Mathematical watershed-scale models are among the best tools available for analyzing water resources (quantityand quality) issues in spatially diverse watersheds since continuous water quality monitoring is expensive and spatially impracticalin mixed land use watersheds. However, models without appropriate validation may lead to misconceptions anderroneous predictions. This study used six years of hydrologic and water quality data to calibrate and validate the capabilityof SWAT (Soil and Water Assessment Tool) model in assessing nonpoint source pollution for a 346 ha watershed in the Piedmontphysiographic region. The evaluation of the hydrology component of SWAT completed in a previous study pointed outthat SWAT has no mechanism to account for subsurface flow contributions from outside the watershed. For this evaluation,all nutrient loadings leaving the watershed were adjusted to subtract the chemical transport via subsurface flow contributionsfrom outside the watershed. Evaluation results indicated a strong agreement between yearly measured and simulated datafor sediment, nitrate, and soluble phosphorus loadings. However, simulations of monthly sediment and nutrient loadings werepoor. Overall, it was concluded that SWAT is a reasonable watershed-scale model for long-term simulation of different managementscenarios. However, its use on storm-by-storm or even on monthly basis may not be appropriate for watersheds withsimilar physiography and size. Additionally, ignoring the subsurface contribution of water and chemicals from outside thewatershed into the watershed aquifer could cause significant errors in model prediction.
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Chu et al. (2004) studied this question.