Evapo-transpiration from vegetation, as well as patterns of precipitation are expected to change as the concentration of CO(sub 2) in the atmosphere continues to rise (f). Water modulates the rates of many biogeochemical processes, and it has been estimated that water directly limits plant productivity over two-thirds of the earth's land surface (2). Water quality and availability are increasingly important practical issues as demands by both agricultural and urban users continue to increase. In a recent Perspective article (3) Farquhar stated that transpiration (water loss) from terrestrial vegetation will decline by 40 to 50% as the CO(sub 2) concentration in the atmosphere approaches twice present levels. He suggested that ''the impending saving of water would be a welcome result of the rising atmospheric CO(sub 2) concentration.'' We can confirm that large reductions in transpiration are expected by terrestrial physiological ecologists. Examining 35 recent articles that discussed the issue of water use while synthesizing research on ecosystem impacts of doubling atmospheric CO(sub 2) (including reviews and crop/natural ecosystem models), we found that 31 articles suggest that reductions in water use of between 25 and 50% are to be expected.
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