Updating and Enhancing the Functionality of
GISHYDRO Including Digital
Terrain and Digital Line Data
MARYLAND DEPARTMENT OF TRANSPORTATION
STATE HIGHWAY ADMINISTRATION
DIVISION OF BRDIGE HYDRAULICS
Dr. Glenn E. Moglen and
Michael J. Casey
Department of Civil and
University of Maryland
College Park, MD 20742
Since the early 1990s, the Maryland State Highway Administration Office of Bridge Development has utilized a Geographic Information System (GIS) to support watershed analyses in the State of Maryland. The system, called GISHYDRO, combines a database of geographic and hydrologic data with software tools for development of hydrologic models.
The University of Maryland Department of Civil and Environmental Engineering, under State and Federal sponsorship, has developed and continues to build upon this system. Close cooperation between MDSHA and the University has allowed the program to continually evolve and improve. The capabilities of the software have been successfully employed by Federal and State agencies as well as private consulting groups. The software has been identified as a high payoff technology by the Federal Highway Administration (FHWA). In January 1999, FHWA sponsored a satellite teleconference on the use of GISHYDRO for hydrologic analysis to six state DOTs, with numerous inquiries into obtaining the software since that time. The software has been distributed to select agencies nationwide and is used routinely in Maryland Department of Environment and Department of Natural Resources for conducting hydrologic analyses.
In the Fall of 1997, the current project, Updating and Enhancing the Functionality of
GISHYDRO Including Digital Terrain and Digital Line Data began to make significant enhancements to the software. Although maintained through the 1990s, the program required updating to take advantage of new GIS data, analysis methods, and software. Divided into two phases, Phase I of the project was completed and submitted in January 1998. The Phase I objectives included adding 1994 Maryland Office of Planning (MOP) land use data, providing compatibility with standard GIS data formats, programming of the 1996 USGS Regression Equations (Dillow, 1996), and allowing the use of SI and English units. The Phase II objectives included migration the software tools to the Windows platform, incorporation of digital terrain data to facilitate automatic watershed delineation, and programming to the USGS Dimensionless Hydrograph model for Maryland. Changes in scope were made to include programming of the Hydrology Panel Time of Concentration regression equations and incorporation of the USGS-based error bound program developed by Gary Tasker. This document describes how these and other objectives were met during the course of the project. Although this report provides a more detailed description of the Phase II objectives, it is intended as an overview for the entire project.
Because of the availability of new data sources, programming tools and experience, it is believed that the GISHYDRO system has improved beyond the scope of the original project. The database of land use data now includes 1970s and 1997 land use conditions as well as a generalized land cover layer derived from satellite imagery. The soils database now includes data from the Natural Resources Conservation Service (NRCS), previously unavailable in digital format. A customized interface to the NRCS TR-20 has been developed for formatting of input and direct execution of the model. Also, the network of Marylands USGS stream gage locations and their attributes has been incorporated into the program. Because of these and other enhancements, the program itself is fundamentally different from earlier versions. The software has therefore been renamed GISHydro2000 to reflect both past success and future innovation. The University of Maryland has maintained close cooperation with MDHSA during the development process. As this new version is tested, improvements will continually be made.
This report is intended as an executive summary of the project. A Users Manual, GISHydro2000 Users Manual, has also been prepared to accompany this report and provide reference for the operation of the software. The Users Manual provides a greater level of detail regarding the structure and operation of the program and includes installation instructions and a sample watershed analysis.
Delivered for this project is a CD-ROM containing the program installation files and database. The functionality of GISHydro2000 has been programmed into a series of scripts for the ArcView GIS program (see Section 2.1). For archival purposes, a description of these scripts, their organization, and their source code is provided in Appendix A.
GISHYDRO in its original form was designed to be used on a dedicated workstation with an attached digitizing table. The program was written in QuickBASIC and relied on specialized data formats for storage and processing of hydrologic data. GISHYDRO was operated by an elaborate menu system and was constrained by the inherent limitations of DOS-based graphic display capabilities. Further, the program could not easily incorporate new GIS data from agencies such as the U.S. Geological Survey.
