The purpose of this study was to evaluate the effects of nonpoint source pollutants on water quality using the HSPF (Hydrological Simulation Program-Fortran) model. Hapcheon dam watershed was selected for study watershed and input data for the HSPF model were established using the watershed, stream, land use, and a digital elevation map and meteorological data and others. The HSPF model was calibrated and validated using the observed runoff data from 2000 to 2016. Parameters for runoff calibration were selected based on the user’s manual and references, and parameter calibration was done by trial and error method. The R2 (determination ceofficient), RMSE (root mean square error), RMAE (relative mean absolute error), and NSE (nash sutcliffe efficiency coefficient) were used to evaluate the model’s performance. Calibration and validation results showed that annual mean runoff was within a ±4% error. The hydrology model performance criteria for calibration and validation showed that R2 ranged from 0.78 to 0.83, RMSE from 2.55 to 2.76 mm/day, RMAE from 0.46 to 0.48 mm/day, and NSE from 0.81 to 0.82. For water quality simulation, we calculated the occurrence load and the emission load of the population, livestock, industry, landuse following the guideline provided by Ministry of Environment. The pollutant data for the study watershed were obtained from NIER (National Institute of Environmental Research). Monthly emission load were estimated by applying the delivery rate. The water quality parameters of HSPF model were also calibrated and validated using the same period of hydrology calibration. Calibration and validation results showed that annual mean BOD has a difference of 0.22 mg/L and an error of ±13%, T-N has a difference of 0.66 mg/L and an error of ±16% and T-P has a difference of 0.027 mg/L and an error of ±13%. To evaluate the nonpoint pollutants management effects, we applied livestock reduction scenarios because livestock consist the largest portion of pollutants. As a result of the 20% of livestock reduction, BOD, T-N, and T-P decreased by 3%, 1%, and 3%, respectively. When 40% of livestock reduction applied, BOD, T-N, and T-P decreased by 5%, 3%, and 4%, respectively. Based on the results of this study, effective pollutant management methods can be applied to improve the water quality and achieve the target water quality of Hapcheon dam watershed.