Modeling the impact of nitrogen fertilizer application and tile drain configuration on nitrate leaching using SWAT

Recently, the Soil and Water Assessment Tool (SWAT) was revised to improve the partitioning of runoff and tile drainage in poorly drained soils by modifying the algorithm that computes the soil moisture retention parameter. In this study, the Revised SWAT model was used to evaluate the sensitivity and long-term effects of different nitrogen (N) application rates and tile drain spacing (SDRAIN) and depths (DDRAIN) on nitrate-nitrogen (NO₃-N) losses through tile drains. Monitoring data for the 1983-1996 period measured on three experimental plots on a poorly drained Webster clay loam soil (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) in southern Minnesota was used. Sensitivity analysis covered the 1983-1996 period and long-term simulations were made for the 1915-1996 period. Sensitivity analysis showed a decrease in tile flow as DDRAIN decreased and/or SDRAIN increased. The predicted NO₃-N losses in tile drain decreased by 16% (from 33.8 to 28.4kgha^-1) and 14% (from 34.0 to 29.4kgha^-1) when SDRAIN was increased by 122% (from 27 to 60m) and 40% (from 1.5 to 0.9m), respectively. However, NO₃-N losses were decreased by 67% (from 33.8 to 11.1kgha^-1) when N application rate was decreased by 50% (from 200 to 100kgha^-1). Long-term simulations results indicated that much greater reductions in NO₃-N losses can be achieved with reduction in the N application rates than with changing the tile drain spacing and depth. Reductions in NO₃-N losses were consistent with the results reported using the Agricultural Drainage and Pesticide Transport (ADAPT) model, which was developed specifically for understanding effects of tile drainage on water quality in the Upper Midwest U.S. Overall, results from sensitivity analysis and long term simulation indicated that Revised SWAT can be used to adequately evaluate the effects of tile drain configurations on drainage and associated NO₃-N losses.