Deterministic optimization of irrigation water allocation in a low-lying paddy area with creek networks

In a low-lying paddy area with creek networks, cyclic use of irrigation return flow can be easily performed. Therefore, water supply in this system can be less than the sum of water requirement of all paddy fields in the area. The deterministic dynamic programming model was used for optimizing the water supply for the system. The model was first formulated with the objective function of minimizing water demand from creek-fields system on the main canal system. Time series of the existing and the optimized water supply to creeks, water application for paddy fields and the change of creek water depth were analyzed as a basis for clarifying the possibility of improving operation of water allocation in the irrigated area. It was shown that the optimum operation yielded better results than the actual condition, in which the effective use of water could be increased significantly. This paper also investigated the characteristics of content ratio of total return flow in creeks (CTRO) as one of indicators for examining the impact of minimizing irrigation water supply to the change of water quality. The results suggested that the further research is needed to take into account CTRO to maintain the quality of irrigation water that is suitable for paddy rice cultivation.