The Hydrological Simulation Program–FORTRAN (HSPF) is a comprehensive model that was developed to simulate many processes related to water flow and quality in watersheds of almost any size and complexity. Paddy rice fields often dominate extensive portions of the landscape in the Asian monsoon region. The hydrological and environmental conditions of paddy fields differ somewhat from those of other land uses, and HSPF may not adequately simulate watersheds in paddy farming regions. HSPF was previously modified to HSPF-Paddy; here, we examined the applicability of the modified model. The model was applied to simulate the water flow and quality of the Saemangeum watershed (2523 km2) in Korea, where paddy rice fields comprise about one-third of the total watershed area. Long-term monitoring data (5 years for water flow, 10 years for water quality) were used in the calibration and verification processes. Model performance was in the range of “very good” and “good” based on model efficiency (R2) and percent difference. The accuracy of the daily simulation was lower than that of monthly simulation for water flow. The water-quality simulation results were encouraging for this sizable watershed with mixed land uses; HSPF-Paddy proved adequate, and its application is recommended to simulate watershed processes in paddy farming regions.

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