Potential Impacts of Climate Change on Streamflow and Flooding in Snow Dominated Forest Basins

Changes in climate resulting from the buildup of greenhouse gases in the atmosphere may impact significantly the frequency and magnitude of flooding in forested watersheds through changes in snowpack, soil moisture, and runoff production. The level of impact will vary considerably among watersheds depending on the climate regime and hydrologic characteristics of the catchments. Two forested watersheds in the Pacific Northwest of the United States, the American River and Middle Fork Flathead River, were studied to examine the influence of long-term global warming on streamflow and flooding in snowmelt dominated basins. These watersheds were selected to compare impacts resulting from changes in the maritime and continental climate regimes associated with the American and Middle Fork Flathead River basins, respectively. Output from a regional climate model was used to drive a distributed hydrologic model under present and future climate conditions corresponding to a doubling of atmospheric CO2. Under the future climate scenario more winter precipitation fell as rain instead of snow producing higher winter flows, a reduced snowpack, and decreased spring and summer flows in the American River. In addition, there was a large increase in the frequency and magnitude of winter flooding, primarily due to an increase in the numbermore » of rain-on-snow events. The change was much less dramatic in the Middle Fork Flathead River where flooding generally occurs during spring snowmelt. In this basin, the seasonal pattern of streamflow remains intact and the incidence of flooding was reduced under the future climate scenario. This study suggests the impacts of climate change on streamflow and flooding in forested watersheds are highly region specific.« less