Environmental and soil-plant parameters for modelling actual crop evapotranspiration under water stress conditions

The evapotranspiration of a crop (ET), in every water condition, is modelled starting from the Penman-Monteith combination equation. The model is subdivided into two submodels, the first one takes into account the behaviour of the crop subjected to environmental demands (the atmospheric thermodynamic status); whereas the second submodel depends only on available energy (this second factor is the ‘equilibrum evapotranspiration’). Thus, the final form of evapotranspiration, on a daily scale, is the product of a coefficient dependent on crop water status expressed by means of predawn leaf water potential and the equilibrium ET. A further simplification of the model is introduced substituting to leaf water potential an experimental function of available water in the soil. Model testing was done by comparing the output (ETcalculated) with actual ET measured by means of the energy balance/Bowen ratio method (ETmeasured), in a location in southern Italy, characterized by semi-arid climate. The results show a very good performance of the model for three crops (soybean, grain sorghum and sunflower) in good water status as well as in water stress. Sorghum, ETcalculated = 0.95 ETmeasured (r2 = 0.98); soybean, ETcalculated = 1.00 ETmeasured (r2 = 0.98); sunflower, ETcalculated = 0.93 ETmeasured (r2 = 0.97).