Introduction
The Watershed Modeling System (WMS) was developed at the Environmental Modeling Research Laboratory (EMRL) in cooperation with US Army Corps of Engineers Waterways Experiment Station (WES) and the Federal Highways Administration. The focus of WMS is to provide a single application which integrates digital terrain models with industry standard runoff models such as HEC-1, HEC-HMS, TR-20, HSPF, TR-55, and the National Flood Frequency program (NFF) regional regression equations. WMS can be used to develop hydrologic data from TINs or grids. More importantly hydrologic data developed in Arc/INFO, ArcView, or WMS can be directly linked to commonly used hydrologic models. Besides being able to export TINs or grids developed in Arc/Info or ArcView to WMS for further hydrologic data development, vector data representing streams and basin boundaries can also be passed between a GIS and WMS. This is done through three primary shapefiles: a polygon shapefile for basin boundaries,a line shapefile for stream networks, and a point shapefile to identify outlet locations. A series of Avenue scripts, developed by ESRI, can be used with the Spatial Analyst extensions to automatically generate these three shapefiles, including population of attribute fields with important hydrologic parameters. An additional ArcView extension WMS-Hydro aids in preparing vector data for import into WMS.
Whether you are using grids, TINs, or vector coverages, these data can then be used to create a model for any of the hydrologic programs supported by WMS. Data entry for the model, including rainfall, job control, or any other parameters not defined as attributes in the shapefiles, can be completed using WMS's hydrologic modeling interface. WMS can be used to post-process and then export results back to the GIS software.
Figure 1. The Watershed Modeling System imports ArcView shapefiles for use in creating HEC-1, TR-20, TR-55, and other hydrologic models.
GIS has become established as an excellent tool for data storage and management. With the creation of GRID in Arc/Info and the Spatial Analyst in ArcView, GIS has become more useful for hydrologic data development as well. However, much of this data, both stored and developed in the GIS, remains locked to hydrologic modelers. Even though GIS holds much promise as a tool for performing spatial hydrologic runoff modeling (particularly on a regional basis), much of the modeling performed must be done using industry standard, lumped parameter models such as HEC-1 and TR-20. While much of the input required to run these models can still be developed using GIS, some parameters such as rainfall, job control, and other model-specific parameters can not.
In order to "unlock" hydrologic data developed/stored in GIS for use in traditional lumped-parameter hydrologic models, a link consisting of three primary shape files has been developed as a joint effort by ESRI and EMRL. The link provides a common gateway to transfer data from a GIS to a hydrologic modeling system such as WMS. These three shapefiles consist of:
Figure 2. A shapefile of stream lines defines the topographic
layout of the watershed.
Figure 3. A shapefile with points defines the watershed
and sub-basin confluence locations.
Figure 4. A polygon shapefiles define sub-basin boundaries.
Figure 5. Both geometric and attribute data stored in ArcView are used in model creation.
WMS supports the processing of GIS data for use in the development of hydrologic models such as HEC-1 and TR-20. WMS-Hydro is designed to pre-process GIS data for import into WMS. When the data are imported into WMS, they are linked to each of the hydrologic models supported in WMS.
In WMS, shapefiles can be imported using two different methods. The Import Shapefile Data option provides the facility to specify each shapefile separately and to map the attributes required for modeling. The other option is to import a ArcView-WMS superfile, which is a collection of ArcView® shapefiles and ASCII grid files. This ArcView-WMS super file can be created using the WMS-Hydro ArcView® GIS extension, developed by EMRL.
Using WMS Hydrologic Extension WMS-Hydro for ArcView® GIS
The hydrologic modeling Avenue scripts developed by ESRI can be used to generate a stream network and basin boundaries from a grid. However, after the basin boundaries and stream network are generated, further editing must be performed before the data is imported into WMS. This editing is facilitated using a seperate Avenue extension developed by EMRL called WMS-Hydro.
