Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 12 4 2008 10.5194/hess-12-1111-2008 http://www.hydrol-earth-syst-sci.net/12/1111/2008/ http://www.hydrol-earth-syst-sci.net/12/1111/2008/hess-12-1111-2008.html http://www.hydrol-earth-syst-sci.net/12/1111/2008/hess-12-1111-2008.pdf 1111 1120 2008-08-14 Mapping and attribution of change in streamflow in the coterminous United States N. Y. Krakauer niryk@berkeley.edu I. Fung University of California at Berkeley, USA An increasing trend in global streamflow has been variously attributed to global warming, land use, and a reduction in plant transpiration under higher CO₂ levels. To separate these influences for the coterminous United States, we use a set of over 1000 United States Geological Survey stream gauges primarily from small, minimally disturbed watersheds to estimate annual streamflow per unit area since 1920 on a uniform grid. We find that changing precipitation, which is not clearly correlated with greenhouse gas concentrations or global warming, explains most of the interannual and longer term variability in streamflow. While streamflow has indeed increased since 1920, this increase has not been steady but rather concentrated in the late 1960s, when precipitation increased. Since the early 1990s, both precipitation and streamflow show nonsignificant declining trends. Multiple regression of streamflow against precipitation, temperature and CO₂ suggests that higher CO₂ levels may increase streamflow, presumably from lower transpiration due to the physiological plant response to CO₂, but that this positive response is offset by concomitant increasing evaporation due to global warming. The net impact of the opposing climate and physiological effects of CO₂ emissions for streamflow is close to zero for the coterminous United States taken as a whole, but shows regional variation. 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