Submitted to: Watershed Management Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2005
Publication Date: 3/1/2005
Citation: Nangia, V., Gowda, P., Mulla, D.J., Sands, G.R. 2005. Field scale application of a water quality modeling approach for alternative agronomic practices. In: Proceedings of the American Society of Agricultural Engineers Watershed Management to Meet Water Quality Standards and Emerging TMDL Conference, March 5-9, 2005, Atlanta, Georgia. p. 364-372. Interpretive Summary: Hypoxia in the Gulf of Mexico is a serious environmental issue which has primarily been attributed to nitrogen enriched waters entering the Gulf from the Mississippi River. The Upper Mississippi River Basin (UMRB) contributes one-third of the total nitrate loading to the Mississippi River, but comprises only about 15% of the total area of the Mississippi River Basin. High nitrate loads from the UMRB are associated with tributaries from agricultural areas in the states of Minnesota, Iowa, Indiana, and Illinois where tile drained row crops receive excess nitrogen from fertilizers and manure. The Committee on Environment and Natural Resources (CENR) recommended a 15-20% reduction in nitrogen discharges to the Gulf mainly through adoption of a 20% reduction in fertilizer nitrogen application. We calibrated and validated a mechanistic model and estimated reductions in nitrate losses associated various alternative nutrient management practices on a typical commercial farm in southern Minnesota. Model predictions were in good agreement for both calibration and validation periods. Nitrate losses were sensitive to timing and rate of fertilizer application as expected. Evaluation of alternative nutrient management practices show that about 8% reduction in nitrate losses are possible when application timing is changed from fall to spring. A further 12% reduction in nitrate losses may require a reduction of 25% in fertilizer applications. These findings are of great interest for watershed managers who are involved in developing alternative management practices to reduce agricultural impact on water quality.
Technical Abstract: This paper evaluates the effectiveness of alternative agronomic practices at improving water quality in agricultural fields. A spatial process model was calibrated using monthly flow and NO3-N losses from 1999-2003. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. Monitoring data from 1999-2001 were used for calibration and data from 2002-2003 were used in validation of the model. For the calibration period, the observed and predicted flow and NO3-N discharges were in good agreement with r2 values of 0.86 and 0.86, respectively. During validation, the observed and predicted flow and NO3-N discharges were in excellent agreement with r2 value of 0.92 and 0.91, respectively. The calibrated model was used to evaluate the effects of rate and timing of N fertilizer application on NO3-N losses. Reductions in NO3-N losses are possible by changing the application timing from fall to spring. For application rate of 180 kg/ha (160 lb/ac), there was 8% reduction in NO3-N losses when such change was made. Further reductions in NO3-N losses require reduction in N application rates. Twelve percent reduction in losses were found when application rate was reduced from 180 kg/ha (160 lb/ac) to 135 kg/ha (120 lb/ac).