watervulnerability

Gallatin National Forest Watershed Vulnerability Assessment, Northern Region (R1)
Exposure
To evaluate exposure, we used the regional downscaled climate and hydrological projections developed
by Littell et al. (2011), which build on research and data from the Climate Impacts Group (CIG) at the
University of Washington. We chose their Ensemble model to examine potential climate change impacts
more closely. This model is composed of the 10 best-fitting global circulation models (GCMs) for the
Upper Missouri River Basin region. The modeled time periods available are 1916-2006 (historic), 2030-
2049 (mid-21 st century), and 2070-2099 (late 21 st century).
The climate projections are downscaled to 6 km 2 resolution and are most appropriately summarized at the
HUC-5 scale. The HUC-6 subwatersheds were overlaid to examine how they may be influenced by these
climate projections. The metrics retrieved for the most current run of the WVA include variable
infiltration capacity (VIC) derived (Liang et al. 1994; Liang et al. 1996) hydrological projections:
combined annual flow, seasonality of flow, and snowpack vulnerability (hydrologic regime). For the
Upper Missouri River Basin, some of the overall trends predicted for the mid- to late 21 st century include
increases in average annual air temperature, increases in seasonal air temperatures, increases in winter
precipitation, and decreases in summer precipitation.
Currently, we have used only the air temperature projections to examine predicted trends. In the future,
these predicted air temperatures, combined with our stream temperature model (in development) and local
air temperature data, may be used to model and predict stream temperatures across the forest.
Potential Impacts to Water Resources
1. Increased instances of low flows and lower flows
• Water uses/diversions would amplify the anticipated low flows
• Culverts currently passable by fish may become barriers during low flows
2. Changes in flow regime
• Increased winter flooding could increase summer low flows; increased/prolonged drought
in the summer will further amplify the effects of changes in flow
• Increased winter scouring of fall spawners (brook trout)
− May favor native cutthroat trout
3. Increased stream temperatures
• Previously unsuitable stream habitats (too cold) may become suitable for fish
• At lower elevations, native cold-water fish will be negatively affected
− More tolerant invasive fish species may outcompete natives
−
4. Increased precipitation events
• Roads would have increased sedimentation into streams
• Culverts may need to be enlarged and/or maintained more frequently to accommodate
higher flow
• Some roads may need more frequent maintenance
5. Increased drought events
• Water use/diversions would exacerbate drought events
• Possible increases in wildfires
41 Assessing the Vulnerability of Watersheds to Climate Change