watervulnerability

White River National Forest Watershed Vulnerability Assessment, Rocky Mountain Region (R2)
BACKGROUND
The White River National Forest is located in west central Colorado, on the western slope of the Rocky
Mountains in the Rocky Mountain Region (R2) of the USFS. Over the 2.3 million acre forest, elevations
start from a low of about 5,500 feet and rise to include several peaks over 14,000 feet. Glaciation has
shaped the higher elevations. Granitic rocks are prevalent on the eastern side of the forest; sedimentary
formations dominate the western side. Most of the precipitation falls as snow in the winter, although
summer thunderstorms are common. Snowmelt from the forest into the Colorado River provides water to
27 million people in 7 states and two countries (Painter et al. 2010). Peak flows are generally associated
with snowmelt, except for the western edge of the forest.
The White River is the most visited National Forest in the country, largely because of winter sports. Most
of Colorado’s largest ski areas (Vail, Keystone, Breckenridge, Aspen, etc.) are permit holders on the
Forest. Consequently, there is a keen interest in how a changing climate may affect air temperatures and
precipitation.
INTRODUCTION
Aquatic biological systems, such as those supported by National Forests, have evolved under certain
climatic conditions. As the climate changes, it is reasonable to anticipate that a watershed’s ecological or
biological values could also change. The analysis described herein is an attempt to apply expected
changes in climate to large portions of the landscape, and determine which areas (and their associated
resource values) are least resilient and therefore most susceptible to adverse effects from a changing
climate.
The objective of this effort is to define a process that sorts blocks of the landscape (HUC-6 subwatersheds
in this case) into categories that express their relative vulnerability to climate change. By way of analogy,
we propose to take all the subwatersheds on the forest and (mentally) shake them through a series of
sieves in order to identify those that have the least resiliency to the anticipated changes in temperature,
precipitation, and runoff.
Because this process is intended to cover large landscapes (2.3 million acres in this case), it is necessary
to rely on existing data. The GIS queries that make up the basis for the assessment rely on common
corporate layers from either the Forest Service or state agencies.
A key step at the outset of this process was the identification of an appropriate scale of analysis. Since
the analysis is aquatics-based, watershed boundaries were chosen. Because subwatersheds generally
coincide with the management scale of most Forest activities, and are also small enough to allow local
expression of factors such as aspect, elevation, vegetation type, etc., they were chosen as the unit of
analysis.
The schematic in Figure 1 shows the general thought process behind the analysis protocol. Resource
values (for example, a sensitive species of trout), are supported by a complex interaction of ecological
landscape-scale drivers. These drivers define the ecological context (environment) of the watershed and
can include such attributes as geology, aspect, precipitation, and glaciation, etc. Changes to this
environment occur constantly, but large changes from anthropogenic or climatic stressors may affect the
resiliency of the resource value of concern. Determining how these ecological and anthropogenic
characteristics interact with anticipated climatic stressors to affect the relative resiliency of each
subwatershed is the objective of this analysis.
113 Assessing the Vulnerability of Watersheds to Climate Change