Aims Land cover changes can disrupt water balance and alter the partitioning of precipitation into surface runoff, evapotranspiration and groundwater recharge. The widely planted Eucalyptus trees in south-western China have the potential to bring about hydrologic impacts. Our research aims to elucidate the hydrologic balance characteristics of the introduced exotic Eucalyptus grandis x Eucalyptus urophylla plantation and to assess whether its high productivity results from high water use efficiency (WUE) or large water consumption. Methods A 400-m(2) experimental plot was established in an E. grandis x E. urophylla plantation in south-western China. Water balance components, including stand transpiration (T-r), evapotranspiration (E-t) and runoff (R) were obtained as follows: T-r was estimated based on sap flow measurements, E-t was estimated as the average of surface transpiration and evaporation weighted by the fractional green vegetation cover using a modeling approach, and R was collected using the installed metal frame. Net primary productivity (NPP) was obtained from allometric equation and annual diameter at breast height (DBH) increment determination. Important Findings Annual E-t and T-r were 430 +/- 31 and 239 +/- 17 mm, respectively. Annual T-r accounts for 56 +/- 8% of total evapotranspiration on average. WUE (NPP/T-r) of the E. grandis x E. urophylla was estimated to be 3.3-3.9 mmol.mol(-1). Based on the comparative analysis of T-r and WUE, E. grandis x E. urophylla had a high productivity due to its high WUE without exhibiting prodigal water use. Meteorological factors including vapor pressure deficit and global solar radiation (R-s) were key factors regulating E-t and T-r in our research site. Annual surface runoff, E-t and canopy interception occupied 7%, 27-30% and 16% of total precipitation, while the remaining 46-50% of precipitation was used for sustaining groundwater recharge and altering soil water storage. The higher runoff coefficient (7.1%) indicated the weaker capability of E. grandis x E. urophylla to reserve water resource than natural forests and less disturbed plantations. The planting and protection of understory vegetation may decrease the surface runoff and exert beneficial effects on water conservation capacity of Eucalyptus plantation.

water balance; evapotranspiration; water use efficiency; sap flow; runoff

South China Botanical Garden, Chinese Academy of Sciences