Estimates of Runoff Using Water‐balance and Atmospheric General Circulation Models1

ABSTRACT: The effects of potential climate change on mean annual runoff in the conterminous United States (U.S.) are examined using a simple water‐balance model and output from two atmospheric general circulation models (GCMs). The two GCMs are from the Canadian Centre for Climate Prediction and Analysis (CCC) and the Hadley Centre for Climate Prediction and Research (HAD). In general, the CCC GCM climate results in decreases in runoff for the conterminous U.S., and the HAD GCM climate produces increases in runoff. These estimated changes in runoff primarily are the result of estimated changes in precipitation. The changes in mean annual runoff, however, mostly are smaller than the decade‐to‐decade variability in GCM‐based mean annual runoff and errors in GCM‐based runoff. The differences in simulated runoff between the two GCMs, together with decade‐to‐decade variability and errors in GCM‐based runoff, cause the estimates of changes in runoff to be uncertain and unreliable.