Submitted to: Transactions of the ASABE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/15/2006
Publication Date: 3/15/2006
Citation: Green, C.H., Tomer, M.D., Diluzio, M., Arnold, J.G. 2006. Hydrologic evaluation of the Soil and Water Assessment Tool for a large tile-drained watershed in Iowa. Transactions of the ASABE. 49(2):413-422. Interpretive Summary: Subsurface drainage is need in over 50% of agricultural land in some states. Tile drains can transport chemicals that degrade nearby water resources. In order to predict chemical movement, the hydrologic components must represent the area. This study includes tile drains to determine if the SWAT2005 model can adequately simulate water distribution within a watershed in Iowa. The tile drainage model routine was found to be pertinent to distributing water correctly so that chemical transport can then be studied. This study also shows that it is important to use a range of precipitation conditions to calibrate the model so that its predictions of water and chemical transport will be better.
Technical Abstract: The presence of subsurface tile drainage systems can facilitate nutrient and pesticide transport thereby contributing to environmental pollution. The SWAT (Soil and Water Assessment Tool) water quality model is designed to assess nonpoint and point pollution and was recently modified for tile drainage. Over 25% of the nation’s cropland required improved drainage. In this study, the model's ability to validate the tile drainage component is evaluated with nine years of hydrologic monitoring data collected from the South Fork watershed in Iowa because about 80% of this watershed is tile drained. This watershed is a Conservation Effects Assessment Program benchmark watershed and represents one of the most intensive regions for row-crop and livestock production in the Midwest. Comparison of measured and predicted values demonstrated that inclusion of the tile drainage system is imperative to obtaining a realistic watershed water balance. The Nash-Sutcliffe E values for the initial simulated monthly and daily flows during the calibration (1995-2000)/validation (2001-2004) periods were 0.8/0.5 and 0.7/0.2, respectively. The year 2000, being extremely dry, negatively impacted both the calibration and validation results. Altering the time periods so that a peak streamflow event was included in each simulation increased the monthly r2/ENS values to 0.9/0.9 and 0.6/0.5, respectively for the calibration (1995-1998) and validation (1999-2004) periods. Each water budget component of the model gave reasonable output which reveals that this model can be used for the assessment of tile drainage with its associated practices. Water yield results were significantly different for the simulations with and without the tile flow component (25.1% and 16.9%, expressed as a percent of precipitation). The results suggest that the SWAT2005 version modified for tile drainage is a promising tool to evaluate streamflow in tile drained regions when the calibration period contains streamflows representing a wide range of rainfall events.