Modeling Flow and Nitrate Losses for Small Watersheds in Southeastern Minnesota

Abstract

Watershed planners and decision makers need tools to determine which best management practices (BMPs) are most effective at reducing nitrate pollution in the Root River Basin of southeastern Minnesota. The Root River Field to Stream Partnership is monitoring three subwatersheds of the Root River (Headwaters, Crystal Creek, and Bridge Creek) for flow and nutrient flux. Additionally, an individual agricultural field in each subwatershed is also being monitored. The Soil and Water Assessment Tool (SWAT) used data from these monitored sites to attempt to create calibrated hydrology and nitrate flux models. Inputs for modeling were obtained using publicly available sources for land cover, soils, elevation, and weather data. Model calibration for hydrology and nitrate flux was done via the trial and error method. Accurate hydrology models were produced for both the Headwaters and Crystal Creek subwatersheds, but a good nitrate model was produced only for the Headwaters due to several complications from underlying karst features in the Crystal and Bridge Creek subwatersheds that were difficult to account for using the SWAT model. Accurate SWAT models could not be developed for Bridge Creek and the field sites. The Headwaters model was then run using ten combinations of BMP scenarios to determine their effectiveness at reducing nitrate losses. Reducing nitrogen fertilizer application rates to economical rates and splitting applications of nitrogen resulted in a fourteen and sixteen percent decrease in nitrate losses, respectively. Applying nitrogen only in the spring and placing filter strips on steeply sloping agricultural land reduced nitrate flux by six percent each.