ArcGIS to HSPF:
A Practical Application, TMDLs of Texas Waterbodies
Jessica L. Watts, CRWR
Table of Contents
The Sandies and Elm Watershed is located in South Central Texas approximately 47 miles southeast of San Antonio. It is currently the subject of a Texas Commission for Environmental Quality (TCEQ) TMDL Study due to its high amounts of bacteria and low dissolved oxygen content. The objective of my project will be to create an accurate simulation of the Sandies and Elm watershed as it pertains to hydrology, hydraulics, pollutant loading, and possible pollutant reduction practices for a Total Mass Daily Load (TMDL) evaluation.
CRWR has been authorized to create an HSPF model to simulate the activities in the Sandies and Elm watershed and their inputs on the rivers therein. The difficulties in modeling this watershed lie in the agricultural aspect of the basin. The nature of the region creates a number of possible non-point source pollution areas and types. Non-point source pollution tends to be applied to the stream during rainstorm events. So, as a first step in simulating the watershed accurately a hydrologic model needs to be created and calibrated which accurately simulates the watershed response to precipitation. This model will be calibrated to the USGS gauging station located southeast of the town of Westhoff.
TMDLs are required for some waterbodies under the 1972
Federal Clean Water Act. A TMDL is a tool for implementing state water quality
standards. It is based on the relationship between sources of pollutants and
in-stream water quality conditions. The TMDL establishes the allowable loadings
for specific pollutants that a waterbody can receive without exceeding water
quality standards, thereby providing the basis for states to establish water
quality-based pollution controls.
A TMDL can be generically described by the following equation:
TMDL = LC = WLA + LA + MOS
where:
LC = loading capacity,
WLA = wasteload allocation,
LA = load allocation, and
MOS = margin of safety
Figure 1: TMDL Breakdown
The loading capacity is the largest pollutant loading a waterbody can receive without exceeding water quality standards for the designated usage. The wasteload allocation is the portion of the TMDL allocated for existing and future point sources. The load allocation is the portion of the TMDL allocated to existing and future non-point sources and natural background. The margin of safety is an accounting of uncertainty about the relationship between pollutant loads and receiving water quality. The margin of safety can be provided implicitly through analytical assumptions or explicitly by reserving a portion of loading capacity.
From EPA’s Protocol for Developing Pathogen TMDLs
The EPA describes Hydrological Simulation Program - FORTRAN (HSPF) as “a comprehensive package for the simulation of watershed hydrology and water quality for both conventional and toxic organic pollutants. HSPF incorporates the watershed-scale ARM and NPS models into a basin-scale analysis framework that includes fate and transport in one-dimensional stream channels. It is the only comprehensive model of watershed hydrology and water quality that allows the integrated simulation of land and soil contaminant runoff processes with in-stream hydraulic and sediment-chemical interactions.”
(Ward, 1999) reports in his paper, which examined forty-six (46) different water quality models and assessed their applicability for use in development of Total Mass Daily Loads (TMDLs) in Texas, that HSPF includes the most capabilities for addressing a range of problems in surface water resource management.
The array of options that is available for depicting various agricultural land treatments and the specialized transport routines for agricultural chemicals within HSPF, make it a superior choice for the Texas watersheds which will be addressed, which are largely agricultural in character.
WinHSPF is a program developed for the EPA by AQUA TERRA Consultants in Decatur, GA and distributed with the EPA's BASINS software. WinHSPF provides a windows-based interface to the extensive input data for HSPF.
A system of preprocessing data for HSPF was present in the BASINS interface which operated within the ArcView 3.x software which worked closely with WinHSPF to develop and post-process HSPF water quality models. Much of the geospatial preprocessing functionality of BASINS is dependent on the ArcView 3.x environment and consequently not available with the most recent ESRI ArcGIS software.
The ESRI ArcGIS software (Arc8.x and Arc9.x) have a file format built around a relational database, a geodatabase. The input file for HSPF is a large text file with very specific formatting requirements making it cumbersome to manipulate and very easy to make mistakes. The timeseries used in HSPF are most often stored in a Watershed Data Management (.wdm) file. The .wdm file is in a binary format, and difficult to work with without extensive knowledge of its structure. However, much of the work to read from and write to .uci and .wdm files has already been done with the Visual Basic code behind WinHSPF and other BASINS software (GenScn and WDMUtil).
Nate Johnson of the Center for Research in Water Resources has created the tools for the transfer of data from an ArcGIS geodatabase to the .uci and .wdm files for use in hydraulic modeling of a watershed. He utilized the public domain Visual Basic Code which supports WinHSPF and other code distributed with BASINS (GenScn and WDMUtil) to develop custom interfaces for interacting with .uci and .wdm files using ArcGIS applications.