Once these shapefiles are ready, they can be exported as a ArcView-WMS super file. A ArcView-WMS super file is a collection of themes (shapefiles) and grids recognized by WMS and can be exported and imported from this extension as well as from WMS.
Figure 6. The coverages and grids are the components of WMS superfile and can be selected while importing in WMS.
Using the 'Import Shapefile' option in WMS
Besides being able to import a ArcView-WMS Superfile into WMS, the outlet, stream and basin shapefiles can be specified as separate files and imported into WMS (many of the same data preprocessing tools available in the WMS-Hydro extension are also available in WMS so you can edit your data in whichever environment you prefer).
Figure 7. Attributes which use specified keywords for item names are automatically mapped when importing shapefiles.
Figure 8. Any attribute can be manually mapped or unmapped to corresponding WMS parameters.
Figure 9. The three shapefiles are used to create a topologic representation of the watershed used in interfacing to hydrologic models.
Figure 10. Typical results of hydrologic models include flood hydrograph and peak flows.
HEC-1, developed by the Hydrologic Engineering Center in Davis, California, has long been one of the industry standard programs for hydrologic analysis. It is a single-event, lumped parameter model, but includes several different options for modeling rainfall, losses, unit hydrographs, and stream routing. The HEC-1 interface in WMS makes it simple to enter and manage input data and process results. All input is managed through a single dialog as shown in the figure below. HEC-1 style inputs for selected basins and outlets is shown and a model checker can be run to verify that data is consistent and properly defined prior to actually running the model. While not completely supported, waterhsed models and data entered as part of an HEC-1 model can be exported for use in HEC-HMS as well.
Figure 11. HEC-1 data not mapped through the shapefiles can be defined using a series of user-friendly dialogs.
The National Flood Frequency (NFF) program was compiled by cooperative effort between the United States Geological Survey, the Federal Highways Administration, and the Federal Emergency Management Agency. It contains a database of regional regression equations that can be used to compute peak discharges for 2, 5, 10, 25, 50, 100, and 500 year events. Watershed data such as area, slope, and median elevation are the primary variables used by most of the regression equations. The link between ArcView and WMS makes it a simple to access equations for any state/region in the US using data easily developed within the GIS. The dialog below illustrates how the interface in WMS is used to compute peak flows and runoff hydrographs with the regional regression equations.
Figure 12. The National Flood Frequency (NFF) program contains state by state regional regression equations.
The Rational Method is one of the simplest and best known methods routinely applied in urban hydrology. Peak flows are computed from the simple equation:
Q = CiA
where:
Q - Peak flow
C - Runoff coefficient
i - Rainfall intensity
A - Catchment area
Both the catchment area, A, and the runoff coefficient, C, are easily computed using GIS. The rational method interface in WMS includes tools to generate intensity-duration-frequency curves to determine i, and several different dimensionless hydrograph methods that can be used for developing runoff hydrographs from peak flows. Routing lag times and level-pool routing through detention ponds can also be specified in order to develop runoff from a large catchment which has been subdivided into several smaller basins. The combination of a GIS such as ArcView and a hydrologic modeling system such as WMS provides a powerful method for analyzing urban drainage scenarios.
Figure 13. An interface to run the rational method can be used for urban hydrology problems.
TR-20, like HEC-1, is a lumped parameter, single event model that was developed by the National Resource Conservation Service (NRCS). Like HEC-1, data developed in ArcView can be passed to WMS using the three primary shapefiles and then remaining input parameters defined using a series of user-friendly dialogs. WMS will then create a properly formatted file and start the TR-20 executable. Results can be viewed in the same fashion as is with HEC-1.
The NRCS TR-55 method of hydrograph computation is a simplified version of the TR-20 method. The TR-55 method is a simple procedure that estimates peak flows and hydrographs for small watersheds and urban areas. Because of the simplicity of the TR-55 method and its time-proven results, many cities and counties use this method to estimate peak flows and generate hydrographs. And because of the GIS-hydrologic data link in WMS, it is simple to generate TR-55 hydrologic models in WMS.