In enhancing the capabilities of the software, some fundamental changes were made to its structure. This section describes the major differences caused by the migration to GISHydro2000.
One of the primary objectives of Phase II was to migrate GISHYDRO to the Windows computer platform. In studying the alternatives for meeting this objective, it was realized that a commercial GIS software package was required. ArcView GIS by Environmental Systems Research Institute (ESRI) was selected as a base onto which the enhancements to GISHYDRO would be built. The program is considered an industry standard for display and analysis of geographic data. Federal agencies including the USGS and Environmental Protection Agency have begun to distribute GIS data in the ArcView file format. An additional extension to the program, ArcView Spatial Analyst was also selected to provide raster-processing capability.
ArcView is a fully compliant, 32-bit Windows application. It is for use with Windows 95/98/NT and the expected Windows 2000. It can be used in a network environment using shared file servers. This compatibility is required for installation of the software at MDSHA. At the time this report was written, ESRI reported that ArcView is a year-2000-compliant application.
ArcView includes an application programming language called Avenue for advanced customization and application development. This language was used to program GISHydro2000 functionality. Necessary geographic data and customizations are embedded into a project file which ArcView loads and interprets. GISHydro2000s functions are stored as scripts within the project file.
The original GISHYDRO program lacked the capability to produce high quality maps and report output. Also, because it was developed as a stand-alone application, it lacked many GIS related functions such as spatial overlay and query. By using a commercial package such as ArcView, access to these functions are directly available. Figure 2-1 shows the opening screen to GISHydro2000. Access to its various functions is provided with standard Windows controls such as menus, buttons, and tools.
Figure 2-1: Screen shot of GISHydro2000 Main Screen.
A description of the function and operation of GISHydro2000 is provided in the accompanying Users Manual. Use of the ArcView system will allow future enhancements to easily be added. Further, as new functions become available in ArcView itself, they will be applicable to the Maryland Hydrologic database.
The original GISHYDRO required that hydrologic features be manually digitized using a digitizing tablet. Difficulty in configuring the digitizer and the tediousness of tracing complex watershed features led to the adoption of new techniques.
Because the new GISHYDRO system, GISHydro2000, employs an internal database of terrain for the State of Maryland, it is possible to delineate hydrologic features automatically using terrain-processing algorithms. The current system operates by allowing the user to simply click the mouse at a watershed outlet or stream channel tip. The watershed boundary and channel flow path are automatically determined, respectively. Figure 2-2 shows an example of a watershed and channel network delineated with GISHydro2000.
Figure 2-2: Example Watershed and Stream Network Delineated with GISHydro2000.
This method is considered superior to the older technique of manually digitizing individual watershed elements. Although not included in this version of the software, the capability exists to interface ArcView with a digitizing tablet for manual entry.
In developing input for hydrologic models such as TR-20, the original GISHYDRO required that hydrologic elements and their parameters be defined explicitly and in the proper sequence. GISHydro2000 provides the capability to translate hydrologic elements such as streams and watersheds directly into model input. Figure 2-3 shows an example of a simple watershed translated into hydrologic elements. This schematic model is subsequently processed to create input for TR-20.
Figure 2-3: Example of Watershed Schematic Generated by GISHydro2000.
Automatic model development is seen as an important capability of the software. It is now possible for engineers to commit resources previously used to developing model input to studying analysis alternatives.
The updating and expansion of the hydrologic database for GISHYDRO was a substantial task for this project. The database was expanded well beyond the original scope, which included only the addition of 1994 MOP land use data. Data from the U.S. Geological Survey (USGS), the Environmental Protection Agency (EPA), and the Natural Resources Conservation Service (NRCS) has been included. Besides land use, the hydrologic database includes digital elevation models (DEMs) and hydrologic soils data. The sections that follow describe the nature of each of these layers.
