ECONOMIC OPTIMIZATION OF FURROW IRRIGATION

A seasonal furrow irrigation model was constructed from soil moisture, kinematic-wave hydraulic, and economic optimization models to study t he effects of heterogeneity in water balance, soil water holding prope rties and rooting depth, and infiltration functions on furrow irrigati on design (flow rate and cutoff time), bean yield, and net return to w ater. Irrigation designs achieving nearly 100% irrigation adequacies w ere unchanged by heterogeneity in water balance, soil water properties , and rooting depth, but both bean yield and net return to water decre ased 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 chara cteristics but consistently decreased with increasing irrigation inter val. Net return to water was less for spatially variable infiltration functions as compared to homogeneous infiltration conditions. Using me an evapotranspiration (ET) of a grass reference crop (ETo) resulted in slightly higher bean yield and net return to water as compared to obs erved 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 c riteria (80% irrigation adequacy at cutoff time). Both ETo conditions led to essentially the same design inflow rates. Errors in inflow volu me were less than 12 and 3.5%, respectively, for individual irrigation s and on a seasonal basis. Irrigation scheduling and surface irrigatio n design can be forecast at the beginning of the growing season for th e conditions studied using historical mean ETo if spatial and temporal variability of infiltration are considered.