Seasonal and maximum daily evapotranspiration of irrigated winter wheat, sorghum, and corn - Southern High Plains

Abstract

Evapotranspiration (ET) is basic information required for irrigation scheduling and for crop growth simulation models. However, many ET models have not been tested for their applicability to the Southern High Plains. In this study, ET was measured for irrigated winter wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.) at Bushland, Texas, in the semi-arid Southern High Plains for various growing seasons from 1988 through 1993. Weighing lysimeters containing Pullman clay loam (Torrertic Paleustolls) monoliths were used to measure ET. Weather data from a nearby station were used to compute daily ET values for several widely used reference or potential ET equations. These computed values were then compared by linear regression with the measured ET values for periods of full groundcover (LAI≤3) and with adequate soil water to permit maximum ET. Measured mean seasonal ET was 877 mm for winter wheat, 771 mm for 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 during a brief period of strong advection in 1990 when corn ET rates exceeded 12 mm d-1. Maximum daily ET for wheat exceeded 10 mm d-1 on many days during the three seasons due to the high vapor pressure deficits and wind speeds at Bushland during the spring and early summer. The Penman-Monteith equation performed consistently better than other combination and/or radiation/temperature based ET equations in estimating maximum daily ET rates for these crops. The leaf diffusion resistance (r(l)) permitting the best agreement between predicted and lysimetrically determined ET was 280 s m-1 for sorghum, 252 s m-1 for corn, and 135 s m-1 for wheat when using the relationship of r(c) = r(l)/(0.5 LAI) where LAI 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 mainly to the differences in planting date and growing season length since the 'apparent' leaf resistances were similar. The even higher seasonal 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 Southern High Plains.