MODELLING OF WATER UPTAKE, TRANSPIRATION AND HUMIDITY IN GREENHOUSES, AND OF THEIR EFFECTS ON CROPS
A review of existing models shows that experimental correlations were used to identify solar radiation and vapour pressure deficit as the main factors affecting transpiration. Based on the Penman-Monteith equation, some more generals models give a good prediction of the transpiration as a function of the humidity inside the greenhouse. Their accuracy is directly linked to their ability to predict the variation of the stomatal conductance with solar radiation and vapour pressure deficit. As there is an interaction between inside humidity and transpiration (feedback effect), these parameters need to be predicted as a function of the outside conditions. Traditionally this is done using complex dynamic models which lack transparency. Recently simplified and accurate water balance models have allowed the prediction, of not only the actual humidity and transpiration but also the additional water and energy to be added or extracted to reach a given set point. More research is required on the link between water and nutrient uptake.
Knowledge about the response of greenhouse crops to humidity and transpiration has recently been obtained. On a short term basis, low humidity slightly affects plant growth by reducing the stomata opening and therefore the photosynthesis rate. On a long term, high humidities modify the leaf development, either by increasing the number of side shoots or by causing low transpiration rates and therefore calcium deficiency, which may lead to leaf area reduction of up to 50%.
For some of the greenhouse crops, models are now available to predict both humidity, transpiration and their effect on the yield. They can already be used for the optimal control of humidity in greenhouses.