Water use by greenhouse evaporative cooling systems in a semi-arid climate

Water availability is a common concern in semi-arid regions, such as Southern Arizona, USA, where greenhouse crop production is popular due to high solar radiation. Hydroponic greenhouse crop production greatly reduces irrigation water use; however, water use by evaporative cooling systems has never been quantified. This project investigated water use by two evaporative cooling systems: pad-and-fan (P&F) and high-pressure-fog (HPF) with fan ventilation and a central overhead line. Water use data were collected from a double-layer polyethylene film-covered greenhouse (28 × 9.8 × 6.3 m) with mature tomato plants (1.7 plants m ^-2) from 08:00-17:00 for each ventilation rate tested under semi-arid climate conditions (T _Out,Avg=35°C, RH _Out,Avg=10%, Solar _Avg=900 W m ^-2). Total daily water use by P&F was 3.2, 6.4, 8.5, and 10.3 L m ^-2 for ventilation rates of 0.016, 0.034, 0.047, 0.061 m ³ m ^-2 s ^-1 respectively. Total daily water use by the HPF system was 7.9, 7.4, and 9.3 L m ^-2 for fan ventilation rates of 0.01, 0.016, 0.034 m ³ m ^-2 s ^-1, respectively. Total greenhouse WUE (water use efficiency) was calculated using hydroponie irrigation (4.5 L m ^-2) and evaporative cooling water use. Total WUE values were comparable to sprinkler-irrigated WUE (15-19 L m ^-2) and flood-irrigated WUE (<12L m ^-2). Using a "closed" hydroponic irrigation system would increase the WUE greatly and should be implemented to conserve water. Furthermore, HPF water use should decrease under natural ventilation.