Application of the Soil and Water Assessment Tool (SWAT) to the Willow River Watershed, St. Croix County, Wisconsin.

Abstract

Identifying critical source areas of sediment and phosphorus nonpoint pollutant loads under alternative land use scenarios is aided by the use of hydrologic models. We applied the Soil and Water Assessment Tool (SWAT) to the Willow River watershed in St. Croix County, Wisconsin, to examine the effects of possible future scenarios and best management practices. The model was calibrated and validated to water year (WY) 1999 and WY2006 datasets, respectively, with land use configured for each year. The model performed well in calibration, but could not simulate conditions outside of the calibration conditions in the validation dataset. Sediment and phosphorus trapping was influenced by trapping between the landscape source and the watershed outlet in closed-drainage lakes, flow-through wetlands, and on-stream reservoirs. The relative contributions of pollutants were related to the landscape position of the source area and the number and trapping efficiency of the intermediate traps in the flow path. We simulated best management practices including lowered soil-test phosphorus, increased conservation tillage, lowered cattle dietary phosphorus, and changed agricultural crop rotations. Simulations demonstrated that conversion to mulch tillage and no-till from conventional tillage could reduce sediment yield on the converted lands by 3% to 27% and phosphorus yield by 5% to 21%. For the current mix of agricultural land management in the Willow Watershed, converting all cropland to mulch tillage would reduce watershed export of phosphorus by 1% and sediment export by 1%. Converting all of the agricultural land to no-till produced a modeled decrease in watershed export of sediment of 2% and a decrease in phosphorus of 7%. Simulations also demonstrated a 22% reduction watershed phosphorus export by reducing average agricultural soil-test phosphorus to 20 ppm. Converting all farm land from a mixture of cash grain rotations to a dairy rotation that included two years of corn and three years of alfalfa caused a modeled reduction in watershed phosphorus export of 15% and a modeled reduction in sediment export of iii 5%. Continued conversion of agricultural land to rural residential land uses produced lower modeled loads of watershed sediment export up to 13% and phosphorus export up to 27% depending on the area developed and the average lot size. Changes in point source phosphorus because of better wastewater treatment caused a decrease in modeled phosphorus delivery of 13% between the calibration and validation time periods. Alternative climate scenarios were also simulated, showing that evapotranspiration was the driver of the altered hydrologic cycle, and thus the driver of reduced sediment and phosphorus export.