Determination of Crop Water Requirements and Crop Coefficients for Onion Crops Using Lysimeter in the Semi‐Arid Regions of Ethiopia

ABSTRACT Accurate determination of crop water requirement (CWR) and crop coefficient (Kc) values is fundamental for optimizing irrigation in water‐scarce regions. This study aimed to establish locally calibrated Kc values and determine the CWR for onion ( Allium cepa L.) in the semi‐arid climate of the Central Rift Valley of Ethiopia. A 3‐year field experiment (2020–2023) was conducted using a drainage‐type lysimeter to directly measure crop evapotranspiration (ETc) through the water balance method. The results showed that the seasonal CWR for onion averaged 472 mm over the 3 years. The derived, site‐specific Kc values were 0.55 for the initial stage (Days 1–15), 0.84 for the development stage (Days 15–40), 1.13 for the mid‐season stage (Days 40–75) and 0.81 for the late‐season stage (Days 75–97). These values peaked at mid‐season due to full canopy cover. Notably, the local Kc values deviated from the standard FAO‐56 guidelines. This difference underscores the influence of local climatic conditions. The study concludes that the application of these locally validated Kc values is crucial for developing precise irrigation schedules, enhancing water productivity and promoting the sustainable intensification of onion production in semi‐arid areas of Ethiopia.