General Equation for Recession of Water in Border Irrigation

Determination of recession curve for evaluation of border irrigation is of prime importance. Due to variability of soil types, as well as initial and boundary conditions in border irrigation, water advance rate varies considerably indifferent borders. In recent years,scaling approach has been adopted to reduce number of measurements, and to formulate governing equations. The aim of this study was to develop a unique equation, independent of initial and boundary conditions, for evaluation of water recession in borders,by using scaling approach. For this purpose,we considered kinematic wave model and Philip’s 2-termequation for border irrigation. The scaling factor was defined, so as the kinematic wave equation is independent of initial conditions and soil properties.Results showed that the scaling factor follows Log-normal distribution. A quadratic equation was fitted to define recession of water. The equation was evaluated for 25 borders under cultivated and uncultivated and inciuding different slopes of 0.001 to 0.005, roughness of 0.017 to 0.211, length of 91.4 to 100 m and flow rate of 0.08 to 0.16 m3/min.m.For indices of coefficient of determination (R2), comparison with perfect line of agreement, percentage of model prediction error (Er), and percentage of average relative error (Ea) were used. The results showed that the proposed equation based on scaling performed well (R2 =0.94, λ=1.046, Er=4.6 and Ea=7.52). Results showed that the mean absolute percent error for recession time was 6.78 and 8.08 for uncultivated andcultivated border, respectively,which are nearly the same. On the overall, border recession was simulated under scaling method which is simple and independent of soil type.