Correctly understanding the hydrological impacts of forest vegetation and climate change is of considerable significance for forestry management and watershed ecological restoration. To investigate the effects of climate and vegetation cover changes on carbon-water cycles, the biophysical/dynamic vegetation model SSiB4/TRIFFID (Simplified Simple Biosphere model version 4, coupled with the Top-down Representation of Interactive Foliage and Flora Including Dynamics model) was coupled with the TOPMODEL (Topographic Index Model) based on the catchment scheme partitions between saturated and unsaturated zones. The coupled model (hereinafter SSiB4T/TRIFFID) was employed to perform long-term dynamic simulations of vegetation succession and carbon-water circulations under different climate scenarios for a subalpine basin (the Soumou River basin that is a tributary of the Yangtze River located in the mountain region of southwestern China). The results of all tests indicate that vegetation fractions initially undergo changes from C3 grass dominance to tundra shrub dominance and then gradually approach equilibrium forest dominance. The results of the control test show that evapotranspiration of the basin increases and reaches its maximum value and runoff reaches its minimum value during the succession period of C3 grasses into tundra shrubs. Additionally, evapotranspiration decreases and runoff increases during the succession period of tundra shrubs into forests. An increase in temperature by 2 ℃ enhances the rate of transpiration and canopy interception evaporation of forests more than those of grasses and tundra shrubs. As a result, the role of forests in increasing runoff is reduced. An increase in temperature by 5 ℃ accompanied by an increase in precipitation by 40% [T+5, (1+40%)P test] will cause forests to reduce runoff because of the considerable increase in water loss through canopy interception evaporation and transpiration of forests. The results indicate that sensitivity to temperature changes of canopy interception evaporation and transpiration of forests are more than those of grasses and shrubs. Such a mechanism of temperature change causes the forest-runoff relationship to change. From the control test to the T+5, (1+40%)P test, the forest net primary productivity (NPP) increases with the increase in temperature. By contrast, the increase in temperature has a slight effect on the NPP of grasses and tundra shrubs. The water use efficiency (WUE), which characterizes the coupling relationship between carbon and water, considerably decreases with the increase in temperature. As elevation decreases (temperature increases) in the mountain region of southwestern China, WUE decreases with the decrease in altitude. Moreover, the role of forests to increase runoff changes to decrease runoff. The vertical zonality of climate controls the spatial variation of the forest-runoff relationship and WUE.