watervulnerability

White River National Forest Watershed Vulnerability Assessment, Rocky Mountain Region (R2)
this reason, projected increases in stream temperatures are not carried forward in this process as a
potential impact.
Step 3. Identify Landscape-Scale Ecological and Anthropogenic Drivers
At this point in the analysis, we have a general idea about the magnitude and direction of effects to
aquatic systems from climate change. From the exposure data, we can see that temperatures will increase,
some elevations will experience more rain than snow, and runoff timing may shift earlier while overall
volume may decrease. With these potential changes in mind, we looked at the landscape-level drivers,
both inherent to the subwatershed and human-created, that could either exacerbate or buffer these effects.
Inherent Attributes of the Project Area Subwatersheds
The resiliency of a watershed to any change is largely a function of parent geology, typical climate,
topography, and vegetation. For this analysis, these factors were subdivided into more specific attributes
that could be queried in GIS by subwatershed. The attributes considered most important for the White
River National Forest are as follows:
Geochemistry of the parent geology. Aquatic systems are intimately linked with the chemistry of the
parent geology. In particular, calcareous geologies contain calcium carbonate (CaCO3), which dissolves
to form ions that influence primary productivity in a stream. The weathering of these rocks also raises the
stream pH and produces carbon dioxide for photosynthesis (Staley 2008). Because of the buffering effects
to aquatic ecosystems from increased productivity, the percentage of a subwatershed with calcareous
parent geology was used as a measure of resiliency to climate change.
Extent of glaciation. Glacial processes have made some landscapes more suitable for wetland and
riparian area developments by flattening the gradient of high mountain valleys and slowing runoff.
Lateral and terminal moraines have created topography that encourages the slow movement and retention
of large volumes of snowmelt-recharged groundwater. Consequently, glaciated environments typically
have the highest densities of high-quality wetlands on the forest. Since glaciation generally led to a
significant local influence on water availability and distribution, the percent of a subwatershed that was
glaciated is used as a measure of inherent resiliency to climate change.
Aspect. In snow dominated systems, aspect is a key factor affecting the size and longevity of the
snowpack. South aspects tend to lose snow to evaporation or sublimation, even in the middle of winter.
Subwatersheds dominated by southern aspects are expected to carry less snow for shorter periods under a
warming climate scenario. Therefore, the percent of a subwatershed with a south, southeast, or southwest
aspect is used as a measure of inherent resiliency to climate change.
Hydroclimatic regime. This refers to the typical precipitation regime for a subwatershed. In the central
Colorado Rocky Mountains, landscapes below about 7,500 feet typically have much of their precipitation
and storm peaks associated with rainfall. Landscapes above about 7,500 feet in elevation typically have
most of their precipitation and storm peaks associated with snowfall and snowmelt. As the climate warms,
we expect that the transition from a snow-dominated to rain-snow-dominated precipitation regime will
migrate upslope. The elevation band from 7,500 to 8,500 feet is considered to be an at-risk zone for
snowpack. For this analysis, the percent of a subwatershed within the at-risk snow elevation band is used
as a measure of inherent resiliency.
Weighted precipitation. This attribute refers to the amount of precipitation that falls on the landscape as
either snow or rain. In the Rocky Mountains, precipitation amount varies significantly with elevation and
orographic effects. The Parameter-elevation Regressions on Independent Slopes Model (PRISM) database
120 Assessing the Vulnerability of Watersheds to Climate Change