Application of SWAT Hydrologic model for TMDL development on Chapel Branch Creek watershed, SC.
Watershed-scale hydrologic/water quality models are frequently used to characterize flow dynamics, pathways, and pollutant loading rates as a function of land use, soils, topography, management practices, and their interactions with variability in climate. SWAT (Soil and Water Assessment Tool) is a widely used GIS-based basin scale model to quantify the impact of land management practices on hydrology and water quality in large, complex watersheds including analysis for development of Total Maximum Daily Loads (TMDLs). In this project supported by SC DHEC’s 319 Grant Program, SWAT has been used to evaluate nutrient loading and allocations based on source area identification and proposed best management practices (BMPs) for developing TMDLs for a 1,555 ha mixed land use watershed draining Chapel Branch Creek (CBC) to Lake Marion near Santee, SC. Lake Marion is an important recreational area for South Carolina, and the town of Santee, SC derives most of its economic activity from tourism. The creek is in SC DHEC’s 2004 (303d) list of impaired water bodies for excessive N, P, chlorophyll-a, and pH. In order to apply the SWAT model for scientifically valid determination of the nutrient loadings, it is first being calibrated for the hydrology component driving these loadings. Two years (2006- 08) of rainfall data from three on-site gauges and weather data from a nearby station were used to simulate the stream flow rates across the watershed. The CBC watershed was delineated into smaller subwatersheds using the Arc-GIS SWAT2005 model with DEMs for topography, SSURGO data base for soils, NAIP2005 imagery for land use, and field observed drainage network. The model is being calibrated using flow rates measured at two stream locations draining major categories of land use. Once the model is calibrated for flow it will further be calibrated with the nutrients measured using auto-samplers at the same two locations. Event-based nutrient data collected at six other locations within the watershed will also be used for source area identification and validating the SWAT model that will be applied for load allocation analysis and TMDL development within the CBC stream. As the TMDL is developed, stakeholders will be included in order to earn their “buy-in” for the proposed BMPs for necessary load reductions. This project not only includes the TMDL development, but also the implementation of BMPs plus educational strategies for nonpoint pollution reduction.