.16770.649.0910.519.0
May 16/7412.0058.328.50056.344.8549.474.0
Aug. 23/740.6751.200.333153.6276.815.58.0
Aug. 23/759.1757.222.66777.726.257.714.0
July 6/777.1751.107.16750.297.4128.049.0
July 31/770.8745.190.333156.7754.2316.231.0
Sept. 24/7710.6760.968.33326.245.9924.845.1

The SCS 24-hour and Chicago design storms were compared with the historical storm series. The design storms were developed from the Bloor Street station IDF curves for return periods of 5, 10 and 25 years. The Chicago storm was 4 hours in duration and was discretized at 10-minute inter-vals. The SCS storm was 12 hours in duration and was discretized at 12 min. interval. The peak intensity and antecedent moisture conditions should be adjusted so that the design storm resembles real storm conditions.

The adjustment is necessary on urban catchments because the peak flows are dependent on the peak intensities. The scatter gram in the figure below shows that the correlation between peak flows and peak intensity is close to 1 in urban areas.


The choice of the time step is important in obtaining a peak intensity close to the peak intensity of real storms. An analysis was conducted to demonstrate the importance of the time step used with a design storm. The 5- and 10-min. intensities were extracted for the highest recorded storms in Toronto (Hogg,1980). These were plotted along with the peak intensities of the Chicago design storm peak intensities discretized at 5- and 10-minute intervals as shown in the figure below. For the 5-minute intensities, the Chicago design storm intensities are higher than the real storm intensities, while for the 10-minute intensities they are slightly lower. Wisner and Gupta (1979) show that there can be a large variation in flows depending on the step size chosen for the design storm. Time steps between 10 and 20 minutes are recommended for use with the Chicago design storm. If the design storm peak intensity is still larger than that of real storms it should be adjusted so that the two peak intensities are similar.


The antecedent moisture conditions are usually not considered as being important when a design storm is used with an event simulation model. Some studies, though, have been conducted to investigate this. Wenzel and Voorhees (1979) tested design storms using both wet and dry antecedent moisture conditions, but they do not recommend a procedure for determining what conditions should be used with a design storm. The Flood Studies Report (NERC,1975) present a procedure for determining the antecedent moisture conditions. Using a relationship between the Ur-ban Catchment Wetness Index and the Standard Average Annual Rainfall the antecedent moisture conditions can be determined for the FSR design storm in any area in the U.K. In the present study the modified curve number is used to represent the antecedent moisture conditions. With the OTTHYMO model an average modified curve number, for a watershed, is used with the design storms.

Watersheds Studies

Three different types of watersheds were examined in this study; a rural watershed, an urban watershed and a mixed land use watershed. Two rural watersheds in southern Ontario were tested, one was large and had an area of 6540 ha. the other was small having an area of 44 ha.

Simulation runs were also conducted on three southern Ontario urban watersheds. The catchment characteristics are summarized in Table 5.


WatershedArea (ha.)Imperviousness (%)
No. 1294.435 & 50
No. 2290.350
No. 3150.530

On two of the catchments the urban area routine URBHYD of OTTHYMO was used. Impervious conditions of 35% and 50% were used on urban catchment No.1 to observe the change in difference between the design storm peak flows. On the third urban catchment the SWMM simulation program was used. A schematic of this catchment is shown in the figure below.


A mixed land use watershed in Metropolitan Toronto, having a rural area of 1597 ha. and an urban area of 5536 ha., as also tested. The flows on this watershed were found using the OTTHY-MO model. The total area contained 21 urban subwatersheds and 12 rural subwatersheds. They ranged in size from 53 ha. to 778 ha. The urban subwatersheds had an imperviousness of 35%.


This section compares the peak flow results of design storms and measurement rainfall data for different types of watersheds.

Rural Watersheds

In this study design storm and historic storm flows were generated on large rural watersheds. The results from the historic storm events were examined to determine if the peak intensity or the antecedent moisture conditions influence the flows on a rural catchment. The peak intensity was found to be independent of the peak flows as shown in the figure below. On the other hand, the antecedent moisture conditions, as measured by the API, are correlated with the peak flows. The correlation coefficient between the API and the peak flows is close to 1.