APPLICATION OF THE PENMAN-MONTEITH MODEL TO CALCULATE THE EVAPOTRANSPIRATION OF HEAD LETTUCE (LACTUCA SATIVA L. VAR. CAPITATA) IN GLASSHOUSE CONDITIONS

In order to minimize the output of nutrients and pesticides to the environment, a plant dependent model for irrigation of head lettuce in greenhouses is being optimized. Estimation of the lettuce evapotranspiration is done by means of the Penman-Monteith concept. The stomatal resistance and the amount of the transpiring surface needed for this model are estimated by the use of two submodels. Stomatal resistance was calculated by fitting a function of solar radiation intensity, vapour pressure deficit and air temperature to porometer measurements of stomatal resistance. The submodel for the transpiring surface was based on the Gompertz equation and uses the outdoor shortwave radiation as the only input parameter. However, special attention has to be paid to the measurement of the real transpiring surface of head lettuce. Calculation of the hourly evapotranspiration rates using the total leaf area as an estimation of the transpiring surface showed a very poor fit with actual water consumption. This presumably can be explained by the fact that head filling leaves of the lettuce have only a limited contribution to transpiration. A very good fit of the model was reached by using the vertical projection of the crop, measured with a video camera, as an estimation of the transpiring surface. The usefulness of the model in various growing conditions is discussed. Extended validation of the model should indicate the need of further refinement.