SEASONAL AND MAXIMUM DAILY EVAPOTRANSPIRATION OF IRRIGATED WINTER-WHEAT, SORGHUM, AND CORN - SOUTHERN HIGH-PLAINS

Evapotranspiration (ET) is basic information required for irrigation s cheduling and for crop growth simulation models. However many ET model s have not been tested for their applicability to the Southern High Pl ains. In this study, ET was measured for irrigated winter wheat (Triti cum aestivum L.), sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.) at Bushland, Texas, in the semi-and Southern High Plains for various growing seasons from 1988 through 1993. Weighing lysimeters c ontaining Pullman clay loam (Torrertic Paleustolls) monoliths were use d to measure ET. Weather data from a nearby station were used to compu te daily ET values for several widely used reference or potential ET e quations. These computed values were then compared by linear regressio n with the measured ET values for periods of full groundcover (LA1 gre ater than or equal to 3) and with adequate soil water to permit maximu m El: Measured mean seasonal ET was 877 mm for winter wheat, 771 mm fo r corn, and 578 mm for sorghum. Maximum daily ET rates rarely exceeded 10 mm d(-1) for the sorghum or corn crops, except for a few days duri ng a brief period of strong advection in 1990 when corn ET rates excee ded 12 mm d(-1). Maximum daily ET for wheat exceeded 10 mm d(-1) on ma ny days during the three semens due to the high vapor pressure deficit s and wind speeds at Bushland during the spring and early summer The P enman-Monteith equation performed consistently better than other combi nation and/or radiation/temperature based ET equations in estimating m aximum daily ET rates for these crops. The leaf diffusion resistance ( r(l)) permitting the best agreement between predicted and lysimetrical ly determined ET was 280 s m(-1) for sorghum, 252 s m(-1) for corn, an d 135 s m(-1) for wheat when using the relationship of r(c) r(l)/(0.5 LA1) where LA1 is the leaf area index and r(c) is canopy resistance in s m(-1) These results indicate that the greater seasonal water use by irrigated corn compared with sorghum in this environment was due main ly to the differences in planting date and growing season length since the ''apparent'' leaf resistances were similar The even higher season al and maximum daily water use of irrigated winter wheat compared with corn and sorghum was due to its longer growing season, its lower leaf resistance, and the high evaporative demand in the spring in the Sout hern High Plains.