ENVIRONMENTAL AND SOIL-PLANT PARAMETERS FOR MODELING ACTUAL CROP EVAPOTRANSPIRATION UNDER WATER-STRESS CONDITIONS

The evapotranspiration of a crop (ET), in every water condition, is mo delled starting from the Penman-Monteith combination equation. The mod el is subdivided into two submodels, the first one takes into account the behaviour of the crop subjected to environmental demands (the atmo spheric thermodynamic status); whereas the second submodel depends onl y on available energy (this second factor is the 'equilibrum evapotran spiration'). Thus, the final form of evapotranspiration, on a daily sc ale, is the product of a coefficient dependent on crop water status ex pressed by means of predawn leaf water potential and the equilibrium E T. A further simplification of the model is introduced substituting to leaf water potential an experimental function of available water in t he soil. Model testing was done by comparing the output (ETcalculated) with actual ET measured by means of the energy balance/Bowen ratio me thod (ETmeasured), in a location in southern Italy, characterized by s emi-arid climate. The results show a very good performance of the mode l for three crops (soybean, grain sorghum and sunflower) in good water status as well as in water stress. Sorghum, ETcalculated = 0.95 ETmea sured (r(2) = 0.98); soybean, ETcalculated = 1.00 ETmeasured (r(2) = 0 .98);sunflower, ETcalculated = 0.93 ETmeasured (r(2) = 0.97). (C) 1997 Elsevier Science B.V.