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 growthsimulation models. However, many ET models have not been tested for their applicability to the Southern High Plains. Inthis 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 1988through 1993. Weighing lysimeters containing Pullman clay loam (Torrertic Paleustolls) monoliths were used to measureET. Weather data from a nearby station were used to compute daily ET values for several widely used reference orpotential ET equations. These computed values were then compared by linear regression with the measured ET values forperiods of full groundcover (LAI=3) and with adequate soil water to permit maximum ET. Measured mean seasonal ETwas 877 mm for winter wheat, 771 mm for corn, and 578 mm for sorghum. Maximum daily ET rates rarely exceeded10 mm d1 for the sorghum or corn crops, except for a few days during a brief period of strong advection in 1990 whencorn ET rates exceeded 12 mm d1. Maximum daily ET for wheat exceeded 10 mm d1 on many days during the threeseasons due to the high vapor pressure deficits and wind speeds at Bushland during the spring and early summer. ThePenman-Monteith equation performed consistently better than other combination and/or radiation/temperature based ETequations in estimating maximum daily ET rates for these crops. The leaf diffusion resistance (rl ) permitting the bestagreement between predicted and lysimetrically determined ET was 280 s m1 for sorghum, 252 s m1 for corn, and135 s m1 for wheat when using the relationship of rc = rl /(0.5 LAI) where LAI is the leaf area index and rc is canopyresistance in s m1. These results indicate that the greater seasonal water use by irrigated corn compared with sorghum inthis environment was due mainly to the differences in planting date and growing season length since the apparent leafresistances were similar. The even higher seasonal and maximum daily water use of irrigated winter wheat compared withcorn and sorghum was due to its longer growing season, its lower leaf resistance, and the high evaporative demand in thespring in the Southern High Plains.