Sensitivity, Calibration, and Validation of SWAT in the Choptank River Basin.

The value of watershed-scale, water quality models to ecosystem management is increasingly evident as more programs adopt these tools to help assess the effectiveness of different management scenarios on the environment. The USDA-Conservation Effects Assessment Project (CEAP) is one such program which was established to quantify the environmental benefits from conservation practices implemented under USDA conservation programs. The Choptank River watershed, located in Maryland on the Eastern Shore of the Chesapeake Bay, is a special emphasis watershed under the CEAP program. Several of its tributaries have been identified as “impaired waters” under Section 303(d) of the Federal Clean Water Act due to high levels of nutrients and sediments.

 The Soil and Water Assessment Tool (SWAT) was utilized to build a model for the German Branch (GB) watershed (~50 km²), a non-tidal tributary basin of the Choptank River. The overall goal of the project is to determine the effectiveness of cover crop programs to reduce nutrient loadings to the Choptank River; however, the initial steps to build the GB model are discussed here. Sensitivity, calibration, and validation analyses were conducted on the hydrology component of the GB model.

 Streamflow output was most sensitive to curve number for moisture condition II (Cn2), deep aquifer percolation fraction (Rchrg_Dp), and soil evaporation compensation factor (Esco). Model performance during the calibration period was very good with RMSE equal to 0.07 and both r² and NSE equal to 0.84. Results for the validation period were not as good. However, an r² of 0.50 indicated satisfactory model performance. This study demonstrated that SWAT is capable of effectively simulating streamflow from tributaries of the Choptank River. The optimal water balance obtained in this study is an essential precursor to simulating realistic sediment and nutrient results in forthcoming GB modeling efforts.