The study at first recalls the concept of "potential evapotranspiration" (PET), originally considered equal to the evaporation climatic demand; then, it reminds the steps of its progressive evolution toward the concept of "reference crop evapotranspiration" (ET0) determined on irrigated grass. A physical analysis conducted on the evaporation process is subsequently reported to help clarifying the links between ET0 and evaporation climatic demand. This analysis clearly demonstrates that the equivalence of ET0 to evaporation climatic demand is not correct, although still common assumption in recent scientific literature, particularly in hydrology. The study also identifies two processes acting in opposite directions in the dynamics of ET0: (1) the climatic variables determining the evaporation demand, and (2) the canopy resistance which slows down the response of irrigated grass to such demand. The analysis of the respective impact of these two processes on ET0 dynamics shows that the available energy is the dominant process. This variable takes into account the 60-70% of the variation of ET0, both at hourly and daily scales, while canopy resistance only explains 10-20% of ET0 variation of irrigated grass. The study regards different climatic situations. Possible effects on practical applications were also discussed in the conclusions, together with comments on the correct canopy resistance modelling.
