Paired watershed studies for nutrient reductions in the Minnesota River basin.

摘要

The purpose of this project was to identify and evaluate best management practices (BMPs) to improve water quality using a paired watershed approach. The project was conducted on two watersheds located in Nicollet County, Minnesota. Each of the watersheds covered more than 1100 ha, with cultivation comprising over 90% of the land use. The project was comprised of four components. The first component consisted of an inventory of farm management practices with 24 producers responsible for crop production on more than 80% of the cultivated area in both watersheds. The survey revealed a wide range of management practices across both watersheds. The survey was instrumental for identifying best management practices for each farm in the treatment watershed to improve water quality. The second component of the project evaluated the influence of BMP implementation during a two-year treatment period on sediment and phosphorus (P) transport from the watershed. Practices were implemented on 49 and 63% of the cultivated area in the 2003 and 2004 crop years, respectively. Changes in storm event transport of sediment and P were not detected as a result of the BMPs implemented throughout the treatment phase of the project. The third component of the project consisted of a field evaluation of the Minnesota P Index to prioritize fields based on risk of P transport to water bodies. Phosphorus index risk ratings of medium, high, and very high were determined for 32, 12, and 6% of the watershed area. The last component of the project evaluated the use of published threshold values of terrain indices to identify critical source areas of nonpoint source pollution. Critical source areas are defined as portions of a watershed that contribute disproportionately to the pollutant load transported at the watershed outlet. Results suggest that the critical source areas are small, spatially distinct areas covering less than 10% of the watershed area.