Transpiration of tomato plant canopy and water use for a fog cooled greenhouse in semiarid climate

The ultimate goal of this collaborative project is to develop an effective environmental control strategy to cool the greenhouses for plant production and minimize the water use in semiarid climate. Using a single-span double-polyethylene greenhouse with tomato plant canopy at The University of Arizona, the canopy transpiration rate and the water balance of greenhouse were investigated. The greenhouse was equipped with high-pressure fog nozzles, roll-up side vents with insect screens, and a roof vent. Fogging was operated cyclically with an air temperature set point of 24°C. Under different vent configurations, the transpiration rate was measured using a stem gage. The amounts of generated fog and non-evaporated water droplets were collected and measured. The natural ventilation rate was measured continuously using SF ₆ gas as a tracer. Preliminary results showed that the transpiration rate increased linearly with an increase in vapor pressure deficit (VPD) of the air. When the ventilation rate was decreased by reducing the vent openings, the total water use in the greenhouse decreased by 13% and relative humidity increased as expected from simulation based on the steady-state energy balance. The decrease in canopy transpiration was driven by the decrease in VPD, and was at a greater magnitude than that of fog evaporation rate under the present experimental conditions with relatively high humidity ranging 70-94%. These results suggest that by optimizing natural ventilation rate, we could effectively cool the greenhouse with less water use.