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Greenhouse crop production in semi-arid climates is desirable because high solar radiation levels are consistent year round. The use of evaporative cooling will further increase yields and crop consistency. However, these regions typically receive less than 500 mm of rain annually, making water use management a critical concern. This study evaluated water use for irrigation (WU(I)) and pad-and-fan (WU(PF)) evaporative cooling systems in a single-span, polyethylene-covered greenhouse in Tucson, Arizona, from March to October 2006. A single-use, non-recirculating irrigation system delivered water to hydroponically grown tomatoes. The pad-and-fan system was computer controlled to maintain day/night air temperatures of 24 degrees C/18 degrees C. The total eight-month WU(I) and WU(PF) were 780 and 1450 L m(-2), respectively. WU(I) increased steadily from 4.3 L m(-2) d(-1) during crop establishment to 7.2 L m(-2) d(-1) when the plants were mature. WU(PF) increased from 1.1 L m(-2) d(-1) during early spring to a peak of 11 L m(-2) d(-1) during the hottest, driest outside conditions. The water use efficiency (WUE, kg yield per m(3) water use) of the irrigation and pad-and-fan cooling systems was 30 and 16 kg m(-3), respectively. When WUE was calculated by combining WU(I) and WU(PF), the total greenhouse WUE was 11 kg m(-3). Theoretically, using a 100% recirculating irrigation system could have produced a greenhouse WUE of 13 kg m(-3). This study demonstrates that although greenhouses achieve high annual yields with low irrigation rates, using an evaporative cooling system reduces greenhouse WUE to field WUE levels. To minimize greenhouse water use while maintaining high crop yields, this study recommends further examination of the use of recirculating irrigation systems, variable-speed fans to improve climate control, alternative cooling systems, and drought-tolerant crops.