The hydrologic database is based on the array of USGS 7.5-minute quadrangles, which cover land draining into the State of Maryland (except Susquehanna River drainage). Spanning this area are 340 quad sheets amounting to 19,682 mi2. Data contained in the database is referenced to these quad sheets. A quad is therefore the unit amount of geographic data that is stored for the DEM, land use, and soils layers.
To facilitate correct overlay of geographic data layers, a standard geographic projection was adopted for the database. All existing data was converted to this standard. Future data can also be converted. ArcInfo software was used to assemble and process geographic data from many sources. A description of the procedures used is beyond the scope of this report, however, reference should be made to the final report for a separate project submitted to MDSHA, Development of Data Handling Tools for Hydrologic Analysis of Small Watersheds (SP 707A4K, January 1998).
The specific coordinate system used includes the following parameters:
The database was stored to this specification to match existing digital data such as the MDSHA Grid Maps. Raster data such as topography, land use, and soils have been stored at a spatial resolution of 100ft pixels. Data assembled from a more coarse scale has been over-sampled to match this resolution. Procedures have been developed to translate the database to SI units (e.g. meters) if necessary.
A complete database of Digital Elevation Models (DEMs) has been assembled for Maryland and surrounding drainage areas in Delaware and Pennsylvania. This database is based on USGS DEMs and is based on native horizontal resolutions of 30 and 90-meters. The 30 meter DEMs represent the best resolution available at large (state) scale. Complete coverage for Maryland was not available at this resolution until October 1999. Before this time, the 90 meter DEMs were used where necessary for terrain analysis and watershed delineation.
In testing GISHydro2000 and its database, errors have been realized in delineating hydrologic features on Marylands Eastern Shore using the 30 meter data. There errors are caused by the relatively low relief present there. Several Maryland counties have begun releasing high-resolution topography in digital format. It is expected that this new data will be used in the future to alleviate these problems once better data becomes available for the Eastern Shore.
The land use database for GISHydro2000 has been expanded to include land use data from Maryland Office of Planning (MOP), U.S. Geological Survey (USGS) and U.S. Environmental Protection Agency (EPA). The MOP database now includes complete State of Maryland coverage for 1990, 1994, and 1997. USGS land use/land cover data (LULC) has been assembled representing 1970s land use conditions. The prior database developed by Dr. Robert Ragan for the previous GISHYDRO is based on MOP land use maps and includes 1985 and 1990 data. EPA land use data (MRLC) derived from Landsat satellite imagery is available for all of EPA Region 3 including MD, PA, and DE. This data represents a mosaic of land cover conditions during the early 1990s.
The database of land use has therefore been expanded well beyond the scope of the original project which required only incorporation of the 1994 MOP land use (completed in Phase I).
The soils database for GISHydro2000 has been expanded to include data from the Natural Resources Conservation Service (NRCS). Two formats of data have been included. The first, called STATSGO, is a low resolution layer showing broad soil associations. The second, called SSURGO is available for selected counties at high resolution. These counties include Carroll, Dorchester, Montgomery, Queen Annes, Washington, Worcester, Baltimore City, and Adams (PA), Somerville (PA) and York (PA). Where available, these data types have been added to GISHydro2000. The original database of soils from GISHYDRO has been preserved and is called simply Ragan Soils. As new counties become available in the SSURGO format, they will be incorporated into the program.
In addition to the primary layers of hydrologic data, additional layers are provided within GISHydro2000 to support watershed analyses. Digital Line Graphs (DLGs) are vector line files that represent geographic themes such as transportation or hydrography. A layer of major roads within the State of Maryland is provided to aid in location of specific transportation design project. The data is attributed with federal, state, and county-level road designations. A layer of hydrographic features including streams, rivers, and lakes is provided. This layer is used to force drainage to known flowpath locations during DEM processing. Digital Raster Graphics (DRGs) have been assembled at a coarse scale for the state. DRGs are scanned versions of USGS topographic maps at, in this case, 1:250,0000 map scale. These are used as a backdrop for locating geographic features and for creating report quality maps.
