River Basin Scale Analysis on the Return Ratio of Diverted Water from Irrigated Paddy Areas

Water flows in irrigated paddy areas are complicated not only by the substantial volumes involved but also by repeated cycles of diversion and return flow. A spatially explicit hydrological model that simulates water diversion and return processes in irrigated paddy areas was used to evaluate the return ratio of diverted water to river basin. Assuming complete mixing of the rainfall and irrigated water over the entire irrigation period, the time-averaged return flow was calculated using the simulated net drainage from the irrigated areas. The return ratio, which is defined as the fraction of the total return flow volume to the total diverted water volume, was 48.5% (ranging from 42.3 to 52.1%) in our case study. These values suggest that most of the water diverted to the irrigated paddies will be discharged except that which is lost through evapotranspiration, because we consider the return ratio during a sufficiently long period and for large irrigation areas. While the largest and smallest return ratios were obtained in the driest and wettest years, respectively, the correlation between rainfall amount and return ratio was not significant, suggesting the need for a more appropriate averaging period that accounts for the short-term variabilities in return flow processes.