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Abstract
The combination of aridity, extensive urbanization and expansion of irrigated fanning have brought about substantial wale demand increase and intensified the gap between rising water demands and limited existing water supply in the Sultanate of Oman. Creenhouse farming has hem adopted as pan of the government effects 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 irrigation rates. (I, 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 we efficiency and cost. Results showed an asymptotic increase of greenhouse cucumber yield with increase in water applications from 1 to 4 mm/day. The 2mm/day applications optimized yields (kg/m3), whereas 3mm/day application maximized yields, with no significant difference from the 2mm/day applications being observed. Yields wee increased by 135% from 27 to 63 t/ha when irrigation was increased from lmm/day to 3 mm/day respectively, and declined thereafter. Field cucumber yields increased linearly as the irrigation 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 2mm/day yields (53 t/ha) obtained in the greenhouse. Optimum yields, were obtained at an average mop factor (Kc) of 0.58 ETo and 1.55 ETo in the greenhouse and the field respectively, indicating that water requirements for the greenhouse cucumber is about one third of that in the open field. The irrigation water use efficiency was higher in the greenhouse than that of the open field because of the Iowa water requirement and higher yields of cucumbers. But the total water use efficiency approached that of the field as the rats wee maximized, bemuse of the high quantity of water used in evaporative cooling The average cooling pad water use was found to be 79.11m-2 day-1 of pad area. ln the greenhouse, irrigation water use efficiency was highest with 2 mm/day applications (31.3 kg/m3), whereas in the open field the highest irrigation water use efficiency obtained was only 7.6 kg/m-3 for the 6 mm/day applications. Treatments close to the cooling pads of the greenhouse wee more moist than the amount of water applied would have indicated, but supposed yields obtained were attributed to the high soil salinity levels washed from the pads and possibly due to the chilling temperature incurred at night.