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Continuous water quality monitoring is expensive and spatially impractical in mixed land use watersheds.Mathematical watershed-scale models are among the best tools available for analyzing water resources (quantity andquality) issues in spatially diverse watersheds. Although existing watershed-scale models provide some reasonableguidelines, their application without proper validation has resulted in some misconceptions about such models. This studyused six years of hydrologic data to calibrate and validate the capability of the SWAT (Soil and Water Assessment Tool) modelin predicting surface and subsurface flow for a 340 ha watershed in the Piedmont physiographic region of Maryland. Previousstudies have indicated that most existing models only handle subsurface flow bounded by the surface topography, thusneglecting the possible subsurface flow contribution from the outside of watershed, which appears to be a great modeldeficiency considering the major pathway of pollutant loadings via subsurface flow. Preliminary simulations showed thatSWAT underestimated subsurface flow and total streamflow, especially during wet periods. A water budget analysis, therefore,was performed to quantify various components of the hydrologic cycle within the watershed. The resulting imbalance in waterbudget analysis suggested a considerable groundwater contribution from outside the watershed, especially during wet years.Adjustments to measured base flow and streamflow were made to exclude the extra groundwater recharge from outside thewatershed, thus comparing the model predictions with appropriate measured data. However, SWAT seemed to be unable tosimulate the extremely wet hydrologic conditions, even after adjustments to measured data. Overall, the hydrology componentof the SWAT model is able to perform an acceptable prediction of long-term simulations for management purposes, but failsto have reasonable predictions for short time intervals (i.e., daily).
Chu et al. (Thu,) studied this question.