watervulnerability

determine changes in groundwater levels and flow rates
to lakes, streams, and wetlands.
The analysis on the GMUG NFs differed from other
assessments, in that results were displayed at a large scale.
Six large geographic areas, stratified by climatic regime
and elevation, were used for graphical analysis. Projected
changes to maximum and minimum air temperatures
and an index of aridity were factors used to rate exposure
in each of these geographical areas. This analysis
technique was at least partially driven by the resolution
of the downscaled exposure data, which is typically on
a grid of 6 km2 (Figure 7). This fairly gross resolution
results in as few as two or three data points for a HUC-6,
making discrimination at this scale inappropriate. As a
result, pilots typically used HUC-6 for distinguishing
differences in resource densities and sensitivity, overlaid
with a larger-scale rating of exposure.
Climate models typically provide predictions of
temperature and precipitation. These data are then
combined with characterizations of watershed
characteristics and vegetation in modeling of
other hydrologic variables. CIG has also developed
predictions of hydrologic change based on the Variable
Infiltration Capacity (VIC) model (Gao et al. in review).
The CIG was extremely helpful in releasing data for
use during the pilot study, and in explaining its utility
and limitations. VIC is a distributed, largely physicallybased
macro-scale model that balances water and energy
fluxes at the land surface and takes into account soil
moisture, infiltration, runoff, and baseflow processes
within vegetation classes. It has been widely used in
13 | ASSESSING THE VULNERABILITY OF WATERSHEDS TO CLIMATE CHANGE
Assessment Principle Five: Don't Get Lost in
Exposure Data
Pilot Forests used exposure data of different
specificity and detail (for example, in one
case, only air temperature change; in another,
predicted stream temperatures). The level of
detail influenced the analysis, but the take-home
message is that all levels of exposure projections
produced useable vulnerability assessments.
Detailed projections at management-relevant
scales are not necessary to gauge relative
vulnerability of watersheds. It is more productive
to move forward with the analysis than to get lost
in the details of refining exposure data.
the western U.S. to study past and potential future
changes to water flow regimes (e.g., Hamlet et al. 2009),
snowpacks (Hamlet et al. 2005), and droughts (Luo and
Wood 2007). Several pilots (Helena, GMUG, Coconino,
and Sawtooth NFs) made use of the VIC model outputs
to evaluate exposure. VIC attributes evaluated by pilots
included runoff, baseflow, and snow water equivalent.
Several pilots employed projections of changes to flow
characteristics. These were selected because of their
important influence on habitat for species of concern.
Flow metrics were also useful in describing relative
exposure of water uses. In contrast, predictions of peak-
and low-flow responses to climate change are limited
and consist primarily of generalized predictions of
higher peaks and more severe droughts with warming
Figure 9. Winter peak flow risk from Sawtooth NF WVA; current data (at left) and projected data for 2040 (at right).
Ratings for subwatersheds are: highest risk (red), moderate risk (yellow), and lowest risk (green). Ratings were developed
by assessing change to frequency of highest stream flows occurring during the winter. Red lines are HUC-4 boundaries.