The effects of potential climate change on mean annual runoff in the conter
minous 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 gener
al, the CCC GCM climate results in decreases in runoff for the conterminous
U.S., and the HAD GCM climate produces increases in runoff. These estimate
d changes in runoff primarily are the result of estimated changes in precip
itation. The changes in mean annual runoff however, mostly are smaller than
the decade-to-decade variability in GCM-based mean annual runoff and error
s in GCM-based runoff. The differences in simulated runoff between the two
GCMs, together with decade-to-decade variability and errors in GCM-based ru
noff, cause the estimates of changes in runoff to be uncertain and unreliab
le.