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This study evaluated hydrologic and water-quality data collected on a coastal-plain research watershed during a series of hurricanes and tropical storms that hit coastal North Carolina in 1999, including hurricanes Dennis, Floyd, and Irene. During September and October 1999, the research watershed received approximately 555 mm of rainfall associated with hurricanes. This was the wettest such period in a 49-year historical weather record (1951-1999). Prior to the hurricanes, the watershed experienced a dry late winter, spring, and summer (565 cm for Feb.-Aug.). This was the third driest such period in the 49-year record. Maximum daily flow rates measured across the research watershed were greater during hurricane Floyd than for any other time in a four-year (1996-1999) study of the watershed. Daily flows observed for an agricultural subwatershed were generally greater than for a forested subwatershed throughout the study, and during the hurricanes of 1999. Daily nutrient loads measured across the research watershed were greater during hurricane Floyd than for any other time in the study. In general, the two-month period of hurricanes produced total nitrogen and total phosphorus loads nearly equal to average loads for an entire year. Total annual nitrogen export from an agricultural subwatershed was 18 kg/ha in 1999, with 11 kg/ha (61%) lost during September and October. Total annual nitrogen export from a forested subwatershed was 15 kg/ha in 1999, with 10 kg/ha (67%) lost during September and October. The nitrogen export observed in the forested subwatershed was high compared to other forested areas, likely due to the highly permeable organic soils in the watershed. Total annual phosphorus export from an agricultural subwatershed was 0.9 kg/ha in 1999, with 0.7 kg/ha (78%) lost during the hurricanes/tropical storms. Total annual phosphorus load from a forested subwatershed was 0.1 kg/ha in 1999, with 74% of the load exported during the months of September and October Hurricanes and floods occur with some regularly in North Carolina, but the effects are infrequently documented. This study provides information that will contribute to greater understanding of how watersheds respond to these events. [ABSTRACT FROM AUTHOR]

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 346 ha watershed in the Piedmont physiographic region. The evaluation of the hydrology component of SWAT completed in a previous study pointed out that 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 contributions from outside the watershed. Evaluation results indicated a strong agreement between yearly measured and simulated data for sediment, nitrate, and soluble phosphorus loadings. However, 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. However, its use on storm-by-storm or even on monthly basis may not be appropriate for watersheds with similar physiography and size. Additionally, ignoring the subsurface contribution of water and chemicals from outside the watershed into the watershed aquifer could cause significant errors in model prediction. [ABSTRACT FROM AUTHOR]

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 and quality) issues in spatially diverse watersheds. Although existing watershed-scale models provide some reasonable guidelines, their application without proper validation has resulted in some misconceptions about such models. This study used six years of hydrologic data to calibrate and validate the capability of the SWAT (Soil and Water Assessment Tool) model in predicting surface and subsurface flow for a 340 ha watershed in the Piedmont physiographic region of Maryland. Previous studies have indicated that most existing models only handle subsurface flow bounded by the surface topography, thus neglecting the possible subsurface flow contribution from the outside of watershed, which appears to be a great model deficiency considering the major pathway of pollutant loadings via subsurface flow. Preliminary simulations showed that SWAT 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 water budget 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 the watershed, thus comparing the model predictions with appropriate measured data. However, SWAT seemed to be unable to simulate the extremely wet hydrologic conditions, even after adjustments to measured data. Overall, the hydrology component of the SWAT model is able to perform an acceptable prediction of long--term simulations for management purposes, but fails to have reasonable predictions for short time intervals (i.e., daily). [ABSTRACT FROM AUTHOR]