Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2003
Publication Date: 1/10/2004
Citation: Chu, T.W., Shirmohammadi, A., Montas, H., Sadeghi, A.M. 2004. Evaluation of SWAT Model's sediment and nutrient components in piedmont physiographic region of Maryland. Transactions of the ASAE. 47(5):1523-1538. Interpretive Summary: Mathematical watershed scale models are among the best tools available for analyzing water resources (quantity and quality) issues in spatially diverse watersheds since continuous water quality monitoring is expensive and spatially impractical in mixed land use watersheds. However, models without appropriate validation may lead to misconceptions and erroneous predictions. This study used six years of hydrologic and water quality data to calibrate and validate the capability of SWAT (Soil and Water Assessment Tool) model in assessing nonpoint source pollution for a 340 ha watershed in the piedmont physiographic region in Maryland The evaluation of the hydrology component of SWAT completed in a previous study has pointed out the presence of subsurface flow contribution from outside the watershed that SWAT has no mechanism to account for. To have a fair evaluation of the SWAT model, all nutrient loadings leaving the watershed have been adjusted to subtract the chemical transport via subsurface flow contribution from outside the watershed. Evaluation results indicated a strong agreement between yearly measured and simulated data for sediment, nitrate and soluble phosphorus loadings. However, model's simulations of monthly sediment and nutrient loadings were poor. Overall, it was concluded that SWAT is a reasonable watershed scale model for long-term simulation of different management scenarios. Its use on a storm-by-storm or even on a monthly basis may not be appropriate. Additionally, ignoring the subsurface contribution of water and chemicals into the watershed aquifer could cause significant errors in model prediction.
Technical Abstract: A continuous time watershed scale model (SWAT, Soil & Water assessment Tool) was applied to a 340 ha watershed in Maryland to test its performance in predicting hydrologic/water quality response. Most existing watershed scale models normally consider that the subsurface water flow is also bounded by the surface topography, missing the potential subsurface flow contributions from outside the watershed boundaries. This assumption lead to inaccurate predictions of pollutant loading via respective pathways. To have a fair evaluation for the nutrient component of SWAT model, all nutrient loadings leaving the watershed have been adjusted to subtract the chemical transport via subsurface flow contribution from outside the watershed. Generally, SWAT performed relatively poorly in predicting monthly nutrient loadings. However, the model showed great success in predicting annual nitrate and soluble phosphorus loadings. Overall, SWAT is a reasonable annual predictor of the watershed responses for assessing the impacts of different management systems on water supplies and nonpoint source pollution. One should be aware of possible subsurface flow contributions from outside of a surface watershed when applying hydrologic models in regions with abundant groundwater or potential aquifer discharge as may occur in the piedmont regions.