GIS Hydro '98 - WMS
Watershed Modeling System
Engineering Computer Graphics Laboratory, Brigham Young University
Introduction
The Watershed Modeling System (WMS) was developed at the Engineering Computer Graphics Lab (ECGL) 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, TR-20, TR-55, and the National Flood Frequency program (NFF) regional regression equations. WMS can be used to develop hydrologic data from TINs or grids using many of the same techniques described in other sections of this CD, in particular Watershed Characterization. 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 (downloadable from the ECGL web site ) aids in preparing vector data for import into WMS. Specifically this extension performs the following critical steps:
Whether you are using grids, TINs, or vector coverages, these data can then be used in WMS 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, and typically are not efficiently entered and stored in the GIS.
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 ECGL. 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.
In addition to the geometry stored in the shapefiles, any number of hydrologic modeling related attributes may be stored as part of the shapefile. These attributes may be developed using the Spatial Analyst or manually entered.
Figure 5. Both geometric and attribute data stored in ArcView are used in model creation.
These three shapefiles can then be imported into any program designed for hydrologic modeling. The combination of outlet points, stream network, and sub-basins will uniquely define the watershed structure, and attributes can be used to set up any hydrologic model for which the application is designed to support.
WMS supports the processing of GIS data for use in the development of hydrologic models such as HEC-1 and TR-20. This section will present an interface designed to import GIS data into WMS. When the data is imported into WMS, it is automatically 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 WMS/ArcView® superfile, which is a collection of ArcView® shapefiles and ASCII grid files. This WMS/ArcView® super file can be created using the ArcView® GIS extension, recently developed by ECGL.
Using WMS Hydrologic Extension for ArcView® GIS - WMSHydro
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 ECGL called WMSHydro.
Development of a hydrological model in WMS from shapefiles is a single step procedure when the shapefiles have certain properties. First, the shapefile attribute fields must be set up so they can be mapped to the WMS attribute fields. Second, the streams must ordered from downstream to upstream. Finally, the outlets points must coincide with nodes or vertices on the stream and sub-basin layers. Shapefiles digitized or generated using ESRI's Hydrologic Modeling extension may not necessarily possess these properties. Therefore, the WMS Hydrologic Extension for ArcView® GIS, WMSHydro, was developed to provide a set of scripts to prepare these shapefiles for hydrologic modeling in WMS. Depending on the needs, the appropriate dialog can be opened by selecting each of the choices under the WMSHydro pull down menu. This menu is added to ArcView®'s View menu bar once this extention is loaded.
Figure 6. WMSHydro pulldown menu added to ArcView®'s View menu bar.
Figure 7. Assigning aliases for shapefile attribute names so that they are automatically mapped in WMS.
Shapefiles created from different sources usually have the attributes required for hydrlogic modeling such as area, basin id, etc. but may have different attribute names. Aliases can be assigned to these attributes so that they are automatically mapped in WMS.
Figure 8. Stream can be ordered using the 'Stream Ordering' fuctionality provided in the extension.
Stream order is important for developing a hydrlogic model in WMS. WMSHydro provides the facility to reorder arcs in the stream layer so that the arc directions are consistently ordered from downstream to upstream. The most downstream outlet point must be selected in order to perform this task.
Figure 9. Before Ordering the most downstream node in lower sub-basin is displayed in red, which should actually be blue in order to be a downstream node.
Figure 10. After Ordering the most downstream node in lower sub-basin is displayed in blue, indicating a downstream node. The intermediate nodes and upstream nodes have been changed accordingly.
The effects of ordering are described in the above figures, where the upstream and downstream nodes of the streams are displayed using red and blue marker symbols, respectively.
Figure 11. An outlet shapefile defining the watershed and sub-basin confluence locations can be generated automatically.
