Submitted to: Agricultural Water Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/6/2013
Publication Date: 9/5/2013
Citation: Moriasi, D.N., Gowda, P., Arnold, J.G., Mulla, D.J., Ale, S., Steiner, J.L. 2013. Modeling the impact of nitrogen fertilizer application and tile drain configuration on nitrate leaching using SWAT. Agricultural Water Management. 130:36-43. Interpretive Summary: Subsurface tile drainage is a commonly used agricultural practice to enhance crop yield in poorly drained but highly productive soils in many other regions of the world. However, the presence of subsurface tile drainage systems also expedites the transport of nitrate-nitrogen (NO3-N) and other chemicals to surface waters. Hydrologic and water quality models such as the Soil and Water Assessment Tool (SWAT) are widely used to simulate tile drainage systems at various spatial scales. Recently, the SWAT model was revised (Revised SWAT model) to improve the partitioning of runoff and tile drainage in poorly drained soils by modifying the algorithm for computing the soil moisture retention parameter. Whereas the Revised SWAT model has been shown to partition the water and nitrogen budgets well, no thorough sensitivity analyses of long-term effects of tile depth and spacing and N application rates on tile flow and NO3-N losses have been carried out to determine how the model performs relative to previous field and model simulation study results. Such studies are needed in order to validate the new Hooghoudt and Kirkham tile drain equations in the Revised SWAT model before they are used for other independent drainage applications. In this study, the Revised SWAT model was used to evaluate the sensitivity and long-term effects of nitrogen (N) application rate, and tile-drain spacing and depth on nitrate-nitrogen (NO3-N) losses through tile drains.in poorly drained watersheds of Upper Midwest U.S. using the validated Revised SWAT model. Long-term monitoring data from southern Minnesota plots in Waseca were used. Study results indicated that predicted NO3-N losses were most sensitive to the variation of N application rates, with predicted NO3-N losses decreasing by 67% (from 34 to 11 kg ha-1) when N application rate was decreased by 50% (from 200 to 100 kg ha-1). Long-term simulation results also indicated that greatest reductions in NO3-N losses can be achieved with reduction in the N application rates. The trends and the general findings simulated by the Revised SWAT model were similar with those reported in the literature. This implies that the Revised SWAT model showed potential to simulate the effects of tile drain configurations on drainage and associated NO3-N losses.
Technical Abstract: 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 for computing the soil moisture retention parameter. In this study, the revised SWAT model was used to evaluate the sensitivity and long-term effects of nitrogen (N) application rate, and tile-drain spacing and depth on nitrate-nitrogen (NO3-N) losses through tile drains. The Revised SWAT model, previously calibrated and validated for tile flow and NO3-N losses using 14 year (1983 – 1996) monitoring data 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. While sensitivity analysis covered the period from 1983 to 1996, long-term simulations were made for a broad range of climatic conditions between 1915 and 1996 to evaluate the effect of drain depth (DDRAIN) and spacing (SDRAIN) and N application rates on tile drainage and NO3-N losses. As expected, sensitivity analysis showed that in general the amount of tile flow decreased as DDRAIN decreased and SDRAIN increased. The predicted NO3-N losses in tile drain decreased by 16% (from 33.8 to 28.4 kg ha-1) when the SDRAIN was increased by 122% (from 27 to 60 m), by 14% (from 34.0 to 29.4 kg ha-1) when DDRAIN was decreased by 40% (from 1.5 to 0.9 m), and by 67% (from 33.8 to 11.1 kg ha-1) when N application rate was decreased by 50% (from 200 to 100 kg ha-1). Results from long-term simulations 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 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 SWAT can be used to adequately evaluate the effects of tile drain configurations on drainage and associated NO3-N losses.