watervulnerability

White River National Forest Watershed Vulnerability Assessment, Rocky Mountain Region (R2)
This abbreviated list was considered comprehensive enough to cover the most significant aquatic issues
while not generating redundant information across a long list of resource values. It became apparent that
narrowing the list of resource values was justified since there is only modest variability in the final
relative vulnerability of the three selected resource values.
Step 2. Quantify the Anticipated Exposure from Climate Change
Exposure is the term used to describe the amount of anticipated change in climate over time. The types of
exposure typically considered for the mountainous West include changes in air temperature, changes in
precipitation, and changes in runoff.
Exposure estimates are not only highly variable but are highly uncertain as well. Variability of exposure
estimates arise primarily from differences in carbon emission scenarios and the time frame of concern.
High (A2) and low (B1) emission scenarios give very different exposure results when modeled at midcentury
(often shown as year 2040 or 2050) versus those modeled at the end of the century.
Uncertainty is also a major factor in estimating exposure. Exposure estimates, whether for temperature,
precipitation, or runoff, are generated from global circulation models that attempt to predict weather
patterns around the globe simultaneously for any given emission scenario. These large-scale global
estimates are then down-scaled to smaller areas of concern, such as a state or some smaller region. A
single model is rarely used to estimate exposure in a given locale. Rather, many different models are run
and the exposure value presented is often the median of the predictions, along with a potential range of
values.
Since water supply is such a significant issue in the arid west, many states have compiled summaries of
climate change predictions in order to assess future water supplies. Colorado is one of those states. For
this analysis, climate change exposure data were taken from the 2008 report for the Colorado Water
Conservation Boards entitled Climate Change in Colorado: A Synthesis to Support Water Resources
Management and Adaptation (Ray et al. 2008).
Predicted changes to temperature, precipitation, snowpack, and runoff (Christensen and Lettenmaire,
2006) are shown below in Figures 2 through 5. Figure 2 shows that air temperatures are predicted to
increase over time. For the high-emission scenario (A2), the median predictions suggest an increase of 2.5
to 4.5 degrees Fahrenheit for mid- and late-century timeframes, respectively. This is in addition to an
estimated 2 degree increase that has occurred over the last 30 years. Summers are projected to warm more
than winters; winter projections show fewer extreme cold months, more extreme warm months, and more
strings of consecutive warm winters (Ray et al. 2008). These warmer temperatures are likely to influence
precipitation type, stream temperatures, and stream flow rates.
115 Assessing the Vulnerability of Watersheds to Climate Change