Water-use efficiency of ecosystem functions and services of the terrestrial biosphere

Background/Question/Methods

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: WUE_C defined as gross primary production (GPP) divided by transpiration, and WUE_S defined as net primary production (NPP) divided by actual evapotranspiration.

Results/Conclusions

Total annual WUE_C and WUE_S of the terrestrial biosphere were estimated as 8.0 and 0.92 g C kg^–1 H₂O, respectively, for the period 1995–2004. Spatially, WUE_C and WUE_S were only weakly correlated. WUE_S 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 (WUE_C, +7% and WUE_S, +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.