An increasing trend in global streamflow has been variouslyattributed toglobal warming, land use, and a reduction in plant transpiration under higherCO2 levels. To separate these influences for the coterminous United States,we use a set of over 1000 United States Geological Survey stream gaugesprimarily from small, minimally disturbed watersheds to estimate annualstreamflow per unit area since 1920 on a uniform grid. We find that changingprecipitation, which is not clearly correlated with greenhouse gasconcentrations or global warming, explains most of the interannual and longerterm variability in streamflow. While streamflow has indeed increased since1920, this increase has not been steady but rather concentrated in the late1960s, when precipitation increased. Since the early 1990s, bothprecipitation and streamflow show nonsignificant declining trends. Multipleregression of streamflow against precipitation, temperature and CO2 suggests that higher CO2 levels may increase streamflow, presumably fromlower transpiration due to the physiological plant response to CO2, but thatthis positive response is offset by concomitant increasing evaporation due toglobal warming. The net impact of the opposing climate and physiologicaleffects of CO2 emissions for streamflow is close to zero for thecoterminous United States taken as a whole, but shows regional variation.Streamflow at a given amount of annual precipitation has decreased in thePacific west, where most precipitation occurs in winter. Suppression of planttranspiration through higher CO2 levels may be particularly important forsustaining high streamflow in recent decades in the Great Plains, whereprecipitation is concentrated during the growing season.
展开▼
