The combination of aridity, cxtensive urbanization and expansion of irigatod farming have brought about substantial water demand increase and intensified the gap between rising water demands and limited existing water supply in the Sultanate of Oman. Greenhouse faming has been adopted as part of the govermment efforts to conserve and augment water supplies. Greenhouse cropping in Oman is mostly practiced at times when crops could tolerate outside conditions. Experiments were conducted for two seasons to determine the effect of four imigation rates (1, 2, 3 and 4 mm/day in greenhouse and 3, 4, 6 and 8 mm/day in field) and evaporative cooling on yields of cucumbers, total water use efficiency and cost. Results showed an asymptotic increase of greenhouse cucumber yield with increase in water applications from 1 to 4 mm/day. The 2 mm/day applications optimized yields (kg/m3), whereas 3 mm/day applications maximized yields, with no significant difference from the 2 mm/day applications being observed. Yields were increased by 135% from 27 to 63 t/ha when irigation was increased from l mm/day to 3 mm/day respectively, and declined therafter. Field cucumber yields increased linearly as the irigation water was increased from 3 mm/day to 8 mm/day. Yields were optimized at 6 mm/day applications (35 t/ha). The 8 mm/day maximized yields (40 t/ha) but fell short of the optimum 2 mm/day yields (53 t/ha) obtained in the greenhouse. Optimum yields were obtained at an average crop factor (K.) of 0.58 ET, and 1.55 ET, in the greenhouse and the field respectively, indicating that water requirements for the groenhouse cucumber is about one third of that in the open field. The imigation water Use efficiency was higher in the greenhouse than that of the open field because of the lower water requirements and higher yields of cucumbers. But the total water use efliciency approached that of the field as the rates were maximized, because of the high quantity of water used in evaporative cooling. The average cooling pad water use was found to be 79.1 1 m-2 day-1 of pad area. In the greenhouse, imigation water use efficiency was highest with 2 mmday applications (31.3 kg/m3), whereas in the open field the highest imigation water use efficiency obtained was only 7.6 kg/m3 tor the 6 mm/day applications. Treatments close to the cooling pads of the greenhouse were more moist than the amount of water applied would have indicated, but suppressod yields obtained were attributed to the high soil salinity levels washed from the pads and possibly due to the chilling temperatures incurred at night.