Many climatologists predict major changes in the earth's climate due to an expected doubling of atmospheric greenhouse gases. Global circulation models have been used to derive alternate climate scenarios under such conditions and the effects on water resources have been estimated with hydrological models. The hydrological models have generally assumed that the watershed itself would remain unchanged. However, it is more likely that a changing climate would alter the distribution and the transpiration efficiency of vegetation types within a watershed. A new hydrological model in which the parameters are based on land use class (effectively, vegetation type) was calibrated and verified for a watershed in the Rocky Mountains of British Columbia. The changes in land use associated with a doubled carbon dioxide scenario were estimated, and the model was rerun incorporating the changed climate and the changed land use. Details of the effects of this scenario on the hydrology of the watershed are given, but it is emphasized that the purpose of the paper is to develop the methodology and not to endorse any particular scenario of climatic change. ¿ American Geophysical Union 1993