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 of daily crop evapotranspiration ET_c. Thus, the first objective of this study was to measure daily, weekly, and seasonal crop ET_c of wheat and sorghum directly from sensitive weighing-type lysimeters. Experiments were conducted in a set of two electronic weighing-type lysimeters, 2 × 2 m in surface area and 2-m deep, to measure the hourly evapotranspiration of wheat and sorghum from 1991 to 1995 at Karnal, India. The average daily ET_c of wheat varied from <1 mm/d in the early growing period to >4 mm/d at milking stage. The peak ET_c 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 seasonal ET_c of wheat was 336 mm. In the case of sorghum, ET_c was 3 mm/d at the initial stage, achieved a peak value of 6 mm/d between 8 and 9 weeks after sowing and declined to 4 mm/d during the maturity stage. The measured seasonal ET_c of sorghum was 495 mm. Precise information on crop coefficients for estimating ET_c, which is required for regional scale irrigation planning, is lacking in developing countries. Therefore, the second objective of this paper was to develop crop coefficients K_c for wheat and sorghum from ET_c measurements and weather data. The estimated values of K_c for wheat by the Penman-Monteith method at the four crop growth stages (initial, crop development, reproductive, and maturity) were 0.5, 1.36, 1.24, and 0.42, respectively, and for sorghum the K_c values at the four growth stages were 0.53, 0.82, 1.24, and 0.85, respectively. In the case of these two crops, actual 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 of this paper was to investigate the relationship between standard Food and Agricultural Organization of the United Nations Penman-Monteith, and other ET_r methods.