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The presence of subsurface tile drainage systems can facilitate nutrient and pesticide transport, therebycontributing to environmental pollution. The Soil and Water Assessment Tool (SWAT) water quality model is designed to assessnonpoint and point source pollution and was recently modified for tile drainage. Over 25% of the nations cropland requiredimproved drainage. In this study, the models ability to validate the tile drainage component is evaluated with nine years ofhydrologic monitoring data collected from the South Fork watershed in Iowa, since about 80% of this watershed is tiledrained. This watershed is a Conservation Effects Assessment Program benchmark watershed and typifies one of the moreintensively managed agricultural areas in the Midwest. Comparison of measured and predicted values demonstrated thatinclusion of the tile drainage system is imperative for obtaining a realistic watershed water balance. Twocalibration/validation scenarios tested if the results differed in how the data set was divided. The optimum scenario resultsfor the simulated monthly and daily flows had Nash-Sutcliffe efficiency (ENS) values during the calibration/validation(1995-1998/1999-2004) periods of 0.9/0.7 and 0.5/0.4, respectively. The second scenario results for the simulated monthlyand daily flows had ENS values during the calibration/validation (1995-2000/2001-2004) periods of 0.8/0.5 and 0.7/0.2,respectively. The optimum scenario reflects the distribution of peak rainfall events represented in both the calibration andvalidation periods. The year 2000, being extremely dry, negatively impacted both the calibration and validation results. Eachwater budget component of the model gave reasonable output, which reveals that this model can be used for the assessmentof tile drainage with its associated practices. Water yield results were significantly different for the simulations with andwithout the tile flow component (25.1% and 16.9%, expressed as a percent of precipitation). The results suggest that theSWAT2005 version modified for tile drainage is a promising tool to evaluate streamflow in tile-drained regions when thecalibration period contains streamflows representing a wide range of rainfall events.
Green et al. (Sun,) studied this question.