Numerical Simulations of Discharge Change Propagation in Open Channel Irrigation Systems

The purpose of this study is to investigate the arrival times of discharge change propagation from upstream to downstream for improvement of irrigation efficiency. The arrival time of discharge change propagation was defined as the time required at a selected section of a canal system under steady flow conditions with a discharge Q to change to steady flow conditions with a discharge of Q ± ΔQ when the upstream end discharge is increased or decreased by ± ΔQ. A one–dimensional gradually varied flow model was constructed and numerically calculated using an explicit finite difference method of the Leapfrog scheme in an artificial irrigation canal of Horikawa–Yousui Irrigation Canal, Japan, and a natural river of Kim Son River, Vietnam. The simulations for validation and scenario analysis were conducted in irrigation periods of 2018 and 2019 for both the canal and river, respectively. In the validation, root mean square errors (RMSE) were used to evaluate the simulation performance, and the calculated RMSEs indicated a good result considering the slight errors between the simulated and observed data. In the scenario analysis, the observed maximum and minimum discharges in both the canal and river in the irrigation periods were utilized in setting scenarios, and the simulations were conducted considering diversion discharges and no diversion discharges. On average, the results of scenario analysis indicated that the arrival time of discharge change propagation from upstream to downstream for the Horikawa–Yousui Irrigation Canal was approximately 1–1.5 h for a length of 4.5 km. However, the discharge change propagation was approximately 10 to 20 h for the Kim Son River that had a length of 14.2 km to achieve a nearly steady state at the downstream end. The most important finding was that higher discharge increasing or decreasing normally led to higher arrival times of discharge change propagation from upstream to downstream. Besides, a comparison between the celerity of surge propagation and the average velocity of discharge change propagation implied that the effects on these propagations due to longer or shorter time of opening gate led to a gradual or sudden discharge change propagation. So far, the one–dimensional gradually varied flow model with its advantages could be applied for a network system with nodes or a cycling route of channels.