Evapotranspiration and crop coefficients of wheat and sorghum

Wheat and sorghum are important cereal crops next only to rice in India and several other Asian countries. Improved water management requires accurate scheduling of irrigation, which in turn requires an accurate measurement o f daily crop evapotranspiration ETc. Thus, the first objective of this stud y was to measure daily, weekly, and seasonal crop ETc of wheat and sorghum directly from sensitive weighing-type lysimeters. Experiments were conducte d in a set of two electronic weighing-type lysimeters, 2 X 2 m in surface a rea and 2-m deep, to measure the hourly evapotranspiration of wheat and sor ghum from 1991 to 1995 at Karnal, India. The average daily ETc of wheat var ied from <1 mm/d in the early growing period to >4 mm/d at milking stage. T he peak ETc of wheat was 4.6 mm/d and it occurred 16 weeks after sowing at the reproductive growth stage when leaf area index was 3.5. The measured se asonal ETc of wheat was 336 mm. In the case of sorghum, ETc was 3 mm/d at t he initial stage, achieved a peak value of 6 mm/d between 8 and 9 weeks aft er sowing and declined to 4 mm/d during the maturity stage. The measured se asonal ET, of sorghum was 495 mm. Precise information on crop coefficients for estimating ETc, which is required for regional scale irrigation plannin g, is lacking in developing countries. Therefore, the second objective of t his paper was to develop crop coefficients K-c for wheat and sorghum from E Tc measurements and weather data. The estimated values of K-c for wheat by the Pennan-Monteith method at the four crop growth stages (initial, crop de velopment, reproductive, and maturity) were 0.5, 1.36, 1.24, and 0.42, resp ectively, and for sorghum the K-c values at the four growth stages were 0.5 3, 0.82, 1.24, and 0.85, respectively. In the case of these two crops, actu al K-c values are significantly different from those suggested by the Food and Agricultural Organization of the United Nations indicating the need for generating these values at the local/regional level. The third objective o f this paper was to investigate the relationship between standard Food and Agricultural Organization of the United Nations Penman-Monteith, and other ETr methods.