Development and evaluation of a paddy module for improving hydrological simulation in SWAT

The soiland water assessment tool(SWAT) is becoming a popular tool for modeling watershed-scale hydrological and chemical transport in Asia, where paddy rice is cultivated in typical agricultural management systems. In this study, a paddy module was developed by modifying an algorithm designedfor pothole landscapes in SWAT. To simulate the percolationprocesses in paddy fields, a new parameter, the ‘potential percolationrate of the paddy field,’ was introduced which determines the upper limit of the rate of percolationinto the subsoil. The potential percolationrate was calibrated to fit the observed flow rate of a stream. In addition, the ponding-releasing process was varied to simulate a winter paddy field. Moreover, the irrigation process was modified to avoid overflows from paddy fieldsduring irrigation management. Furthermore, the evaporation process was modified in accordance with the evaporation rate observed at a paddy field. The developed paddy module was tested by applying it to a 3km2 watershedin which paddy fieldscomprise 18% of the total area. It was concluded that the water balance in the irrigated paddy fieldswas reasonably modeled by the modified SWAT with the developed paddy module and that the modified SWAT is effective for watershed-scale modeling for watershedscontaining paddy fields.