Abstract
Increasing water use efficiency is one of the main challenges of sustainable fruit tree production. From 2013 to 2015 we measured actual evapotranspiration (ETa) using eddy covariance in a well-irrigated apple orchard located in in South Tyrol (Italy), a sub-humid environment. We assessed the experimental crop coefficient (Kcexp) and analyzed the dependency of Kc on specific environmental variables at a daily time scale. Kcexp values changed throughout the season following a bell-shaped trend and were generally lower than the FAO tabular values corrected for local climatic conditions. In the mid-season phase, when LAI and tabular Kc are supposed to be constant, the average experimental Kc (Kc¯exp) was 1.01, 86% of the Kc value reported by FAO (1.18). Mid-season Kc residuals (Kcexp - Kc¯exp) were positively correlated with daily vapor pressure deficit (VPD) (ρ = 0.45), suggesting that the daily Kc variability observed is due, at least in part, to changes in the evaporative demands of the atmosphere. We explain these results by considering the relatively humid environment, the high water availability and the fact that leaves on apple trees are more tightly coupled to the atmosphere with respect to a smoother grass surface. © 2019 The Authors