Development of the HSPF-Paddy model to estimate watershed pollutant loads in paddy farming regions

The Hydrological Simulation Program-FORTRAN (HSPF) was modified to simulate watershed pollutant loads in paddy farming regions. Dike height and the Dirac delta function were added to a modified HSPF code (HSPF-Paddy) to incorporate the behavior of ponded water and nutrients in rice paddy fields. HSPF-Paddy was validated by examining its application to a rice paddy plot (3000 m 2) and a watershed (1803 km 2) that contained a significant proportion of rice paddy fields (30%). The model efficiency at the plot scale was very high showing 0.52, 0.93, and 0.93 for ponded water depth, and accumulation of total nitrogen, and total phosphorus, respectively, and simulated values and observed data were in close agreement. Model efficiency was somewhat lower at the watershed scale showing 0.82, 0.78, 0.44, 0.61, and 0.63 for stream flow, temperature, 5-day biochemical oxygen demand, total nitrogen, and total phosphorus, respectively. However, model performance was generally within expectations, and generally model efficiency greater than 0.5 is deemed acceptable considering the complexity of the watershed in terms of size and land use. This comprehensive, dynamic model is applicable to a wide range of scales, from plot to watershed, and can be used to simulate various paddy field and watershed environmental conditions, including loads from point and nonpoint source pollution. It may be a practical alternative to the unit-load method for estimates of watershed pollution loads, especially in paddy farming regions, where nonpoint source pollution loads may be poorly estimated because of highly site-specific conditions. The application of HSPF-Paddy is recommended for watershed management and best management practices (BMPs) evaluation in paddy farming regions.