Determination of evapotranspiration and crop coef®cients of rice and sun ̄ower with lysimeter

Lysimeter experiments were conducted on rice during rainy season (July±October) and sun ̄ower during summer seasons (March±June) in a set of two electronic weighing type lysimeters of 2 m 2 m 2 m size to measure the hourly evapotranspiration of these crops from 1994 to 1995 at Karnal, India. The average weekly ET of rice varied from <3 mm per day in the early growing period to >6.6 mm per day at milking stage. The peak ETc was 6.61 mm per day and it occurred 11 weeks after transplanting at reproductive stage when LAI was 3.4. In case of sun ̄ower, ETc was <1.0 mm per day at the initial stage, achieved a peak value of 14.1 mm per day between 8 and 9 weeks after sowing (WAS) and it declined to 3 mm per day during maturity phase. Precise information on crop coef®cients, which is required for regional scale irrigation planning is lacking in Asian countries. Crop coef®cients (Kc) for rice and sun ̄ower from ETc measurements and weather data have been developed. The estimated values of crop coef®cient for rice at the four crop growth stages (initial, crop development, reproductive and maturity) were 1.15, 1.23, 1.14 and 1.02 and corresponding Kc values for sun ̄ower were 0.52, 1.1, 1.32 and 0.41. The estimated Kc values of sun ̄ower is 11.6±74.2% higher than the values suggested by FAO. Relationship between standard FAO±Penman±Monteith with other reference evapotranspiration methods has also been established. # 2000 Elsevier Science B.V. All rights reserved.