Measurement of canopy transpiration or evapotranspiration in greenhouses by means of a simple vapour balance model

Abstract

Canopy transpiration of a greenhouse crop was calculated by means of a simple vapour balance model based on the vapour loss from the greenhouse by ventilation and condensation. The vapour loss by ventilation was calculated from the greenhouse air humidity and ambient humidity and the volume flux of air from the greenhouse. Vapour loss by condensation was calculated from the greenhouse air humidity, glass temperature and the greenhouse air temperature. Two corrections are required for the condensation model: the maximum possible condensation on the cover has to be allowed for, and in summer a correction is applied for the thermal expansion coefficient. Comparison of measured and calculated canopy transpiration shows that this method is suitable to calculate canopy transpiration on a daily as well as on an hourly basis. However, in the region of transpiration rates below 0.1 kg m −2 h −1 the accuracy decreases. This might be improved by a more accurate ventilation rate model.