Watershed Characterization Module
Center for Research in Water Resources, University of Texas at Austin
Introduction
Watershed characterization describes a set of methods for doing calculations on a digital elevation model (DEM) of the land surface to derive drainage patterns, delineate streams and watersheds, and determine their interconnectivity. There are standardized tools in the Arc/Info Grid system to perform these functions, and these have also been incorporated into the Spatial Analyst extension available for Arcview Version 3.0. This page describes several programs that use terrain analysis functions.
CRWR-PrePro, a system of ArcView scripts developed to extract hydrologic, topographic and topologic information from digital spatial data of a hydrologic system, and to prepare these data for input into other modeling systems, can also be used for watershed delineation. Starting with a DEM, CRWR-PrePro delineates streams and watersheds, and determines their interconnectivity.
The Web Grid Calculator illustrates some of the basic hydrologic functions -- such as "flow direction" and "flow accumulation" -- used for terrain analysis.
In this section, you will find articles, reports and class exercises that focus on:
Stream and Watershed Delineation
Two exercises related to terrain analysis, that are part of a GIS course taught by David R. Maidment in the Department of Civil Engineering of the University of Texas at Austin, have been included in GIS Hydro '98. Delineation of Watersheds and Streams describes the delineation process using Arc/Info Grid, while Spatial Hydrology of the Urubamba River System describes it using ArcView 3 with the Spatial Analyst extension.
The Urubamba River exercise, additionally, illustrates the burning-in streams process used to improve the stream delineation. When using DEM's, it is often the case that the relief is not accurate enough to delineate the stream networks in flat areas. In such cases, it is useful to "burn" digitized streams into the DEM so that the elevation model reveals a more accurate representation of the drainage network. A simple methodology for burning-in streams is to rasterize the digitized stream network and then add a constant value to all non-stream cells , while maintaining the elevation of the stream cells. There may be problems with this method that need to be corrected manually if the digitized streams being used are not at the same scale as the DEM. Burning streams is a very useful technique for identifying stream locations in flat areas where high resolution DEMs are not available, but this technique should be used with caution.
A list of the current class exercises is provided under the Class Exercises Module on this CD-ROM.
Identification of Inland Catchments
The Arc/Info Grid FILL function or the equivalent ArcView 3 Spatial Analyst function eliminates pits in the land surface. Filling pits is typically the first processing step taken after a DEM is projected into the chosen map projection. Pits are cells or groups of cells which are completely surrounded by cells of higher elevations. Some pits are artificial while others represent real inland drainage basins. It is sometimes difficult to distinguish between real pits and artificial pits automatically by using the DEM alone. Two criteria that are useful for identifying real pits are the pit depth and area. An Arc/Info Grid procedure for identifying inland basins using these criteria is provided by Francisco Olivera. The Inland Catchment Identification documentation, AML code, and a sample exercise are included with the GIS Hydro '98 CD-ROM.
CRWR-PrePro: An ArcView Pre-Processor for HEC's Hydrologic Modeling System
CRWR-PrePro -- a system of ArcView scripts and associated controls -- has been developed to extract hydrologic, topographic and topologic information from digital spatial data of a hydrologic system and to preprocess these data for input into non-GIS modeling packages. Though it will be able to work with other models in the future, CRWR-PrePro at this time is only able to prepare an input file for the Hydrologic Modeling System (HMS) developed by the Hydrologic Engineering Center (HEC) of the United States Army Corps of Engineers. Starting with a DEM and a SCS curve number grid, CRWR-PrePro delineates streams and watersheds, calculates parameters for each of them, determines their interconnectivity, and prepares an input file for HMS that includes the computed hydrologic parameters.
The first attempt to connect GIS data layers with HEC's HMS was developed by F. Hellweger and D. Maidment. This first version of CRWR-PrePro takes stream and subbasin coverages and analyzes them to produce a schematic model which defines and spatially connects seven types of hydrologic elements in the landscape: subbasins, reaches, junctions, diversions, reservoirs, sources, sinks. Graphics depicting some Exciting Success Stories illustrate the application of CRWR-PrePro to basins of the City of Austin in Texas, of the Upper Mississippi and Missouri Rivers, and of the Niger River in West Africa.
Further development of CRWR-PrePro led to a more complete system that -- besides the capabilities of the first version -- delineates streams and watersheds and calculates hydrologic parameters of each of them. At present, CRWR-PrePro is able to calculate the required hydrologic parameters for determining abstractions according to the Soil Conservation Service curve number method, watershed routing according to the Soil Conservation Service dimensionless unit hydrograph, and stream routing with the Muskingum and lag methods.
To run CRWR-PrePro, the following files have to be retrieved from the directory GISHyd98/watchar/hecfiles:
Additional information on CRWR-PrePro can be found at:
HEC-PrePro: An ArcView Pre-Processor for HEC's Hydrologic Modeling System prepared by Francisco Olivera, Seann Reed and David Maidment, and presented at the 1998 ESRI User's Conference, San Diego, California, 1998. (Please note that at the time of this paper's writing, CRWR-PrePro retained the name HEC-PrePro.)
Spatial Hydrology of Portugal and Spain prepared by Francisco Olivera.
Connecting Digital Elevation Models (DEM) to the Hydrologic Modeling System (HMS) Computer Program prepared by Francisco Olivera, Aubrey Dugger, David Maidment and Edward Holley for the Hydrologic Engineering Center (HEC).
Digital Watershed Delineation and CRWR-PrePro (Class Exercise) prepared by S. Ahrens and D. Maidment.
Primary Contact
Francisco
Olivera
University of Texas at Austin
Center for Research in Water Resources
Austin, Texas 78712
e-mail: folivera@mail.utexas.edu
Phone: (512) 471-0570
Fax: (512) 471-0072
These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 1998 Center for Research in Water Resources.