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© 2001, Dodson & Associates, Inc. - Houston, Texas, USA - www.dodson-hydro.com - All rights reserved.

The Use of HEC-GeoHMS and HEC-HMS To Perform Grid-based Hydrologic Analysis of a Watershed

Christopher A. Johnson and Andrew C. Yung

Dodson & Associates, Inc.

Kenneth R. Nixon and David R. Legates Computational Geosciences, Inc.

Introduction There have been numerous advances in Geographic Information Systems (GIS) in recent years. These

advances include the ability to share spatial information from government agencies and others, and the

development of GIS and hydrologic software that enhance hydrologic modeling of watersheds. It is believed

that these advances will provide for a more efficient and a more accurate alternative to traditional methods

for studying watersheds. The U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC) has

recently made HEC-GeoHMS available to the public. This software is intended to serve as a geospatial

hydrology toolkit for engineers and hydrologists.

Dodson and Associates, Inc. (Houston, Texas) and Computational Geosciences, Inc. (Norman, Oklahoma)

collaborated on an effort to utilize weather radar data, GIS data, HEC-GeoHMS, and HEC-HMS to perform a

hydrologic case study watershed analysis. The purpose of the study was to determine the practical

applicability of this automated approach for widespread use.

Lumped Basin Models

The traditional (non-grid-based) hydrologic model is a lumped basin model. It assumes that each sub-basin

within the watershed can be adequately represented by a number of hydrologic parameters. In effect, these

parameters are a weighted average representation of the entire sub-basin. Any variation within a sub-basin is

ì lumped-intoî the sub-basin total and an average value are used in the analysis.

The key hydrologic ingredients for such an analysis include precipitation depth and temporal distribution,

various geometric parameters (such as length, slope, area, centroid location, etc.), and information used to

characterize abstractions (such as soil types and land use/land cover descriptions). If these items can be

defined for each sub-basin, a traditional lumped basin model can be developed.

To perform a hydrologic analysis, a model is prepared. Raw data is manually processed by a hydrologist to

produce the information needed in a format appropriate for a software application (such as HEC-1 or HEC-


HMS). The raw data typically includes topographic and zoning maps, aerial photographs, field surveys, soil

surveys, precipitation gage data, intensity-duration-frequency data, etc.

Distributed Basin Model

A distributed basin model (or grid-based model) approach allows for a hydrologic analysis of a watershed to

a grid-cell level of detail. If sub-basins exceed the size of the grid-cell, additional detail can be added to the

model with no change in the size of sub-basins. Currently, the most common grid-cell format is the HRAP

(Hydrologic Rainfall Analysis Project) grid. It was developed by the National Weather Service (NWS) to

facilitate atmospheric modeling. Though not uniform in size or configuration, it is approximately 4 km. X 4

km. Therefore, by using a distributed basin model, an analysis of a watershed with sub-areas larger than

about 16 sq. km. (5.8-6.5 sq. mi.) would be expected to have a greater level of precision.

The key hydrologic ingredients for an analysis using the distributed basin model would be same as for the

lumped basin model. If digital data for these ingredients is available, then there is a potential for performing

an analysis using the distributed basin model approach. However, as the case study discussed below

indicates, there are certain constraints that limit the practical application of HEC-HMS and HEC-GeoHMS to

grid-based hydrologic analysis in the near term.

Similar to the traditional approach, a hydrologic model must be prepared. Raw data must be collected and

processed to develop appropriate model input to HEC-HMS. The difference is that the data must be a grid-

based digital file. There are currently readily available sources for gridded precipitation date (NWS

NEXRAD weather radar), topographic and land/use land cover data from the United States Geological

Survey (USGS), and soils data from the National Resource Conservation Service. Data processing can be

done semi-automatically without the HEC-GeoHMS extension by a skilled GIS specialist using GIS software

(such as ArcView) or automatically by someone with a basic working knowledge of ArcView using the

HEC-GeoHMS ArcView extension.

Case Study

A hydrologic model of the 325 square mile East Fork San Jacinto River watershed upstream of USGS stream

gage number 08070000 near Cleveland Texas was prepared. An observed hydrograph from the October 17-

19, 1998 event was used as a basis of comparison with computed model results. The purpose of the study

was to evaluate the current practical application of using the HEC-GeoHMS and HEC-HMS to perform grid-

based hydrologic modeling analysis. With recent technological advances, hydrologists have anticipated that

the modeling process can be improved by adding precision to the analysis, and simplifying the data


processing effort. The case study relied heavily on readily available digital GIS data, minimized the use of

other resources, and did not include field surveys, hydraulic modeling, or a site visit by the authors.

