Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2006
Publication Date: 2/20/2007
Citation: Gowda, P., Dalzell, B.J., Mulla, D.J. 2007. Model based nitrate TMDLs for two agricultural watersheds of Southeastern Minnesota. Journal of the American Water Resources Association. 43(1):254-263. Interpretive Summary: Hypoxia in the Gulf of Mexico is a serious environmental issue which has been attributed primarily to nitrogen enriched waters entering the Gulf from the Mississippi River. The Minnesota River Basin located in southern Minnesota, with more than 30 percent of its crop land artificially drained, has been identified as one of several relatively high contributors of nitrogen into the Upper Mississippi River. The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force set a coastal goal of reducing areal extent of hypoxia in the Gulf to 5,000 sq. km. by 2015. They estimated that this would require a 30 percent reduction in nitrogen discharges from the Mississippi and Atchafalaya Rivers to the Gulf. A modeling study was conducted to evaluate the reductions in nitrate losses possible with several alternative management practices in two agricultural watersheds in south-central Minnesota. Simulation results indicated that the recommended reduction in nitrate losses can be achieved just by switching the timing of fertilizer application for fall to spring. However, trends towards increases in tile drainage density imply that attaining nitrate reduction recommendation in the future may require other alternative management practices in addition to fertilizer management such as partial conversion of cropland to pasture. These findings are of great interest to farmers and policy makers seeking to reduce agricultural impacts on water quality while maintaining farming profits.
Technical Abstract: In this study, a set of nitrogen reduction strategies were modeled to evaluate the feasibility of improving water quality to meet Total Maximum Daily Loads (TMDLs) in two agricultural watersheds. For this purpose, a spatial-process model was calibrated and used to predict monthly nitrate losses (1994-1996) from Sand and Bevens Creek watersheds located in south-central Minnesota. Statistical comparison of predicted and observed flow and nitrate losses gave r2 coefficients of 0.75 and 0.70 for Sand Creek watershed and 0.72 and 0.67 for Bevens Creek watershed, respectively. Modeled alternative agricultural management scenarios included: six different N application rates over three application timings and three different percentages of crop land with subsurface drainage. Predicted annual nitrate losses were then compared with nitrate TMDLs assuming a 30 percent reduction in observed nitrate losses is required. Reductions of about 33 (8.6 to 5.8 kg/ha) and 35 percent (23 to 15 kg/ha) in existing annual nitrate losses are possible for Sand and Bevens Creek watersheds, respectively, just by switching the timing of fertilizer application from fall to spring. Trends towards increases in tile drainage density imply that attaining nitrate TMDLs in the future may require other alternative management practices in addition to N fertilizer management such as partial conversion of crop land to pasture.