Management measures in orchard plantations, such as no-tillage, uneven spacing and incomplete vegetation coverage, result in a two-tier vertical configuration comprising understorey vegetation and fruit tree canopy; this spatial organisation makes it difficult to measure different components of orchard evapotranspiration flux (Williams et al., 2004) and WUE. During the robust period in an irrigated olive grove, when LAI was in the range of 0.3–1.9, average hourly grove-scale WUE was reported to be highly correlated with VPD (Testi et al., 2004).