Nigusie Abebe*, Elias Kebede, Yonas Derese, Fikadu Robi and Kebede Nanesa
The development of crop coefficients (KC), the ratio of crop evapotranspiration (ETc) to reference evapotranspiration (ETo) is important for estimating irrigationwater requirements in relation to specific crop phenological development. This research was conducted to determine growth-stage-specific Kc and crop water use for onion at Werer Agricultural Research Center, Middle Awash Valley Ethiopia during the main season (July–October) and during cool cropping seasons (March–June) from 2012-2015. Three non weighing lysimeters of 1.6 m x 1.6 m in surface area and 2 m deep were used to measure crop water use and local weather data were used to determine the reference evapotranspiration (ETo).The results showed that the obtained Kc values for main cropping season planted onion during initial, crop development, mid season and late-season stages were 0.57, 0.78, 1.03 and 0.77 respectively. Meanwhile, the corresponding Kc values for cool cropping season planted onion were 0.49, 0.90, 1.01, and 0.79in the respective growth stages. The seasonal crop evapotranspiration was 525.83mm and 465.57mm during main and cool cropping season respectively. The measured Kc values were significantly different from the FAO-56 reported values. Therefore, local calibration of crop coefficients is an essential for efficient irrigation water management and precise water applications.
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