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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 (200608) 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. INTRODUCTION Chapel Branch Creek is a small tributary of the Santee River in central South Carolina. It has been listed by the South Carolina Department of Health and Environmental Control (SCDHEC) under EPA approved SC 2004 303(d) list of water bodies for impairment of aquatic life (AL) due to elevated chlorophyll-a, TN, TP, and pH. This conclusion was based on water quality data sampled by Santee-Cooper for the SC Department of Health and Environmental Control (DHEC) at Station (SC-014) about 1.5 km upstream from Lake Marion from 1996-2000. The watershed is small (1555 ha) by comparison to others but it has wide variety of potential nutrient sources. The US Forest Service in conjunction with Clemson University and the College of Charleston are developing a Total Maximum Daily Load (TMDL) assessment and implementation project on Chapel Branch Creek using both a monitoring and modeling approach (Williams et al., 2007). Monitoring involves measuring stream flow in strategic locations on the watershed and water quality from potential source areas. The subsequent modeling approach includes calibration and validation procedures with site specific topographic, soil, land use, and vegetation spatial data along with measured climatic and stream flow data. Model outputs will be used to better understand the watershed hydrologic processes and thus to make decisions on alternative best management scenarios designed to reduce the negative impact of anthropogenic activities on water quantity and quality (Parajuli et al., 2008; Santhi et al., 2008; Moriasi et al, 2007). We have chosen SWAT as the primary hydrologic and water quality model for development of the TMDL. SWAT will be used to estimate flow and dissolved nitrogen and phosphorus loads on this diverse watershed. SWAT was chosen since much of the watershed is still agricultural. SWAT allows segregation of land use into hydrologic management units and spatial distribution into subcatchments. SWAT has been integrated into the GIS program in ARC-GIS 9.1 with user interfaces to aide in watershed delineation and parameterization. In this paper we will discuss the procedures used with the Arc-SWAT model to develop sub-watersheds for its hydrologic calibration including difficulties in watershed delineation due to very flat terrain, and also hydrology and water quality monitoring approaches for its application for