Modeling Hydrologic Processes and NPS Pollution in a Small Watershed in Subhumid Subtropics Using SWAT

The soil and water assessment tool (SWAT) has been calibrated and validated to predict stream flow, and to transport sediment and non-point source (NPS) pollutants to the downstream water resources from a small (1,695 ha) watershed in sub-humid subtropics that receives variable monsoon rains. Observed rainfall, temperature, stream flow, and sediment yield data for three years have been utilized to test the model's prediction capability for daily stream flow and sediment yield during the monsoon months from June to October. Because of the variability of monsoon rains, the model has been calibrated for a normal rainfall year ($\mathrm{M}\mathrm{\text{-}}\mathrm{SD}<\mathrm{RF}<\mathrm{M}+\mathrm{SD}$ and $\mathrm{RF}={1.2}^{*}\mathrm{M}$) and then validated for a relatively dry ($\mathrm{RF}<\mathrm{M}\mathrm{\text{-}}\mathrm{SD}$) and a medium rainfall year ($\mathrm{RF}>\mathrm{M}\mathrm{\text{-}}\mathrm{SD}$ and $\mathrm{RF}<0.8\mathrm{M}$). The results reveal that a calibrated model for a normal rainfall year can be used successfully for predicting hydrologic processes and NPS pollution for a relatively dry rainfall year. However, for the medium rainfall year the model prediction shows more deviations from the measured values. The Nash-Sutcliffe efficiencies in dry and medium rainfall years are 0.70 and 0.62 for daily stream flow and 0.73 and 0.69 for daily sediment yield. NPS pollutants simulation results indicate that a calibrated SWAT model is used in estimating hydrologic responses related to water quality problems of watersheds situated in monsoon regions in which the nature of rainfall shows varying characteristics every year. The results of the study have implications for watershed management to reduce the sediment and NPS pollutants load into downstream water bodies.