A MODEL FOR PREDICTING ACTUAL EVAPOTRANSPIRATION UNDER SOIL-WATER STRESS IN A MEDITERRANEAN REGION

In this paper a model for estimating actual evapotranspiration is deve loped and tested for field crops (grain sorghum and sunflower) maintai ned under water stress conditions. The model is based on the Penman-Mo nteith formulation of ET in which canopy resistance (r(c)) is modeled with respect to the crop water status and local climatological conditi ons. The model was previously tested on reference grass; in this last case no reference was made to soil water conditions and r(c) was model ed only as a function of climatological parameters. Here r(c) is expre ssed as a function of available energy, vapour pressure deficit, aerod ynamic resistance and crop water status by means of predawn leaf water potential. Results, obtained with various crop water stress intensiti es, show that, on a daily scale, calculated ET is 98% and 95% of the m easured ET for sorghum and sunflower respectively. The correlation bet ween daily calculated and measured ET is very high (r(2) = 0.95 for so rghum and r(2) = 0.98 for sunflower). On an hourly scale, the model wo rks very well when the crops were not stressed and during the senescen ce stage. In case of weak and strong stress the model has to be used w ith some precautions.