Modelling evapotranspiration of six irrigated crops under Mediterranean climate conditions

Two models based on the Penman–Monteith equation have been used to calculate the evapotranspiration, on hourly and daily scales, for six crops (grass, tomato, soybean, sunflower, grain sorghum, sweet sorghum), grown in a Mediterranean region, in well watered conditions. These crops presented different sensitivity to the air vapour pressure deficit, due to their height ranging between 0.1 and 2.8 m. In the first model (model 1), the canopy resistance was considered variable and modelled as function of climatic variables. The experimental tests showed that this model simulated well the evapotranspiration for the six crops, both at hourly and daily scales. On the other hand, the simulations obtained by this model 1 at daily scale are clearly more accurate than those obtained by using the standard FAO 56 method. In the second model (model 2), the canopy resistance, locally determined, is considered constant. The experiments showed that the model 2 simulated acceptably the evapotranspiration only for the short crops: the grass (at hourly and daily scales) and the tomato (only at daily scale), but with lower performances than the model 1. For the other four crops (soybean, sunflower, grain sorghum and sweet sorghum) the model 2 seems to be not adapted to calculate correctly the evapotranspiration. In the conclusions we discuss the advantages linked to the use of the model 1 for the direct calculation of the evapotranspiration.