WATERSHED MODELING EXTENSIONS OF THE WATERSHED CHARACTERIZATION SYSTEM

The Watershed Characterization System (WCS), which is available for the eight states in Environmental Protection Agency (EPA) Region 4 — Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee — was originally developed to facilitate the physical characterization of watersheds, evaluate water quality conditions, and assess potential sources of impairment using spatial databases and geographic information system (GIS) technology. This was achieved by providing users the capacity to automatically summarize information in tabular and map format in a Microsoft Word document. WCS has now evolved beyond its original objective as a repository of watershed data and a characterization tool. Several modeling extensions have been developed to support advanced TMDL development, including a sediment budget model, mercury loading model, Stormwater Management Model (SWMM) GIS interface and input preprocessor, and Nonpoint Source Model (NPSM) GIS interface.The sediment budget model is based on a spatially distributed estimation of erosion (using the Universal Soil Loss Equation, USLE), and sediment load (using any of four alternative sediment delivery equations). It can be used to (1) estimate the extent and distribution of potential soil erosion in the watershed; (2) estimate potential sediment delivery to receiving waterbodies; and (3) evaluate the effects of land use, best management practices (BMPs) and road networks on erosion and sediment delivery. The mercury loading model uses the grid-based GIS modeling technology to calculate total soil mercury concentration and mercury loadings from direct atmospheric deposition, surface runoff, soil erosion, and point sources. The SWMM GIS interface uses existing GIS data such as the National Hydrography Dataset (NHD), Multi-Resolution Landuse Characteristics (MRLC), and digital elevation model (DEM) to automatically prepare the input for SWMM modeling. Similarly, the NPSM GIS interface uses commonly available GIS data sets to perform HSPF modeling.