Examination of the water balance of irrigated paddy fields in SWAT 2009 using the curve number procedure and the pothole module

The Soil and Water Assessment Tool (SWAT), a basin-scale hydrological and water quality simulation model, has become popular in Asia for assessing the impacts of land use and human activities including paddy rice (Oryza sativa L.) cultivation which is a typical agricultural management system in Asia. The water ponding and drainage management in paddy fields should significantly affect the regional hydrology and water quality; however, the suitability of SWAT for simulating paddy hydrology at a field scale has not been thoroughly examined. In this study, the water balance of irrigated paddy fields in SWAT was examined for a small watershed where actual daily irrigation data were available. Two approaches available in SWAT to calculate hydrology in a watershed containing paddy fields, the curve number procedure and the pothole module, were applied with the regional paddy rice management standard. The water balance components estimated using the pothole module were significantly different from the actual hydrology in paddy fields. The estimated percolation of water was zero on most days even under ponded water conditions. Any of the percolation, surface runoff and evapotranspiration (ET) was estimated to be zero during the drainage period. The estimated ET was too small on a number of days during the ponding period. As a result, the watershed-scale Nash-Sutcliffe model efficiency (NSE) for the daily river flow rate at the outlet of the watershed was less than zero, indicating low model efficiency. On the contrary, no significant problems were apparently found in the estimated water balance components in paddy fields using the curve number procedure, yielding a higher NSE value of 0.58 at the watershed scale. However, the curve number procedure that in principle cannot simulate the ponded water conditions is obviously impossible to use to reflect the various paddy water management scenarios in the field. In conclusion, neither of these two approaches is suitable for simulating paddy field hydrology, indicating the need for the development of a paddy module in SWAT.