Evapotranspiration in Greenhouses

Abstract

<p>A physically based evapotranspiration model has been developed and tested in an experimental greenhouse. Good agreement was found between hourly model estimates and mass balance measurements of the latent heat flux. The model recognizes the advective nature of the greenhouse microclimate and thus represents an improvement over empirical model estimates of evaporation based on the measurement of radiation alone. Although radiant heating is the dominant mechanism responsible for evapotranspiration it does not represent a constant proportion on an hourly or daily basis. As a result, the Bowen ratio varies over time. Most of the variation was attributable to advection, and to a lesser extent, the sensible and latent heat fluxes at the glazing. During the daytime, the evapotranspiration process utilized in excess of 70% of the net available energy at the surface. However, model estimates and empirical evidence indicate this proportion can equal or exceed 100%. Variations in the latent heat flux are shown to depend on greenhouse design and the ambient microclimate. Simulation of the greenhouse humidity environment using 10 year hourly climatic means for Woodbridge, Ontario demonstrates the effect of modifying ventilation rates, glazing transmission and intake humidity on potential evapotranspiration. A relation is presented which permits the real-time adjustment of ventilation resistance from meteorological measurements of solar radiation and dry and wet-bulb air temperature. The maintenance of potential evapotranspiration for optimal crop productivity is shown to be incompatible with the collection and storage of sensible heat of the exhaust air as a means of defraying greenhouse heating costs.</p>