Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2007
Publication Date: 3/3/2008
Citation: Gowda, P., Mulla, D.J., Jaynes, D.B. 2008. Simulated long-term nitrogen losses for a midwestern agricultural watershed in the United States. Agricultural Water Management. 95:616-624. Interpretive Summary: Hypoxia in the Gulf of Mexico is a serious water quality issue. It has been attributed primarily to nitrogen-enriched waters from the Mississippi River entering the Gulf. High nitrate loadings from the Upper Mississippi River Basin are associated with tributaries from agricultural areas in the states of Iowa, Minnesota and Illinois where a high percentage of agricultural land is in row crops. 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% 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 fertilizer management practices in a small agricultural watershed near Ames, Iowa. Modeled annual nitrate losses show that a 17% reduction in nitrate losses may be possible by reducing the fertilizer application rate by 20% and switching the application timing from fall to spring. Further reductions in nitrate losses require conversion of row cropland to pasture and/or replacement of the prevailing nutrient thirsty continuous corn or corn-soybean rotation systems with alternative crops.
Technical Abstract: Adequate knowledge on the movement of nutrients under various agricultural practices is essential for developing remedial measures to reduce nonpoint source pollution. Mathematical models, after extensive calibration and validation, are useful to derive such knowledge and to identify site-specific alternative agricultural management practices. A spatial-process model that uses GIS and the ADAPT, a field scale daily time-step continuous water table management model, was calibrated and validated for flow and nitrate-N discharges from a 365 ha agricultural watershed in central Iowa, in the Midwest United States. This watershed was monitored for nitrate-N losses from 1991-1997. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. The first half of the monitored data was used for the calibration and the other half was used in validation of the model. For the calibration period, the observed and predicted flow and nitrate-N discharges were in excellent agreement with r**2 of 0.88 and 0.74, respectively. During the validation period, the observed and predicted flow and nitrate-N discharges were in good agreement with r**2 of 0.71 and 0.50, respectively. For all six years of data, the observed and predicted annual nitrate-N losses for the entire simulation were in excellent agreement with nitrate-N losses of 26 and 24.2 kg h**-1, respectively. The calibrated model was used to investigate the long-term impacts of nitrate-N losses to changes in the rate and timing of fertilizer application. Results indicate that nitrate-N losses were sensitive to rate and timing of fertilizer application. Modeled annual nitrate-N losses show that a 17% reduction in nitrate-N losses may be possible by reducing the fertilizer application rate by 20% and switching the application timing from fall to spring. Further reductions in nitrate-N losses require conversion of row cropland to pasture and/or replacement of the prevailing nutrient thirsty continuous com or com-soybean rotation systems with alternative crops.