As mentioned earlier, in addition to stream and sub-basin boundary shapefiles, an outlet shapefile is also required. However, these outlet points must coincide with nodes or vertices on the stream and sub-basin layers so that these layers can be tied together at outlet points in WMS. WMSHydro provides the facility to create an outlet layer intersecting a stream and a sub-basin boundary. At the same time, WMSHydro inserts vertices at the same location in those two layers which otherwise could have been a difficult task to perform manually.
Figure 12. Shapefiles can be exported as a WMS superfile after mapping to an appropriate coverage recognized by WMS.
Once these shapefiles are ready, these can be exported as a WMS/ArcView® super file. A WMS/ArcView® super file is a collection of coverages and grids recognized by WMS and can be exported and imported from this extension as well as from WMS.
Figure 13. The coverages and grids are the components of WMS superfile and can be selected while importing in WMS.
The WMS superfile created from the WMSHydro extension can be read directly into WMS using the Import WMS Superfile option. In addition to the outlet, stream and sub-basin shapefiles, other GIS attribute layers stored as shapefiles or grids can be imported into WMS through this superfile format.
Using the 'Import Shapefile' option in WMS
Besides being able to import a WMS/ArcView® Superfile into WMS, the outlet, stream and basin shapefiles can be specified as separate files and imported into WMS.
Figure 14. Attributes which use specified keywords for item names are automatically mapped when importing shapefiles.
As the shapefiles are read by WMS, key words for database item names such as area, slope, curve number etc. are checked against several defined names in order to automatically "map" these variables for use within WMS. If the shapefiles were created using the customized scripts for the ArcView Spatial Analyst (described in more detail below) then they will be created with the proper item names to automatically map in WMS. If the item names to match the pre-defined names then the user can manually map the item names in the database (dbf) file to their corresponding use in WMS using the dialog shown below.
Figure 15. Any attribute can be manually mapped or unmapped to corresponding WMS parameters.
Once the mapping has been defined the shapefiles are read in and converted to a digital and topological representation of the watershed as shown in the next figure.
Figure 16. The three shapefiles are used to create a topologic representation of the watershed used in interfacing to hydrologic models.
As mentioned above, any variables read in through the shapefile interface can be mapped to their corresponding values in WMS. As a minimum, basin areas and stream lengths will be defined. Since WMS was designed specifically for hydrologic modeling applications it is not important that all parameters be computed in ArcView prior to exporting the shapefiles for use in WMS. Missing data can be defined inside of WMS and properly formatted for any of the supported models. The hydrologic analysis can then be done in WMS and results sent back through the same three shapefiles for storage/query from ArcView.
Figure 17. Typical results of hydrologic models include flood hydrograph and peak flows.
In order to facilitate the development of hydrologic data from grids in ArcView some sample extensions have been created by the ESRI development team. Some of the scripts were included as a sample application with the initial release of Spatial Analyst 1.0. These functions have been combined with several other commands in order to provide a more complete tool for developing hydrologic data in preparation of using industry standard hydrologic models such as HEC-1 and TR-20. The scripts include the following capabilities:
Initially, the following attributes for the three different shapefiles will be exported:
For stream line shapes
For outlet point shapes
For polygon basin shapes
Other attributes for the different shapes will likely be added as the project progresses.
While WMS has been used to show how the resulting shapefiles can be linked to hydrologic models, the format is open and can be processed by any program designed specifically to interface to rainfall/runoff programs.
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.
Figure 18. 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 19. 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 20. 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 21. 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 22. 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 23. Computed flow accumulation are automatically converted to stream lines.
User defined outlet locations and watershed and sub-basin delineation.
Figure 24. 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 25. 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 26. 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 27. 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 WMS home page. Pay attention to information on this site for availability and location where the hydrologic modeling extension scripts for the ArcView Spatial Analyst can be downloaded.
These materials may be used for study, research, and education, but please credit the authors and the Engineering Computer Graphics Laboratory, Brigham Young University. All commercial rights reserved. Copyright 1998 Engineering Computer Graphics Laboratory.
