A watershed model was constructed using the Hydrological Simulation Program Fortran to predict the water quality, especially chlorophyll-a concentraion, at major tributaries of the Nakdong River basin, Korea. The BOD export loads for each land use in HSPF model were estimated at 1.47~8.64 kg/km 2 /day; these values were similar to the domestic monitoring export loads. The T-N and T-P export loads were estimated at 0.618~3.942 kg/km 2 /day and 0.047~0.246 kg/km 2 /day, slightly less than the domestic monitoring data but within the range of foreign literature values. The model was calibrated at major tributaries for a threeyear period (2008 to 2010). The deviation values ranged from -31.5~1.6% of chlorophyll-a, -24.0~2.2% of T-N, and -5.7~34.8% of T-P. The root mean square error (RMSE) ranged from 4.3~44.4 ug/L for chlorophyll-a, -0.6~1.5 mg/L for T-N, and 0.04~0.18 mg/L for T-P, which indicates good calibration results. The operational water quality forecasting results for chlorophyll-a presented in this study were in good agreement with measured data and had an accuracy similar with model calibration results.