watervulnerability

Chugach National Forest Watershed Vulnerability Assessment, Alaska Region (R10)
As a result, the main data gap was an estimate of the future change in streamflows, snowpacks, runoff
timing, and other parameters. My assumption is that this information is available from VIC and other
models for the lower 48 states, but I am not aware that such data are available for Alaska yet. The limited
numbers of stream-gauging stations, limited duration of station operation, and the limited number of
weather data sites in remote areas may be part of the reason. In any case, the data did not appear to be
readily available, so I turned my focus to qualitative assessments.
Other data gaps included long-term water temperature data and stream height/flood level data. Having
more specific data would have added more certainty to some statements and conclusions, but overall I
think the general concepts are valid.
The accuracy of the data provided by the models appeared to be a little questionable at times. For some
areas near Cordova, the maps don’t always fit the topography, which may reflect the extrapolations
between distant weather stations or distance from the ocean. The 2 km cells may also add some
uncertainty if one is trying to analyze a relatively small area. However, if one is only looking for trends,
small discrepancies may not be a concern.
The variation among models also raises some questions. The SNAP website states that the variability
among the models is generally in the range of 0-4 °F and 0-0.7 inches for precipitation. Four degrees is a
large range when one is looking at winter temperatures that are near freezing. For Hope, where conditions
are relatively dry, the range of variability for precipitation is often greater. There is also the question of
whether an average of five models is any more accurate than any single model. Thus, if one were to do a
quantitative analysis, there may be problems. However, the models all agree in the general trends, which
should be sufficient for some types of analysis.
Assessing Risk
One of the suggested methods for assessing overall watershed vulnerability was to create a risk matrix,
comparing various attributes such as road density or slope, values such as fish populations, predicted
climate change parameters, and then assign risk levels on a low to high scale. The total scores would be
used to determine the most vulnerable watersheds. This process did not appear to be applicable for the
Chugach National Forest, where most of the watersheds are undisturbed, road densities are uniformly low
or zero, and the risks to fish and other wildlife from the predicted climate changes are unclear.
Assigning different levels of risk seemed to be subjective, given the wide differences between the
ecosystems. While winter temperatures are expected to increase by about 3.7 °C for both Hope and
Cordova, the effect in Cordova will be much greater since low-elevation winter temperatures are hovering
around the freezing point. Similarly, larger precipitation increases in Cordova are probably less
meaningful, given the currently high precipitation. Also, some watersheds may have greater fire hazards,
while others may have more valuable fish, so the comparisons may not be equal.
With the limited number of developed watersheds, it didn’t seem necessary to rank them to determine
which are the most vulnerable. For the Chugach, it seems simpler to identify the specific issues for each
watershed on its own, since there are only a few to analyze.
The other problem is determining the magnitude of adverse effects from climate change over existing
conditions. As discussed, the predicted increases in temperature and precipitation are well within the
historical variability, although more extreme weather events are expected. While one can intuitively say
that greater precipitation could lead to greater erosion and landslides, it may be difficult to argue that
another 6 inches of rain will increase landslides in a watershed that already receives a mean of 177 inches.
299 Assessing the Vulnerability of Watersheds to Climate Change