Determining the Optimum Reach Length for Numerical Simulation of Unsteady Flow in Irrigation Networks

Unsteady flow occurs in irrigation systems due to the water delivery operation and has direct effect on hydraulic performance. The most important characteristics of unsteady flow in open channels is the response time that is usually used to provide the irrigation schedule. There are some methods for numerical simulation of the unsteady flow in open channels, such as the Preissman four-point scheme. In this study, a computer model was developed to simulate the unsteady flow based on the Preissman scheme and verified with the observed data. The effects of the space step length, Δx, and time weighting coefficient, θ, on accuracy of the response time’s calculation were investigated using the numerical model. By increasing the computational reach length, the accuracy of the calculation of response time was reduced. Also suitable value of the weighting coefficient θ for simulation of the flow in irrigation network was estimated as 0.55. For determining the optimum computational reach length, 130 conditions of unsteady flow were simulated with numerical model, and the influences of Froude number, KF2, side slopes and the percentage of the inflow rate variations on optimum Δx were investigated. The results showed that KF2 and side slopes had a direct and Froude number had an inverse relationship with the optimum computational reach length, and the rate of sudden changes of discharge had no effect on optimum Δx. In this study, a nonlinear relationship for calculating Δx was presented with the Mean Percentage Error value of less than 4.3%.