SCHEDULING DEFICIT WHEAT IRRIGATION WITH DATA FROM AN EVAPOTRANSPIRATION NETWORK

A twoyear deficit irrigation study was conducted to determine the water use efficiency of winter wheat (Triticumaestivum L.) irrigated with fractions of crop evapotranspiration during the spring irrigation season. Fully irrigatedtreatments received sufficient irrigation to meet crop evapotranspiration, as calculated by a PenmanMonteith grassreference evapotranspiration equation and locally derived crop coefficients. Deficit irrigation treatments received 0%, 25%,50%, or 75% of the fully irrigated amount on the same days. Irrigation was applied with a lateral move sprinkler systemequipped with two LEPA methods (doubleended socks and bubblers) and two spray methods (abovecanopy spray andoverhead spray). All four sprinkler devices were spaced 1.52 m apart and located at the appropriate height for the sprinklermethod. Cultural practices were similar to those used for highyield wheat production in the southern Great Plains. Grainyields increased significantly with irrigation amount but not with sprinkler method. The full and 50% irrigation treatmentsaveraged 7.00 and 6.32 Mg/ha for the two years. With 50% irrigation, seasonal water use efficiency averaged 0.95 kg/m 3 ,and spring irrigation water use efficiency averaged 1.70 kg/m 3 , both being larger than for all other irrigation amounts. Thewinter wheat, deficitirrigated with fractions of spring crop ET, efficiently utilized seasonal water consisting of irrigation,rainfall, and stored soil water over the entire 25% to 100% irrigation range. Spring irrigation was most efficiently appliedat the 25% and 50% irrigation amounts. Deficit irrigation, as a fraction of spring ET, is well adapted to an irrigation watersupply from lowproducing wells and provides efficient water use over a range of both rainfall and irrigation amounts.