Crop coefficient and evaptranspiration of onion at Awash Melkassa, Central Rift Valley of Ethiopia

Improved water management through precise crop water requirement determinationis needed to improve the efficiency of water use in agricultural production. As aresult, appropriate irrigation scheduling which can lead to water saving, improvements inthe yield and income can be designed. In this study, three non-weighing lysimeters havingdimensions of 2 m×1 m×2 m were used to determine water requirement (ETc) and cropcoefficient (Kc) of onion (Bombay Red cultivar). Reference crop evapotranspiration (ETo)was determined using weather data recorded at the site. The measured ETc values were51.3 mm, 140.5 mm, 144.8 mm, and 53.9 mm during the initial, development, mid-seasonand late season growth stages respectively. Crop coefficient (Kc) values, calculated as ratioof ETc to ETo, were 0.47, 0.99, and 0.46 during the initial and mid-season stages and end oflate season. Furthermore, third-order polynomials were fitted well to predict the cropcoefficient values as functions of growing degree-days (GDD).