Modeling water flows in a serial irrigation reservoir system considering irrigation return flows and reservoir operations

•We presented a modeling system that simulates water flows.•Water flows in a serial irrigation reservoir system were quantified.•The SSARR model's parameters were automatically calibrated using the SCE-UA.•Return flow rates of irrigation water ranged from 28.0% to 35.0%.•Joint-operation rules were successfully assessed using the modeling system. A water flows network modeling system was constructed to quantify water flows in the Balan Watershed, South Korea, which has two serial irrigation reservoirs. A modified version of the Synthetic Streamflow and Reservoir Regulation (SSARR) model was used to simulate the water flows, taking into account irrigation return flows and reservoir operations. The model parameters were calibrated using a heuristic search method, Shuffled Complex Evolution method developed at the University of Arizona (SCE-UA). The simulated streamflows and reservoir water levels were in good agreement with the observed. The irrigation return flows from paddy fields considerably affected the flow regimes of the streams. Specifically, the return flow rates of the irrigation water ranged from 28.0% to 35.0%. In addition, the water supply capacities of the irrigation reservoirs were evaluated using the modeling system, and joint operation rules were assessed. The results showed that these could have negative influences on the water supply of existing irrigation districts, and the potential effects of these rules on water management should be investigated. The modeling system is useful for identifying the states of water flows in multiple irrigation reservoir systems and for providing information about reservoir operations to water managers and stakeholders.