watervulnerability

Helena National Forest Watershed Vulnerability Assessment, Northern Region (R1)
divided into six overall pathways (Table 3). Each of the pathways is categorized in terms of functionality;
either Functioning Appropriately (FA), Functioning at Risk (FAR), or Functioning at Unacceptable Risk
(FUR). The final rating is based on a suite of metrics which are either (1) quantitative metrics of collected
field data or GIS driven attributes (e.g. road density) or (2) qualitative descriptions based on field reviews,
professional judgment, etc.
The composite watershed sensitivity based on the baseline analysis is depicted in Figure 3. Based on these
parameters, the Helena National Forest has three subwatersheds rated as FA, ten rated as FAR and five
rated as FUR. These rating is applied to only those subwatersheds where there are known populations of
bull trout. Evaluation of those watersheds that could have potential for bull trout habitat but do not
currently have viable populations were not included in this analysis.
Exposure
Summer maximum air temperature predictions were used as a surrogate for stream temperature because
stream temperature data was not widely available. At the time of our analysis this was our best available
dataset, in the future, it might be better to use mean summer temperature as better correlations have been
found between air-water temperatures using the mean vs. max, even though these were very strongly
correlated (Wenger et al. 2011a). Summer maximum air temperatures were predicted to increase by
approximately 2 °C uniformly across the forest for the 2030-2059 predictive period and approximately 5
°C uniformly for the 2070-2099 predictive period. Consequently, it is predicted that not any one
watershed will be more impacted by this change in summer maximum air temperature than another.
However, we can develop conservation strategies based on current conditions in order to buffer more
highly valued watersheds.
Summer baseflow was considered as an exposure element, but not carried forward because Wenger’s
(2011a) work showed temperature to be the key climate change variable related to bull trout habitat. Bull
trout are likely sensitive to increase in winter high flows (Wenger 2011b), but this data is available at the
reach level and time at this point does not allow for this kind of analysis. Winter 95 represents the number
of days during winter that are among the highest 5% (respectively) of flows for the year. Winter 95 was
used as the variable for winter high flows which would affect bull trout and brook trout, but not the spring
spawning Westslope cutthroat trout.
Watershed Vulnerability
By overlaying the climate exposure data to the bull trout fisheries baseline data we see which habitat
currently supporting bull trout populations is most likely to be adversely impacted by changes such as
increased temperatures. Research has found bull trout currently inhabit the coldest available headwater
streams which leaves little potential to shift to higher elevation habitats to avoid temperature increases
(Wenger 2011a). Because the predicted temperature changes on the Helena National Forest are very
uniform across all bull trout habitat, we assumed that it all has similar potential to be impacted by changes
in climate. However, forest managers have the capability to maintain or increase the resiliency of
watersheds that support the most valued bull trout fisheries. These areas can be selected as high priority
for management activities. Because exposure to increased air temperatures is essentially uniform across
the Forest, composite watershed vulnerability for bull trout habitat is equal to the watershed sensitivity
analysis (Figure 3) or the current condition of the fisheries habitat. Incorporation of other climate change
indicators may or may not change the overall potential vulnerability of these watersheds.
52 Assessing the Vulnerability of Watersheds to Climate Change