The development of GISHydro2000 was centered on understanding the process used by MDSHA in conducting watershed analyses. The State Hydrology Panel is currently in the process of codifying this process for MDSHA projects. The following is an excerpt taken from the executive summary of a Watershed Analysis Report conducted for the Little Gunpowder watershed in September 1999. It captures the objectives and purpose of a typical analysis and outlines the overall analysis procedure.
Several hydrologic methods were used to estimate the 2-, 10-, 25-, 50-, 100-, and 500-year peak discharges at the subject crossing. Among them were the USGS Regression Equations, the Tasker program, and the TR-20 model.
Studies have shown that the TR-20 model often predicts peak discharges that are not consistent with peaks that have been measured by Maryland stream gages. A major contributor this problem is that it is very difficult to select the Manning roughness coefficient and the typical cross section that best represents the watershed conditions. Errors in these selections lead to incorrect estimates of the time of concentration and storage conditions and, therefore lead to peak flow predictions that are too high or too low. The ideal scenario is to calibrate the TR-20 model using long term rainfall and runoff records collected by gages located in the watershed. In most instances, there are no gages in the watershed being modeled. In order to achieve a level of calibration that will ensure that TR-20 peak discharges are representative of Maryland conditions, the Hydrology Panel has recommended that the TR-20 peak discharges be compared with those predicted by the USGS regression equations. The USGS regression equations use a statistical analysis of 219 stream gages located in Maryland and adjacent states to provide a synthetic flood frequency series for the 2, 10, 25, 50, and 100-year pear discharge as a function of the watershed area. The approach recommended by the Hydrology Panel is for the engineer to use maps, field investigations, and all available information to define the input parameters required by the TR-20 model. The TR-20 is then implemented using these parameters. The target is to have the TR-20 fall within one standard error (the 67% prediction interval) of the peak estimated by the USGS equations. The Tasker program determines the intervals. If the TR-20 peaks are significantly outside of the 67% interval, the Hydrology Panel has recommended procedures to adjust the TR-20 parameters such that the existing TR-20 peaks move within, or at least close to, the 67% interval. Any parameter adjustment must be consistent with sound engineering practice. The object is not to force every TR-20 discharge within the 67% window. The object is to use the window as a verification that the input parameters are appropriate and, therefore, the TR-20 peaks are consistent with Maryland conditions.
As in the original version of GISHYDRO, the database is organized into files based on 7.5-minute quadrangle sheet. The databases are stored in zip format using standard compression software. For a given analysis, data corresponding to user-specified quad sheets is extracted from the database and assembled for each hydrologic layer (DEM, land use, soils). A customized dialog box is used to select quads of data and appropriate data formats. Figure 4-1 shows this dialog.
Figure 4-1: GISHydro2000 SelectQuads Dialog Box.
The dialog contains options for DEM and curve number processing as well as a warning system for indicating where data is lacking for a particular format. The GISHydro2000 Users Manual provides a detailed description of how to select quadrangles and choose analysis options.
Once data corresponding to the desired Area of Interest is assembled, a watershed is delineated by clicking in the GISHydro2000 display window on a desired outlet point. The watershed and streams draining to that point are automatically delineated. Tools for estimating watershed parameters are organized on a Hydro menu. After the watershed is delineated, hydrologic quantities such as drainage area, land use composition, impervious area, time of concentration, basin relief, and basin-averaged curve number are determined.
The 1996 version of the USGS regression equations have been programmed into GISHydro2000. (refer to Phase I Final Report) The equations provide estimates of the 2- through 500-year peak discharge for streams in Maryland based on selected watershed parameters. These parameters include drainage area, curve number, basin composition, and basin relief, a difficult quantity to estimate manually. For a given watershed within the state, estimates of these discharges can be calculated based on its location relative to the six physiographic provinces shown in Figure 4-2. GISHydro2000 can also evaluate complex cases in which a watershed may span one of these provinces.