Arc Hydro has two input requirements: a digital elevation model and streamlines. These two inputs were downloaded from Seamless and USGS – National Hydrograpy Dataset respectively.
The NHD High Resolution Flowlines that had a name associated with them (named streams) were burned onto the DEM and Arc Hydro was used to create the flow direction raster. WRAP Hydro GIS Tools was used to create a catchment for each of the named streams. There are only 54 named streams in this watershed, but with this method 107 catchments are created, including breaks at the required monitoring points.
The number of catchments, or subbasins, is limited by HSPF by the number of distinct land use types. Because of the limits of the HSPF code, the sum of all the catchments multiplied by the pervious and impervious types can not be greater than 500. I chose to utilize six pervious and one impervious land use types.
Y + (1 impervious x Y) + (6 pervious x Y) < 500
Where Y = the number of subbasins
Which is why I was therefore limited to less than a total of 62 subbasins, which created a difficulty that I solved by creating an Upper and Lower Sandies HSPF model. The Upper Sandies section consists of the 62 catchments above the confluence with Elm as well as those that drain into Elm Creek itself. The Lower Sandies section consists of the 45 catchments below the confluence.
After creating the subbasins the next step was to get the GIS information into HSPF. To do this I had the use of some wonderful tools and model builder, created by Nate Johnson, a graduate student at the University of Texas.
These tools convert information from GIS into an HSPF UCI file. UCI stands for User’s Control Input. It sets up internal information instructing the system regarding the sequence of operations to be performed. It stores the initial conditions and the parameters for each operation in the appropriate file and creates an instruction file which will ensure that time series are correctly passed between operations, where necessary.
- HSPF User’s Manual Version 12
The crwrHSPFTools create the files needed to create the UCI file in WinHSPF. The ArcHydro2HSPF model builder models fill in the information needed for these files. This is, of course, a more complex process than is shown here, as it is Mr. Johnson's master's thesis research. For more information on this topic see the tutorial ArcGIS to HSPF Tutorial.
The forcing data required for a simple hydrologic model in HSPF are precipitation and evaporation. HSPF allows unique timeseries for the required forcing data to act over different areas. This property of HSPF was utilized to great advantage. NEXRAD data gives a very good spatial representation of precipitation which can not be achieved with point source data. South Central Texas is especially susceptible to localized storm systems which can be completely missed by precipitation gaging stations as seen in the figure below.
The above image represents the precipitation over the Sandies and Elm basin for one 24 hour period in August 2001. The white dots are the locations of the NCDC gaging stations in the area. As is noticable in the above image, the most intense area of precipitation is completely missed by the area rain gages. Therefore, for my project NEXRAD data was used to create the precipitation forcing data timeseries. The NEXRAD data was input over each watershed and an hourly timeseries was created. These timeseries were then transformed into a WDM file and the MetSegs were assigned in the HSPF UCI file. This process is explained further in the NEXRAD TimeSeries into HSPF tutorial.
After the ArcGIS to HSPF process was run as explained in the ArcGIS to HSPF tutorial, an HSPF file can be opened in WinHSPF.
Figure 6: WinHSPF view of Sandies & Elm model
The next step will be to apply different modeling parameters so that the USGS Gage at Westhoff, Texas is being simulated on a reasonable timescale.
There is still a lot more to do with this modeling project, but the process of transforming the Arc Hydro and GIS data into HSPF has been greatly simplified with the use of the tools created by Nate Johnson of CRWR. These tools take advantage of the Arc Hydro TimeSeries in creating the needed forcing data and also utilize the georeferencing attribute of ArcGIS to help the modeler in simulating the watershed environment.
The next step in this project will be to calibrate the HSPF parameters so that the stream gage flow is simulated.
U.S. Environmental Protection Agency. 2001. Protocol for Developing Pathogen TMDLs. EPA 841-R-00-002. Office of Water (4503F), United States Environmental Protection Agency, Washington DC. 132 pp.
Bicknell, B.R. et. al. 2001. HSPF Version 12 User’s Manual. AquaTerra Consultants, Mountain View, California. 873 pp.
Copy the zip files that can be downloaded below into your C:\ drive, Unzip, and Run Install.bat
This page was created by:
Jessica L. Watts, P.E.
Center for Research in Water Resources
The University of Texas at Austin
jessica.watts@mail.utexas.edu
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 2005 Center for Research in Water Resources.