Carbon and water cycles are intimately coupled in terrestrial ecosystems, and water-use efficiency (WUE, carbon gain at the expense of unit water loss) is one of the key parameters of ecohydrology and ecosystem management. In this study, the carbon cycle and water budget of terrestrial ecosystems were simulated using a process-based ecosystem model, called Vegetation Integrative SImulator for Trace gases (VISIT), and WUE was evaluated: WUEC defined as gross primary production (GPP) divided by transpiration, and WUES defined as net primary production (NPP) divided by actual evapotranspiration.
Results/Conclusions
Total annual WUEC and WUES of the terrestrial biosphere were estimated as 8.0 and 0.92 g C kg–1 H2O, respectively, for the period 1995–2004. Spatially, WUEC and WUES were only weakly correlated. WUES ranged from <0.2 g C kg–1 H2O in arid ecosystems to >1.5 g C kg–1 H2O in boreal and alpine ecosystems. The historical simulation implied that biospheric WUE increased from 1901 to 2005 (WUEC, +7% and WUES, +12%) mainly as a result of the augmentation of productivity in parallel with the atmospheric carbon dioxide increase. We examined that WUE of human activities on the basis of human carbon use (e.g., NPP appropriation) and water use (e.g., agricultural freshwater withdrawal). Country-based analyses indicated that total NPP is largely determined by water availability, and human appropriation of NPP is also related to water resources to a considerable extent. These results have implications for (1) responses of the carbon cycle to the anticipated global hydrological changes; (2) responses of the water budget to changes in the terrestrial carbon cycle; and (3) ecosystem management based on optimized resource use.