Sensitivity of a high-elevation Rocky Mountain watershed to altered climate and CO2

We explored the hydrologic and ecological responses of a headwater mountain catchment, Loch Vale watershed, to climate change and doubling of atmosphe ric CO2 scenarios using the;Regional Hydro-Ecological Simulation System (RH ESSys). A slight (2 degrees C) cooling, comparable to conditions observed o ver the past 40 years, led to greater snowpack and slightly less runoff, ev aporation, transpiration, and plant productivity. An increase of 2 degrees C yielded the opposite response, but model output for an increase of 4 degr ees C showed dramatic changes in timing of hydrologic responses. The snowpa ck was reduced by 50%, and runoff and soil water increased and occurred 4-5 weeks earlier with 4 degrees C warming. Alpine tundra photosynthetic rates responded more to warmer and wetter conditions than subalpine forest, but subalpine forest showed a greater response to doubling of atmospheric CO2 t han tundra. Even though water use efficiency increased with the double CO2 scenario, this had little effect on basin-wide runoff because the catchment is largely unvegetated. Changes in winter and spring climate conditions we re more important to hydrologic and vegetation dynamics than changes that o ccurred during summer.