Development and test of SWAT for modeling hydrological processes in irrigation districts with paddy rice

The water movement in irrigation districts, especially for paddy rice cultivation, is characterized by complicated factors. Soil and Water Assessment Tool (SWAT) is a popular tool for understanding the hydro-agronomic processes. However, it fails to simulate the hydrological processes and crop yields in paddy rice areas. In this study, we develop the SWAT model by incorporating new processes for irrigation and drainage. The evapotranspiration process in paddy fields is simulated on the basis of water storage conditions, and a controlling irrigation scheme is introduced to manage the irrigation and drainage operations. The irrigation function of local water storages, such as ponds and reservoirs, is extended for these storages in order to provide water in a timely manner to paddy fields. Moreover, an agronomic model is incorporated to estimate crop yields when available data sets are not satisfactory. The model is tested in Zhanghe Irrigation District, China. The simulated runoff matches well to the measurements and the results indicate the developed model is preferable to the original edition of SWAT. The estimate of the paddy rice yield is acceptable and the dynamics of water balance components approximately characterize the state of water movements in paddy fields. Therefore, the developed framework for SWAT is practical and capable of representing the hydrological processes in this irrigation district. Further work is still needed to more broadly test the model in areas with paddy rice cultivation.