Water use efficiency (WUE) is an important variable used in climate change and hydrological studies in
relation to how it links ecosystem carbon cycles and hydrological cycles together. However, obtaining
reliable WUE results based on site-level flux data remains a great challenge when scaling up to larger
regional zones. Biophysical, process-based ecosystem models are powerful tools to study WUE at large
spatial and temporal scales. The Integrated BIosphere Simulator (IBIS) was used to evaluate the effects of
climate change and elevated CO2 concentrations on ecosystem-level WUE (defined as the ratio of gross
primary production (GPP) to evapotranspiration (ET)) in relation to terrestrial ecosystems in China for
2009–2099. Climate scenario data (IPCC SRES A2 and SRES B1) generated from the Third Generation
Coupled Global Climate Model (CGCM3) was used in the simulations. Seven simulations were implemented
according to the assemblage of different elevated CO2 concentrations scenarios and different
climate change scenarios. Analysis suggests that (1) further elevated CO2 concentrations will significantly
enhance the WUE over China by the end of the twenty-first century, especially in forest areas;
(2) effects of climate change on WUE will vary for different geographical regions in China with negative
effects occurring primarily in southern regions and positive effects occurring primarily in high latitude
and altitude regions (Tibetan Plateau); (3) WUE will maintain the current levels for 2009–2099 under
the constant climate scenario (i.e. using mean climate condition of 1951–2006 and CO2 concentrations
of the 2008 level); and (4) WUE will decrease with the increase of water resource restriction (expressed
as evaporation ratio) among different ecosystems.