Water Quality Modeling for the Raccoon River Watershed Using SWAT

Abstract

The Raccoon River watershed (RRW) in west-central Iowa has been recognized as exporting some of the highest nitrate-nitrogen loadings in the U.S. and is a major source of sediment and other nutrient loadings. An integrated modeling framework has been constructed for the 9,400 km2 RRW that consists of the SWAT (Soil and Water Assessment Tool) model, the interactive SWAT (i_SWAT) software package, the Load Estimator (LOADEST) computer program, and other supporting software and databases. The simulation framework includes detailed land use and management data, such as different crop rotations, and an array of nutrient and tillage management schemes, derived from the USDA National Resources Inventory (NRI) databases and other sources. This article presents the calibration and validation of SWAT for the streamflow, sediment losses, and nutrient loadings in the watershed, and an assessment of land use and management practice shifts in controlling pollution. Streamflow, sediment yield, and nitrate loadings were calibrated for the period 1981-1992 and validated for the period 1993-2003. Limited field data on organic nitrogen, organic phosphorus, and mineral phosphorus allowed model validation for the period 2001-2003. Model predictions generally performed very well on both an annual and monthly basis during the calibration and validation periods, as indicated by R2 and Nash-Sutcliffe efficiency (E) values that exceeded 0.7 in most cases. A set of land use change scenarios depicting conversion of cropland into land set-aside resulted in large reductions of sediment yield at the watershed outlet. A second scenario set found that reductions in nutrient applications of 10% to 20% resulted in similar predicted percentage reductions in nitrate loadings at the watershed outlet and in corresponding corn yield reductions of 3% to 6%.