Previous work has demonstrated that constant water use per unit leaf area results from the combined control of stomatal and hydraulic conductance holds for plant water use; however, few studies have ever explored the water use of a popularly planted fast-grown tree species, eucalyptus, in southern China. In this study, seasonal variations in hydraulic traits and ecophysiological parameters were monitored via the sap flux, leaf water potential (ψleaf) and associated environmental variables to investigate the water use of a five-year-old Eucalyptus grandis × Eucalyptus urophylla plantation in Guangxi province, China, in July and October 2012 and January and April 2013. The results show that predawn ψleaf was similar among all months, suggesting an abundance of soil water in the study site. There was a significant seasonal variation in midday ψleaf. Moreover, canopy stomatal conductance (Gs) was higher in July than in October and linearly decreased with the natural logarithm of the vapor pressure deficit (ln(VPD)). Furthermore, the hydraulic conductance from soil to leaves (K) per unit sapwood area in the dry season (October) was relatively high compared to that in the wet season (July). Whole-tree water use per day was estimated to be 7.7 kg d−1 and 6.7 kg d−1 in July and October, respectively, and linearly increased with leaf area. However, the slope of the regression line between whole-tree water use per day and leaf area was similar in July and October, clearly indicating that this eucalyptus stand had a constant water use per unit leaf area, which confirms the generally accepted concept. The findings of this study should help address the increasing ecohydrological and water resource concerns related to the rapid expansion of Eucalyptus spp. plantations in southern China, which recently underwent a severe drought.