Evaluation of Alternative Cropping and Nutrient Management Systems with Soil and Water Assessment Tool for the Raccoon River Watershed Master Plan

The Soil and Water Assessment Tool (SWAT) ecohydrological model was used to assess a suite of adaptive nutrient management, alternative cropping system and combination scenarios in support of a Master Plan that was developed to help guide policy makers, land owners and other stakeholders in improving water quality for the 9,400 km² Raccoon River watershed in west central Iowa, located in the Corn Belt region of the United States. Calibration and validation of SWAT showed that the model was able to accurately replicate most streamflow and pollutant loss conditions, with r² and Nash-Sutcliffe modeling Efficiency (NSE) statistics exceeding 0.5 for the majority of indicators. The largest reduction in predicted nitrate losses at the overall watershed scale for the adaptive nutrient management scenarios was slightly over 12%, in response to a scenario in which 100% of the anhydrous ammonia fertilizer application was applied in the watershed four weeks after planting of corn. Considerably larger impacts on nitrate loss and other sediment and nutrient indicators, with some reductions exceeding 50%, were predicted for scenarios representing widescale conversion of corn-soybean rotations to six-year rotation of Corn-Soybean-Corn-Alfalfa-Alfalfa-Alfalfa (CSCAAA) or combinations of adaptive nutrient management, conversion to CSCAAA rotations and the insertion of a rye cover crop on a portion of the area managed with corn and soybean within the CSCAAA rotations.