It is very important to control and schedule irrigation inside greenhouses to achieve the maximum yield and to prevent soil-borne diseases. Irrigation of greenhouse crops is mainly managed by farmer's experiences. The main objective of this study was to identify the cucumber water requirement inside greenhouses located in an arid region in comparison to the outside requirement. Two different structures of greenhouses, seawater and conventional greenhouse were used in this study. Four lysimeters were installed in each greenhouse. Irrigation was controlled and scheduled based on drainage collected at the bottom of the lysimeters. Sensors of relative humidity, temperature and solar radiation were installed at three different locations inside each greenhouse. Wind speed was measured manually using hand held anemometer. Outside weather parameters were obtained from a weather station installed outside of each location. Class A evaporation pans were installed both inside and outside of each structure. Daily water consumption from both irrigation system and cooling pads was measured using water meters. Daily drainage was also collected from different lysimeters to calculate the actual crop evapotranspiration. Data obtained showed that SWGH structure was able to reduce the ambient temperature by 4.8°C while CGH reduced it by 7.4°C. Both greenhouses increased inside relative humidity by 20-23% more than ambient humidity. Relative humidity inside SWGH was always higher than inside CGH; hence SWGH was more favourable for cucumber growth. Solar radiation inside both structures was always high (4.3-4.7 kW/m2) which indicated that shade or different plastic cover is required to reduce solar radiation for better cucumber growth. Penman-monteith (P-M) equation was used to calculate reference evapotranspiration inside CGH and SWGH and the result obtained was an average of 5.5 and 4.3 mm/day respectively. The Kpan factor inside and outside CGH was adjusted based on P-M equation. The results obtained showed that Kpan inside and outside CGH was 0.82 and 0.9 respectively. The value of Kpan outside SWGH was 0.78.