Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/1/2005
Publication Date: 10/4/2005
Citation: Gowda, P., Mulla, D.J., Dalzell, B.J. 2005. Evaluating feasibility of nitrate TMDLs on two agricultural watersheds in Southern Minnesota [abstract]. Mississippi River Basin Nutrients Workshop. St. Louis, Missouri. 2005 CDROM.
Technical Abstract: Hypoxia in the Gulf of Mexico is a serious environmental issue that has been attributed primarily to nitrogen enriched waters entering the Gulf from the Mississippi River. According to the Gulf of Mexico Watershed Nutrient Task Force, a 30% reduction in nitrogen discharges from the Mississippi and Atchafalaya rivers to the Gulf is needed to be consistent with their coastal goal, i.e. reduce the areal extent of hypoxia to 5000 km2 by 2015. In this study, a set of nitrogen reduction strategies was developed to evaluate the feasibility of complying with the coastal goal. For this purpose, a spatial-process model was calibrated and used to predict monthly nitrate losses (1994-1996) from Sand Creek and Bevens Creek watersheds located in south-central Minnesota. Statistical comparison of the predicted and observed data on Sand Creek watershed gave r2 coefficients of 0.75 and 0.70 for flow and nitrate losses, respectively. Similar results were found for Bevens Creek watershed. The calibrated model was used to evaluate the effects of alternative agricultural management scenarios, including changes in the rate and timing of N-fertilizer application rates and increases in the percentage of crop land with subsurface drainage. Alternative management practices include: six different N application rates (by changing the baseline rate by -20, -10, 0, +10, +20, and +30% over three different timings: fall, spring, and 50% in fall and 50% in spring) and three different percentages of crop land with subsurface drainage (base, +10 and +20%). Predicted nitrate losses were then compared with nitrate TMDLs for compliance. The nitrate TMDLs were calculated assuming that a 30% reduction in observed nitrate losses is required to meet the TMDLs. Evaluation of alternative management practices showed that annual nitrate losses decreased by 23% (from 7.8 to 6 kg/ha) and 19% (21 to 17 kg/ha) for Sand Creek and Bevens Creek watersheds, respectively, when fertilizer application rates were reduced from +30% to -20% of the baseline rate. Reductions of about 33% (8.6 to 5.8 kg/ha) and 35% (23 to 15 kg/ha) in annual nitrate losses are possible for Sand Creek and Bevens Creek watersheds, respectively, just by switching the timing of fertilizer application from fall to spring. Trends towards increasing tile drainage density suggest that attaining nitrate TMDLs in future may require other alternative management practices such as partial conversion of crop land to pasture.