watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
By 2080, only one (2.0%) subwatershed remains in a low risk category and three (5.9%) subwatersheds
remain in a moderate risk category (Figure 9). The remaining 47 (92.1%) subwatersheds are in a high risk
category, with Elk, Meadow, Pettit Lake, Smiley, Stanley Lake, and Upper Redfish Lake Creeks showing
over a 5-day increase in winter peak flows.
Summer Baseflows (Mean Summer)
Figure 10. Summer baseflow risk in 2040; moderate
(yellow); and lowest (green)
169 Assessing the Vulnerability of Watersheds to Climate Change
The VIC model projects that summer baseflows
may decrease from current conditions (1977-1997)
by 22% in 2040 and 29% in 2080, for the entire
Sawtooth NRA under the A1B emission scenario.
This is not unexpected, because air temperatures
and evapotranspiration are expected to increase.
Increasing winter air temperatures will reduce the
amount of snow (e.g., more precipitation falling as
rain than snow), as already observed in several
parts of the western United States (Aguado et al.
1992; Dettinger and Cayan 1995). Higher spring
temperatures will also initiate earlier runoff and
peak streamflows in snowmelt-dominated basins
(Aguado et al. 1992; Cayan et al. 2001).
By 2040, 18 (35.3%) of the 51 subwatersheds
analyzed are predicted to see moderate risks (20-
40%) from decreases in baseflow, and 33
subwatersheds will see low risk (< 20%) (Figure 10).
By 2080, only 5 (9.8%) subwatersheds are predicted
to remain in a low risk category and 42 (89.4%)
subwatersheds in a moderate risk category (Figure
11). The remaining 4 (7.8%) subwatersheds (Beaver,
Elk, Fishhook, and Park-Hanna) are predicted to be
in a high risk category, with baseflow decreases of
37% or greater. These model predictions, however,
should not be viewed as absolute changes, but
instead as more reflective of a general trend of
declining baseflows. This is because the VIC does
not model groundwater, which causes it to
underestimate summer flows where groundwater
contributes. Conversely, the model also
overestimates summer flows in drainages that lose
stream flow.
Figure 11. Summer baseflow risk in 2080; highest
(red); moderate (yellow); and lowest (green)
Still, the prediction of lower baseflows is consistent with other studies. Since 1950, stream discharge in
both the Colorado and Columbia River basins has decreased (Walter et al. 2004). Regonda et al. (2005)