Rising atmospheric CO 2 concentration affects the water balance through climatic changes and through changes in transpiration, vegetation structure and distribution. This study quantifies CO 2 effects upon evapotranspiration, soil moisture and runoff under conditions of potential natural vegetation globally, using a biosphere model forced by observed climate . Isolated CO 2 impacts were inferred from a 2 × CO 2 scenario, assuming no climate change. Global effects are moderate: evapotranspiration reduces by 7%, whilst runoff and soil moisture increase by 5% and 1%, respectively. But the effects vary regionally and seasonally, owing to the complex interplay between water and vegetation dynamics. For example, reduced leaf-level transpiration is outweighed by increased transpiration resulting from enhanced vegetation cover in semiarid areas; and seasonal transpirational water savings result in year-round increases in soil moisture in boreal regions. Overall, CO 2 effects impart significant variability to hydrological processes at all scales, so they merit consideration in assessments of future water resources.

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