Evaluation of the Swat Model on a Coastal Plain Agricultural Watershed

The Better Assessment Science Integrating point and Nonpoint Sources (BASINS) system was developed by theU.S. Environmental Protection Agency to facilitate developing total maximum daily loads (TMDLs). The Soil Water AssessmentTool (SWAT) is one of the watershed-scale simulation models within BASINS. Because of the critical nature of the TMDLprocess, it is imperative that BASINS and SWAT be adequately validated for regions on which they are being applied. BASINSand SWAT were tested using six years of hydrologic data from a 22 km2 subwatershed of the Little River in Georgia.Comparisons were made between water balance results obtained using high and low spatial resolution data as well as thoseobtained using default initial parameters versus those modified for existing groundwater conditions. In general, all scenariossimulated general trends in the observed flow data. However, for the years with lower precipitation, the total water yieldssimulated with the low spatial resolution data and the default initial conditions were overpredicted by up to 27% of the annualprecipitation input. Total water yields simulated using the high spatial resolution input data were within 20% of the observedyields for each year of the assessment. Nash-Sutcliffe model efficiencies (E) for monthly total water yields were 0.80 usingthe high spatial resolution data with the modified initial conditions and 0.64 using the low spatial resolution data with thedefault initial conditions. While the model simulated general streamflow trends, discrepancies were observed betweenobserved and simulated hydrograph peaks, time to peak, and hydrograph durations. A one-day time lag between the simulatedand observed time to peak was the primary cause of large errors in daily flow simulations. Model modification and moreextensive calibration may be necessary to increase the accuracy of the daily flow estimates for TMDL development.