Watershed Characterization System (WCS) Modeling Extensions The Watershed Characterization System (WCS) is an Ar- cview-GIS based system that was originally developed to facilitate the physical characterization of watersheds, evalu- ate water quality conditions, and assess potential sources of impairment. This was achieved by providing users the capa- bility to automatically summarize information in tabular and map formats in a Microsoft Word document. WCS now also provides a platform for specialized modeling extensions sup- porting TMDL development. These extensions provide new watershed loading models such as the Sediment Budget Model, and the Mercury Loading Model, and the development of GIS-based interfaces to prepare data for existing models such as Stormwater Management Model (SWMM) and Loading Simulation Program in C+ + (LSPC, a TMDL-focused version of HSPF). WCS and its extensions are currently available for EPA Region 4. Sediment Budget Model The Sediment Budget Model is a WCS extension for estimat- ing soil erosion and sediment loads from watersheds. EPA Region 4 developed the model in cooperation with the U.S. Department of Agriculture's (USDA) Natural Resources Con- servation Service (NRCS) and Forest Service. The model uses the Universal Soil Loss Equation (USLE) to estimate the total potential erosion and its spatial distribution in the watershed. The fraction of the potential erosion that reaches the stream (e.g., sediment delivered) is estimated by using one of the four sediment delivery equations. These equations express sediment delivery as a function of the distance of the source grid cell from the stream, and/or difference in elevation. The distance is computed by tracing the most likely path of the sediment from the source grid cell to a stream grid cell (e.g., grid cell to grid cell routing) based on the steepest descent method. The model can be used to evaluate the magnitude, extent and spatial distribution of erosion and sediment load in the watershed under historical, existing, and managed conditions. An initial set of data that may be used to represent existing conditions is distributed with the model. By using Arc View's standard and WCS customized tools, users can easily supple- ment the initial data set with their own data sets that may be more recent and of higher resolution. In addition, users are provided with customized utilities allowing them to quickly formulate and evaluate alternative management scenarios con- sidering such things as land use changes; road and silvicultural practices; riparian zone characteristics; human disturbances (e.g., construction areas); on-site BMPs such as cropping practices; and off-site BMPs such as ponds and filter strips. The model has been successfully used for developing sediment TMDLs in the states of Georgia, Alabama and Tennessee. Mercury Loading Model The WCS Mercury Loading Model is based on a soil-mercury mass balance model, IEM v2.05, developed by EPA's Office of Health and Environmental Assessment and Environmental Research Lab, Athens. The soil-mercury mass balance model calculates surface soil concentrations in dissolved, sorbed, and gas phases. The model accounts for three routes of contaminant entry into the soil: • Deposition of particle-bound contaminant through dry fall • Deposition through wet fall • Diffusion of gas phase contaminant into the soil sur- face. DATA watershed boundary, land uses, streams, elevation, soils and roads Erosion calculation using USLE Distances, slopes, streams Roads, BMPs, management structures Sediment transport and delivery OUTPUTS Erosion map Sediment delivery map Sediment delivery to assessment points Automated reports (maps and tables) United States Environmental Protection Agency ------- The model also accounts for four dissipation pro- cesses that remove mercury from the surface soils: • Volatilization (movement of gas phase out of the soil surface) • Runoff of dissolved phase from the soil surface • Leaching of dissolved phase through the soil horizon • Erosion of particulate phase from the soil sur- face. The watershed mercury load is calculated as the total of mercury contributed by sediment, runoff from both pervious and impervious surfaces, direct deposition on water surfaces, and point sources. The algorithm for estimating erosion and sediment transport is based on the WCS Sediment Budget Model. Runoff is calculated for each land use grid using the USDA-NRCS curve number method that requires rainfall as an input and calculates other hydrologic components such as evapotranspiration and infiltration. The WCS Mercury Loading Model was used to develop mer- cury TMDLs for the Middle and Lower Savannah River. WCS SWMM Extension EPA's Storm Water Management Model (SWMM) is a com- prehensive computer model for the analysis of water quantity and quality problems associated with urban runoff. SWMM can simulate all aspects of the urban hydrologic and water qual- ity cycles, including rainfall, snowmelt, surface and subsurface runoff, flow routing through drainage networks, storage, and treatment. The WCS SWMM extension is designed to simplify and facilitate the creation of SWMM input files, especially for PC SWMM (Computational Hydraulics Int.). It consists of two coupled parts: a GIS interface and a SWMM input processor. The GIS interface processes spatial databases and prepares two important data sets, land use distribution and stream network connectivity, for the SWMM input processor. The SWMM input processor has a Visual Basic interface that reads GIS output and user-specified simulation information to generate the required input files for the SWMM model. WCS LSPC Extension The WCS LSPC extension preprocesses GIS, spatial data, and time series data for input to the Loading Simulation Program in C+ + (LSPC). The LSPC system is a stand-alone watershed modeling system that includes algorithms from the Hydrologic Simulation Program - FORTRAN (HSPF) plus other key features to support large-scale watershed modeling and TMDL calculation. LSPC is used for simulating watershed pollutant loadings as well as flow and water quality routing through stream reaches. Hg -i ) Diffusion Volatilization Dry Fall Wet Fall Leaching The WCS LSPC extension provides many key data processing benefits. After delineating the watershed, the LSPC extension (1) packages GIS layers for the LSPC GIS interface, (2) com- piles landuse and reach geometry/network information by de- lineated subwatershed, (3) locates and compiles USGS gage data and STORET water quality data for model calibration, (4) locates weather stations, (5) locates and compiles point source information from the Permit Compliance System (PCS), (6) queries the STATSGO database to identify the predominant hydrologic soil type in each delineated subwa- tershed, and (7) assigns initial hydrologic model parameters based on the STATSGO query results. WCS LSPC extension has been used to develop TMDLs in Alabama, Tennessee, and South Carolina. Visit the Watershed & Water Quality Modeling Technical Support Center Website http://www.epa.gov/athens/wwqtsc/index.html United States Environmental Protection Agency ------- |