Figure 14. An interface to run TR-55 can be used for small watersheds.
The TR-55 input parameters include sub-basin drainage area, time of concentration, rainfall, curve number, and pond/swamp area (if applicable). WMS provides an interface for entering these parameters for each sub-basin in a watershed model. And since most of the parameters required for TR-55 are easily computed in WMS or imported from a GIS, using the TR-55 method is quick and easy.
WMS was initially developed as a surface modeling tool. Surface representation was in the form of triangulated irregular networks or TINs. Watershed and sub-basin boundaries can be determined from a TIN and all hydrologic parameters (area, slope, runoff distances, etc.) that can be computed from a TIN are done so automatically. These parameters are then used in conjunction with defining the models described above. Since WMS can import Arc/Info TINs, a TIN developed in the GIS can be used inside of WMS for hydrologic data development.
Figure 15. Black River Watershed (western New York state) delineated using a TIN.
Besides interfacing with ArcView and Arc/Info through shapefiles, WMS can be used for further hydrologic data development from grids. Two exported grids from either ArcView (export a grid as ASCII) or Arc/Info (use the GRIDASCII command) are required as input to WMS: 1) an elevation grid, and 2) a flow direction grid. Given these two grids WMS can be used to compute much of the same data as the scripts developed by ESRI to work inside of ArcView, including:
Computation of flow accumulation grid
Figure 16. Computed flow accumulation are automatically converted to stream lines.
User defined outlet locations and watershed and sub-basin delineation.
Figure 17. The combination of stream lines, specified outlet points, and a flow direction grid are used to delineate basins.
Computation of important hydrologic parameters such as area, average basin slope, maximum runoff distances, etc.
Figure 18. Basin parameters can be computed from delineate sub-basins for use in hydrologic models.
Raster watershed boundaries can be converted to polygons and exported as shapefiles (with accompanying computed attributes) for storage in a ArcView.
As with any of the watershed data developed in WMS, the hydrologic data developed can be used with any of the supported hydrologic models. Dean Thomas, a graduate assistant working with Dr. David Maidment at the University of Texas, successfully built an HEC-1 input file as part of an independent pilot study using these tools in a beta version of WMS.
Digital elevation data and watershed model data can be used in WMS to automatically generate time computation arcs. These arcs can be used to compute either the time of concentration for a basin or the travel time from a sub-basin outlet point to the watershed confluence point. Specific attributes can be assigned to these arcs, but length and slope (the two variables most often used with time of concentration equations) are mapped automatically from the digital terrain model and flow path segments. Besides length and slope, some of the other attributes include: arc equation type, Manning's n-value, rainfall intensity (solved iteratively as a function of tc). Besides a library of equations such as those used in TR-55 and by the FHWA, user defined equations may also be used.
Figure 19. Overland flow arcs, channel flow arcs, and travel time arcs can be automatically defined and used to compute travel times and times of concentration.
When a basin is selected, the Time of Concentration is computed for the selected basin by summing the times of travel for all flow arcs in the basin. When an outlet is selected, the Travel Time is computed by summing the travel times for the arcs to the next downstream outlet point.
Shapefiles or ASCII grid files representing Land Use and Hydrologic Soil Type can be imported into WMS and used to determine composite curve numbers for subbasin boundaries as illustrated in the following diagrams. The same type of operation is also available for mapping other soil and land use parameters such as hydraulic conductivity, runoff coefficient, and impervious zones as used by the different models supported in WMS.
Figure 20. Polygon shapefiles of soil type and land use can be used to create composite curve numbers.
More Information
More information, and a free demonstration version of WMS can be downloaded from the EMS-I home page.
These materials may be used for study, research, and education, but please credit the authors and the Environmental Modeling Research Laboratory, Brigham Young University. All commercial rights reserved. Copyright 1999 Environmental Modeling Research Laboratory.