Figure 4-2: Physiographic Provinces for Maryland.
The Tasker program is a separate executable program written by Gary Tasker of the USGS. It provides statistical confidence intervals on estimates of peak discharge calculated using the USGS regression equations. GISHydro2000 automatically interfaces with the program and reports these intervals when calculating the USGS predicted peak discharges.
The SCS Lag Formula is an empirical equation used to estimate time of concentration. The equation has been programmed into TR-20 as an option for calculating tc. Future versions of the program will incorporate TR-55 methods for estimating time of concentration.
Mr. Wilbert Thomas has formulated a set of regression equations for estimating time of concentration (tc) within the State of Maryland. Similar to the USGS regression equations, they use selected watershed parameters to predict values of tc. GISHydro2000 uses these equations to provide an estimate of tc when calculating basin statistics.
The USGS Dimensionless (or Unit) Hydrograph is a model developed to complement the 1996 USGS regression equations. It provides a simulated unit hydrograph based on selected watershed parameters. An option is included in GISHydro2000 to calculate the dimensionless hydrograph based on selected watershed parameters and a user specified return period/peak discharge.
For complex watershed analyses involving multiple subwatersheds, routing, or storage, a rainfall-runoff simulation model such as TR-20 is required. Development of model input parameters for TR-20 requires subdivision of the watershed to reflect varying runoff and storage conditions. The subdivided watershed is typically translated into a schematic diagram for determination of the correct sequence of hydrologic elements (see Figure 2-2). In a GIS system, this translation is accomplished by processing the topological relationships between watershed elements.
GISHydro2000 uses a hydrologic pre-processor called CRWR-PrePro to determine watershed topology and generate watershed model schematics. The pre-processor was developed by the Center for Research in Water resources at the University of Texas at Austin. It has been adapted to function within GISHydro2000 by permission.
The TR-20 Interface within GISHydro2000 is designed to translate the attributed watershed model schematic developed by CRWR-PrePro into a properly formatted TR-20 input file. The Interface is also designed to execute the model and display the resulting output file.
A TR-20 Control Panel was developed to set user input/output options. Figure 4-3 shows this dialog box:
Figure 4-3: TR-20 Control Panel Dialog Box.
The TR-20 Interface allows for different analysis alternatives to be evaluated quickly, freeing the engineer from much of the tedium of model development and allowing more time for critical thinking and sensitivity analysis.
This purpose of this project was to update and enhance the GISHYDRO system. Pursuant to this purpose were the following objectives; update land use database to include 1994 conditions, allow operation in both English and metric units, program the 1996 USGS regression equations, integrate digital terrain and line data, migrate the software to the Windows platform, and program the calculation of the USGS Dimensionless Hydrograph model.
The project was divided into two phases. Phase I included incorporation of the 1994 MOP land use data, providing for compatibility with other GIS data formats, and English-to-metric conversion capabilities. Phase II included the remaining objectives. To meet the stated objectives, the GISHYDRO system was migrated into an ArcView GIS-based application. Functionality existing in the original program was recreated in the new version. In making this migration, some fundamental changes in the analysis procedure changed. Hydrologic features such as streams and watersheds are no longer digitalized manually but are instead extracted automatically from topography. Improved graphics and output capabilities have also resulted from this migration.
GISHydro2000 provides data and powerful tools for performing hydrologic analyses. Significant resources had previously been committed to developing watershed models for estimating peak discharge and hydrographs. GISHydro2000 greatly reduces the time effort necessary to develop these models. It does not however replace or reduce the role of engineering judgment in performing these analyses. It is expected that the savings in resources realized from using GISHydro2000 will be applied to studying various alternatives and to performing sensitivity analyses on model parameters.
The GISHYDRO system has been recognized by Federal and State agencies as a valuable tool for conducting watershed analyses. This project has made significant enhancements to the software and has helped to insure its continued success.