Improved Surface Volume Estimates for Surface Irrigation Volume-Balance Calculations

This article reviews procedures for estimating surface storage in surface irrigation volume balance calculations. Those procedures are based on the assumption of a power law relationship for flow depth as a function of distance along the stream. The analysis uses zero-inertia simulation and a system of dimensionless variables to examine how the depth profile varies as a function of hydraulic conditions when infiltration is given by the empirical extended Kostiakov equation. Alternatives for approximating the exponent of the depth profile power law ($\beta$) are suggested. The magnitude of the resulting errors relative to zero-inertia model predictions is quantified. Results show that the range of variation for the parameter $\beta$ increases with field slope, with increasing advance length relative to the maximum advance distance, and when infiltration rates are relatively constant with time during the irrigation event. Estimating $\beta$ as a function of advance distance is most challenging under these conditions. Potentially large errors in the determination of $\beta$ do not undermine the proposed procedures when the surface volume represents only a small fraction of the applied volume. Users of volume balance procedures need to be aware of conditions in which uncertain surface volume calculations can lead to potentially large volume balance errors and, thus, in which results need to be interpreted carefully.