Economic optimization of furrow irrigation

A seasonal furrow irrigation model was constructed from soil moisture, kinematic-wave hydraulic, and economic optimization models to study the effects of heterogeneity in water balance, soil water holding properties and rooting depth, and infiltration functions on furrow irrigation design (flow rate and cutoff time), bean yield, and net return to water. Irrigation designs achieving nearly 100% irrigation adequacies were unchanged by heterogeneity in water balance, soil water properties, and rooting depth, but both bean yield and net return to water decreased with increasing heterogeneity. Irrigation designs were sensitive to infiltration characteristics and to irrigation interval. At a given irrigation interval, bean yield was insensitive to infiltration characteristics but consistently decreased with increasing irrigation interval. Net return to water was less for spatially variable infiltration functions as compared to homogeneous infiltration conditions. Using mean evapotranspiration (ET) of a grass reference crop (ETo) resulted in slightly higher bean yield and net return to water as compared to observed ETo (1992 season values of grass reference crop ET). Spatial and temporal variability in infiltration gave the same optimal irrigation interval (10 d) under both ETo conditions for the chosen irrigation criteria (80% irrigation adequacy at cutoff time). Both ETo conditions led to essentially the same design inflow rates. Errors in inflow volume were less than 12 and 3.5%, respectively, for individual irrigations and on a seasonal basis. Irrigation scheduling and surface irrigation design can be forecast at the beginning of the growing season for the conditions studied using historical mean ETo if spatial and temporal variability of infiltration are considered.