Assessing the performance of SWAT and AnnAGNPS models in a coastal plain watershed, Choptank River, Maryland, U.S.A.

Abstract

This study was conducted under the USDA-Conservation Effects Assessment Project (CEAP) on the Choptank River watershed which is located within the Chesapeake Bay watershed on the Delmarva Peninsula in Maryland, U.S.A. The watershed is nearly 1036 km2 and is dominated primarily by corn and soybean production with extensive poultry production operations. Portions of the watershed have been identified as "impaired waters" under Section 303(d) of the Federal Clean Water Act due to high levels of nutrients and sediment. In recent years, a significant number of state and federal incentive programs have been implemented for water quality improvement in this watershed, but environmental benefits from these programs have never been quantified. Two of the most widely used USDA watershed-scale models, Soil and Water Assessment Tool (SWAT) and Annualized Agricultural Non-Point Source (AnnAGNPS) were applied to quantify the environmental benefits widely used practices like winter cover crops. Five years (1991-1995) of detailed observed flow and water quality data were used to provide baseline calibration and validation for the two models. Simulation results showed significant differences in base-flow estimations for the two models. This difference may be considered to be a significant factor in model selection to estimate nutrients and sediment loads in regions of fairly flat landscapes such as the Coastal Plain physiographic region. This study concluded that both SWAT and AnnAGNPS models performed well for simulating hydrologie conditions, however, for nitrate loads, AnnAGNPS NC coefficients were relatively lower, slightly above an acceptable 0.5 value.