Effect of canal flow variability on irrigation performance.

Abstract

Flows in an irrigation canal were analyzed to evaluate the effects of lateral flow fluctuations on tailend spill losses and on-farm application losses. The flow variabilities at the farm turnouts due to lateral heading inflow variabilities were examined through CanalCAD modeling simulations of the Palm Lateral in the Imperial Irrigation District. The magnitude of flow fluctuations at the turnouts were dependent primarily on the localized flow conditions (i.e. flow depths) and types of structures at the bifurcations. Simulations of fluctuating lateral heading inflows with coefficients of variations (CV) ranging from 0.0 to 0.15 were evaluated to quantify the effect of inflow variability on tailend spillage. A positive relationship between lateral inflow fluctuations and tailend spill was obtained. The increased spill as inflow variability increased was also related to the localized flow conditions at the bifurcation structures. Significantly higher spill ratios were evident only when inflow CV's were greater than 0.10 and the head differences at the turnouts were relatively large. On the farm, 104 recorded irrigation events from the District were randomly selected. Multiple linear regression analyses were performed on the field data with the tailwater quantities (tailwater volume and fraction) as the dependent variables and the inflow variability (CV) and some other field parameters (e.g. inflow volume, date of irrigation, and crop type, etc.) as independent variables. The best-fit regressor variables were CV, delivery volume, month, and crop for the regression equation to predict tailwater volume. It had an R-squared value of 0.393 which indicated a weak correlation. Two on-farm management conditions were simulated and analyzed for border irrigation using the SRFR model. One set of inflow hydrographs had equal flow volume and duration and were used in simulations with "irrigation by the clock" (i.e. fixed application duration) management method. The results showed that the inflow variability did not affect the irrigation performance (i.e. tailwater ratio, distribution uniformity, application efficiency, etc.). For the other set of hydrographs, the flow volumes were varied. But, the hydrographs were applied with an irrigation method where the application time was determined by the advance of the wetting front. This "irrigation by the advance" method yielded similar performance among the different ranges of inflow variability.