ABSTRACT The Soil and Water Assessment Tool (SWAT) has been developed to evaluate the effectivene... more ABSTRACT The Soil and Water Assessment Tool (SWAT) has been developed to evaluate the effectiveness of agricultural management practices on watershed water quality. Many studies have indicated that watershed subdivision can affect the accuracy of model predictions. Most of them used the minimum drainage area (MDA) to delineate subwatershed and varied value of MDA is used depending on the size of the watershed being modeled. Instead of MDA, we suggested to use the National Hydrography Dataset Plus (NHDPlus) to delineate the watershed. NHDPlus is an integration of the best features of the National Hydrography Dataset (NHD), Watershed Boundary Dataset (WBD), National Elevation Dataset (NED), and the National Land Cover Dataset (NLCD). The Kaskaskia River watershed in Illinois was selected to investigate the individual effects of subwatershed and hydrologic response unit (HRU) delineations on predicted streamflow, total suspended sediment (TSS) and total nitrogen (TN) losses at two USGS gages. Besides a MDA value of 3000 hectare, four levels of stream order (the 2nd, 3rd, 4th and 5th order) were evaluated. Three levels of HRU thresholds (5%, 10% and 15%) were used for each stream order model. The results showed that the stream order had little effects on predicted streamflow, but great impact on TSS and TN losses and the impact of HRU delineation became greater when a higher stream order was used to delineate the watershed. As a higher stream order was used, fewer streams were recognized in SWAT simulations, which resulted in less sediment routing and channel processes. Less channel processes lead to less deposition in the channels in this case, thus high sediment losses were obtained at the watershed outlet. However, less channel processes lead to less in-stream N processes in this case, thus less TN losses. Overall, the SWAT simulations performed the best when the 2nd stream order was used for delineations comparing with USGS observed data, followed by the 3rd stream order. Therefore, in order to fully depict the watershed characteristics to perform SWAT simulations, a stream order that is higher than 3rd stream order is not suggested for watershed delineation.
ABSTRACT The Soil and Water Assessment Tool (SWAT) has been developed to evaluate the effectivene... more ABSTRACT The Soil and Water Assessment Tool (SWAT) has been developed to evaluate the effectiveness of agricultural management practices on watershed water quality. Many studies have indicated that watershed subdivision can affect the accuracy of model predictions. Most of them used the minimum drainage area (MDA) to delineate subwatershed and varied value of MDA is used depending on the size of the watershed being modeled. Instead of MDA, we suggested to use the National Hydrography Dataset Plus (NHDPlus) to delineate the watershed. NHDPlus is an integration of the best features of the National Hydrography Dataset (NHD), Watershed Boundary Dataset (WBD), National Elevation Dataset (NED), and the National Land Cover Dataset (NLCD). The Kaskaskia River watershed in Illinois was selected to investigate the individual effects of subwatershed and hydrologic response unit (HRU) delineations on predicted streamflow, total suspended sediment (TSS) and total nitrogen (TN) losses at two USGS gages. Besides a MDA value of 3000 hectare, four levels of stream order (the 2nd, 3rd, 4th and 5th order) were evaluated. Three levels of HRU thresholds (5%, 10% and 15%) were used for each stream order model. The results showed that the stream order had little effects on predicted streamflow, but great impact on TSS and TN losses and the impact of HRU delineation became greater when a higher stream order was used to delineate the watershed. As a higher stream order was used, fewer streams were recognized in SWAT simulations, which resulted in less sediment routing and channel processes. Less channel processes lead to less deposition in the channels in this case, thus high sediment losses were obtained at the watershed outlet. However, less channel processes lead to less in-stream N processes in this case, thus less TN losses. Overall, the SWAT simulations performed the best when the 2nd stream order was used for delineations comparing with USGS observed data, followed by the 3rd stream order. Therefore, in order to fully depict the watershed characteristics to perform SWAT simulations, a stream order that is higher than 3rd stream order is not suggested for watershed delineation.
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