Reuse of return flows and its scale effect in irrigation systems based on modified SWAT model

Return flows in irrigation systems are often reused contributing to overall efficiency. To investigate the fate of return flows and the scale effects of reuses, the SWAT (Soil and Water Assessment Tool) model was modified to better represent the characteristics of paddy rice irrigation systems, which includes a simulation module for automatic multi-source irrigation (AMSIM). The modified SWAT model was used to simulate the hydrological processes in the Yangshudang (YSD) watershed of the Zhanghe Irrigation System (ZIS) in China. Furthermore, we proposed a method to calculate the amounts of return flows and the reused amount based on the output of the model. The sub-basins nesting method was used to divide the study area into six scales. We calculated the rainfall & irrigation water reuse rates (eta(I+P)) and the irrigation water reuse rates (eta(I)) at different scales and analyzed the changes of these two indicators over different scales. The results revealed that the modified SWAT model succeeded in simulating hydrological processes in a paddy rice irrigation system. eta(I+P) and eta(I) increased with the increase of scale. eta(I+P) was higher in the wet years and lower in the dry years, while n i was higher in the dry years and lower in the wet years. The reason for increase of eta(I+P) and eta(I) as the scales increases were due to the fact that the return flows was repeatedly intercepted by downstream paddy fields, farm ponds, and drainage channels at larger scales, This reuse rates however reach the upper limit at a scale of 3500 ha, after which eta(I+P) and eta(I) no longer increase.