Distributed or Lumped Basin Model?

Seeking the benefits of greater precision, it was decided to attempt the preparation of a distributed basin

model. HEC-GeoHMS documentation includes a very important conditional statement concerning the ability

to create grid-based basin models.

ì If the hydrologic model employs the distributive techniques for hydrograph transformation,

i.e. ModClark, and grid-based precipitation, then a grid-cell parameter file and distributed

basin model at the grid-cell level can be generated.î (Doan, 2000)

Unfortunately, HEC-GeoHMS and HEC-HMS do not currently process the grid-based precipitation data so

that HEC-HMS can use it. It is not practical for most hydrologists to process the grid-based precipitation

independently. The HEC-HMS program requires gridded precipitation data in a HEC-DSS (the HECís Data

Storage System) file format. The current version (July 2000) of the HEC-GeoHMS Userís Manual does not

include documentation of the DSS file format and neither HEC-HMS nor HEC-GeoHMS converts the data

into a DSS file format. Therefore, grid-based precipitation cannot readily be used to meet the conditions of

the Userís Manual quote above and a distributed basin model cannot be readily created. HEC representatives

have indicated that a utility package to convert NEXRAD weather radar data to DSS file format is

forthcoming, but have not provided a schedule for its release. Since preparation of a distributed basin model

was not practical, the case study proceeded using a lumped basin model approach.

Source Data and Analytical Methods

Data sources for the case study consisted of a 30-meter DEM from the USGS, land use/land cover data

produced by the USGS using 30-meter Landsat thematic mapper ô data, and STATSGO soils data from the

Natural Resource Conservation Service. NEXRAD weather radar data was taken from the Houston-

Galveston radar located in League City, Texas. Computational Geosciences, Inc calibrated it to all NWS and

Harris County Office of Emergency Management recording gages within the radar coverage umbrella. After

calibration, significant differences were observed between the non-recording gages and the radar data.

Subsequently, non-recording precipitation gages were used to adjust total rainfall depths within each sub-

basin.

Hydrologic analysis methods included Clarkís unit hydrograph (Montgomery County method) parameters,

and Muskingum routing methodology. ArcView and the HEC-GeoHMS extension were used to process

digital data sources within the GIS environment and determine most of the basin and river characteristics

needed (length, slope, area, etc.) to estimate unit hydrograph and stream routing parameters.


Modeling Results

Modeling results indicated a reasonably close reproduction of observed peak discharge (within 4%) and total

runoff volume (within 7%). However, there were notable differences in the shape of the two hydrographs. It

appeared that improved stream routing based on hydraulic modeling, more knowledge of the site, and field

survey data of the channel would have helped produce a closer match with the observed hydrograph.

Conclusions

Several practical lessons related to the application of HEC-GeoHMS and HEC-HMS to grid-based

hydrologic modeling were learned as a result of performing this case study, including:

1. HEC-GeoHMS is not as complete a product as it may first appear.

2. HEC-GeoHMS has some practical application for most hydrologists, although limited.

3. Digital data is not a substitute for site reconnaissance.

4. If possible, non-recording gages should also be considered when defining precipitation using NEXRAD

data.

5. The HEC-GeoHMS extension is currently practical for processing digital data to define watershed and

sub-basin boundaries, river lengths, longest flow paths, slopes, centroid locations and lengths to

centroids, and the HEC-HMS basin schematic and map file.

6. Other digital data can potentially (at the discretion of the modeler) be used along with ArcView to

develop sub-basin average information (precipitation, soils, land use, etc.) for a lumped basin model.

7. It is not currently practical for most hydrologists to use HEC-GeoHMS and HEC-HMS to prepare a

distributed (grid-based) hydrologic model. However, upon the HECís release of additional

documentation and the anticipated file conversion utilities, the full capabilities of these applications will

be realized.

Reference

Doan, James H. 2000. Geospatial Hydrologic Modeling Extension, HEC-GeoHMS Userís Manual: US

Army Corps of Engineers, Hydrologic Engineering Center.

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