On the other hand, Yan et al. (2006) found that seasonal WUE was positively correlated with seasonal precipitation in an alpine meadow during the robust growth season. Using the eddy covariance technique, we found that diurnal variation in WUE at the orchard-scale was significantly influenced by VPD in the rapidgrowth stage, but there was no such relationship in the flower ingstage. This may be related to the characteristics of the orchard ecosystem, the extent of ground vegetation cover at different times of the year and meteorological factors such as differences in vapour pressure. Unlike most field crops, the orchard ecosystem has a two-tier structure comprising surface vegetation and the tree canopy. The understorey vegetation does not fully cover the groundsurface over the whole growth period. LAI values and understorey vegetation cover during the flowering and early fruit enlargementstages were relatively low, and soil evaporation was significant lyelevated. However, during the fruit enlargement and ripen in gstages, physiological water demand and water consumption were highest. In the post-harvest stage, air temperature was relatively low, and LAI and net CO2 flux at the orchard-scale were relatively high. Thus, WUE differs between pre- and post-harvest stages.