Alternative Agricultural Ditch Designs, NO₃-N Treatment, Construction Costs, and Benefits—Mower County, Minnesota, USA

The Minnesota Nutrient Management Strategy has identified waters with the highest amounts of nutrients that could potentially enter the Mississippi River. Mower County contains the headwaters of the Cedar River Basin, a large nutrient loading basin in the upper Mississippi River Basin. Joint efforts of the UMN (University of Minnesota), the Nature Conservancy, and Mower County, MN implemented alternative agricultural ditch designs to improve water management. Designs consisted of a TSD (Two-Stage Ditch), a rock trench, a rock inlet, and two in-ditch linear treatment systems. These features were implemented to increase ditch stability and to reduce the loading of sediment and nutrients. Important costs were the possible removal of productive ag-land, construction costs, and the break-even discount (interest) rates. Benefits included lower maintenance cost, reduced sediment and phosphorus loads with more stable ditches, and enhanced removal of NO₃-N (Nitrate-Nitrogen). Results suggest costs of the TSD construction may be supplemented with subsidies to break-even with conventional ditch design costs. Subsidies based on NO₃-N removal show the TSD compares well to other NO₃-N best management practices. The break-even NO₃-N removal costs vary greatly but are lowest when the floodplain bench is inundated for longer time periods and when the discount rate, cleanout interval of the conventional design, and construction costs of the TSD are low.