Nitrate Loss from Subsurface Drains in an Agricultural Watershed Using SWAT2005

Abstract

Many soils in the Midwest need artificial drainage for economical crop production, but nitrate loss from subsurface tile drains can cause environmental problems downstream. The impact of tile drains on nitrate loss at the watershed scale is not well quantified. SWAT2005, with its modified tile drain component, was used to evaluate nitrate loss through tile drains in a heavily drained watershed, Sugar Creek watershed in Indiana. Monthly streamflow predictions resulted in Nash-Sutcliffe efficiency values of 0.88 for both the 5-year calibration period and the 6-year validation period. Monthly nitrate-N loads were also predicted well, with Nash-Sutcliffe efficiency values of 0.63 in the calibration period and 0.67 in the validation period. Nitrate-N load from tile drains was added as an output, and the simulated nitrate loss through tile drains was compared with nitrate losses from surface flow, groundwater, and lateral flow. The estimated median percentage of monthly nitrate loss entering Sugar Creek that flows through tile drains ranged from 0% to 37% and was more than 30% for three months, April to June. Results like these could be used in estimating potential nitrate reductions at the watershed level that would be achieved by implementing drainage-related conservation practices throughout the watershed.