Applying SWAT for TMDL programs to a small watershed containing rice paddy fields

The main objective of this study was to apply the soil and water assessment tool (SWAT) to develop total maximum daily load (TMDL) programs for a small watershed containing rice paddy fields in the Republic of Korea. The total maximum daily load system (TOLOS), based on ArcView SWAT (AVSWAT) using geographic information system (GIS) and remote sensing (RS), was incorporated with the SWAT model to simulate water balance and water quality from irrigated paddy fields. Model parameters related to hydrology and water quality were calibrated and validated by comparing model predictions with the field data collected for 4 years. The results indicated that the simulated runoff and water quality values were acceptably close to the observed data. Water quality parameters also appeared to be reasonably comparable to the field data. The applicability of the system for TMDL development was tested in terms of TMDL allocations and the redistribution of load reductions to 23 sub-areas within the watershed. The results demonstrated that the urbanized subwatershed #2, with residences and other community activities, required the largest allocation of road reduction. TOLOS thus appears to be a useful tool for planning TMDL for a small watershed including rice paddies in Korea.