The present study was conducted to compare the reference evapotranspiration (ETo), crop coefficient (k_c), applied irrigation water (AIW) and water productivity (WP) under the open field and greenhouse conditions for cucumber crops. Precise estimation of ETo, k_c,ET_c daily is important to apply water through a drip system in the open field and greenhouse. The ETo was estimated in the open field using “FAO-56 Penman-Monteith method" whereas, inside the greenhouse ETo was estimated using “Makkink FAO-24". The  value represents crop specific water use and it is a requirement for accurate estimation of irrigation requirements for cucumber crops. The accumulated heat units (HUs) were used to determine the k_c daily for each growing stage. The results revealed that the monthly ETo values under greenhouse were less than those under open field for all months. Mean values of k_c in the open field during the Four growth stage (Initial, Development, Mid and late stage) was 0.30, 0.60, 1.01 and 0.81 while it was 0.32, 0.79, 1.03 and 0.56 under greenhouse, where the Mid stage recorded the highest value in both cultivation systems. The estimated results of total AIW indicated that significant irrigation water saving occurs through the cultivation of cucumber under the greenhouse as compared to open field cultivation by nearly 33.5 % as the value of AIW inside the greenhouse was 903.9 m³/fed, while the value outside was 1359.7 m³/fed. Regarding the WP result, values were 33.46 and 15.97 kg/m³ under the greenhouse and open field conditions respectively.