Evaluation of the Penman-Monteith evapotranspiration model for plant dependent irrigation control in greenhouses

A new concept for plant dependent irrigation management in greenhouses was developed and tested for Ficus benjamina. For precise irrigation control, two plant parameters where introduced in the Penman-Monteith model:the stomatal resistance and the amount of the transpiring surface. For stomatal resistance, a function of two driving micro-climatic variables, i.e. solar radiation and leaf-air vapour pressure deficit was fitted to porometer measurements of stomatal resistance. To calculate the total amount of the transpiring surface, a Richards growth function based on cumulated values of short wave radiation was used instead of time. It was shown that such a growth function provided very good estimates of leaf surface during both the summer and winter season. To validate the plant dependent Penman-Monteith model, daily sums of calculated evapotranspiration values for 31 selected days were compared to the actual water consumption measured with an automatic weighing system. The deviation between the measurements and the model predictions was very small, being only about 4 %. Hence, the implementation of this model on a climate computer, together with specific information on transpiring leaf surface and stomatal resistance, will allow a very accurate estimation of the actual water needs and will ultimately lead to a method for plant dependent irrigation control.