| Title: | Hydrologic evaluation of the soil and water assessment tool for a large tile-drained watershed in Iowa |
| Authors: | Green, C.H., M.D. Tomer, M. Di Luzio and J.G. Arnold |
| Year: | 2006 |
| Journal: | Transactions of the ASABE |
| Volume (Issue): | 49(2) |
| Pages: | 413-422 |
| Article ID: | |
| DOI: | 10.13031/2013.20415 |
| URL (non-DOI journals): | |
| Model: | SWAT |
| Broad Application Category: | hydrologic only |
| Primary Application Category: | tile drainage effects and/or processes |
| Secondary Application Category: | modified runoff curve number approach |
| Watershed Description: | 775 km^2 South Fork of the Iowa River, located in the Des Moines Lobe region in north central Iowa, U.S. |
| Calibration Summary: | scenario 1 (1995-98) r2/E values: annual = 1.0/.70 monthly = .90/.90 daily =.70/.70 -------------------------- scenario 2 (1995-2000) r2/E values: annual = .90/.90 monthly = .90/.80 daily =.70/.70 |
| Validation Summary: | scenario 1 (1999-2004) r2/E values: annual = .70/.60 monthly = .60/.50 daily =.50/.40 -------------------------- scenario 2 (2001-2004) r2/E values: annual = .70/-.80 monthly = .60/.50 daily =.30/.20 |
| General Comments: | Inclusion of an improved tile drainage routine in the SWAT2005 simulation was found to be essential to accurately simulated the water balance of the watershed (significantly different water yield results were found between simulations with and without tiles). Scenario 1 was found to be the optimal scenario which reflected the distribution of peak rainfall events represented in both the calibration and validation periods. The model had difficulty capturing the extremely dry conditions of the year 2000; this year negatively impacted the simulated results when used in either the calibration or validation period. Reasonable results were found for all predicted water balance components. |
| Abstract: | The presence of subsurface tile drainage systems can facilitate nutrient and pesticide transport, thereby contributing to environmental pollution. The Soil and Water Assessment Tool (SWAT) water quality model is designed to assess nonpoint and point source 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, since about 80% of this watershed is tile drained. This watershed is a Conservation Effects Assessment Program benchmark watershed and typifies one of the more intensively managed agricultural areas in the Midwest. Comparison of measured and predicted values demonstrated that inclusion of the tile drainage system is imperative for obtaining a realistic watershed water balance. Two calibration/validation scenarios tested if the results differed in how the data set was divided. The optimum scenario results for 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 monthly and 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 and validation periods. The year 2000, being extremely dry, negatively impacted both the calibration and validation results. 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. |
| Language: | English |
| Keywords: | AVSWAT-X, CEAP, Hydrologic modeling, SWAT, Tile drains, Watershed |