Farming options for The Netherlands explored by multi-objective modelling

Efficient irrigation management requires a good quantification of evapotranspiration. In the case of olive orchards, which are the dominant crop in vast areas of southern Europe, very little information exists on evaporation. Measurements of aerodynamic conductance and evaporation above and below an olive orchard allowed the calibration of a transpiration model of olive trees based on the Penman¯Monteith equation. The model was combined with Ritchieʹs soil evaporation model and tested against an independent data set, indicating its validity unless a substantial fraction of the soil surface is wetted by irrigation emitters, which is not taken into account by the model and deserves further research. Simulated crop coefficients of olive orchards in southern Spain changed during the year in response to changes in vapor pressure deficit (VPD) and evaporation from the soil surface. The average annual crop coefficient (0.62) was rather low due to the low ground cover and to the enhanced control of canopy conductance by stomatal responses to VPD. According to our results the crop coefficient will vary among locations and even among years, depending on rainfall and temperature.