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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.

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The Soil and Water Assessment Tool (SWAT) is an ecohydrological watershed-scale model which was initially developed in the early 1990s to simulate the impacts of land use, management systems, and climate on hydrology and/or water quality. First adopted in the U.S., the use of the model then spread to Europe and then later to Asia and other regions. The range of applications that SWAT has been applied to have also expanded dramatically, which influenced ongoing model development which has been virtually continuous over the past two decades. A key component of many SWAT applications in Asia is accounting for rice paddy production that is common in some subregions within the continent. However, most of these studies do not provide explicit details of how rice production was simulated in SWAT. Other research has revealed that significant problems occur when trying to represent rice paddy systems in standard versions of SWAT, due to limitations in algorithms based on the runoff curve number approach or the pothole option. In response, key modifications have been made to SWAT in recent studies that have resulted in a more accurate representation of rice paddy systems. These developments point to the need for the incorporation of an enhanced rice paddy module within SWAT to better capture rice paddy hydrological and pollutant dynamics, which would support improved use of the model in Asia and other rice production regions. Subtopics related to simulating rice production in SWAT are discussed as follows: 1) an overview of global rice production; 2) history of SWAT development; 3) typical approaches for simulating rice production; 4) problems associated with the typical approaches; 5) recent code modifications to address deficiencies in replicating rice paddy systems; 6) recommendations for developing a standard rice paddy module for future SWAT codes.

No abstract