watervulnerability

White River National Forest Watershed Vulnerability Assessment, Rocky Mountain Region (R2)
needs to be weighted in order to combine them into a meaningful aggregate score. The processes for
assigning categories and relative weights are as follows.
Determination of High, Moderate, and Low Categories for Subwatershed Attributes
In order to apply a simple mathematical ranking system by subwatershed, each of the previously
discussed attributes required binning into categories. The amount of influence that an attribute exerts
within a given subwatershed was categorized as high, moderate, or low.
Upon inspection, most of the attributes or influences have no physical threshold to suggest a breakpoint
between categories. For example, we don’t have any data to suggest how many diversions per square mile
a subwatershed can contain and still have a low influence on aquatic systems. Since the objective of this
analysis was to determine relative vulnerability between subwatersheds, a simple and objective approach
was used. For each attribute listed, the distribution of the 166 subwatersheds was plotted and the quartiles
determined. By definition, the first quartile is the 25 th percentile of the ranked data, the second quartile is
the median, and the third quartile is the 75 th percentile of the ranked data. Subwatersheds below the first
quartile (lowest 25%) were ranked as low influence; subwatersheds between the first and third quartile
(middle 50%) were ranked as moderate; subwatersheds above the third quartile (top 25%) were ranked as
high. See the example plot for road density below in Figure 6.
Figure 6. A sample histogram of diversions per square mile across all subwatersheds, and the use of quartiles to
categorize the relative influence on resiliency as high, moderate, or low
Determination of the Relative Weights of Inherent and Anthropogenic Attributes
While each of the attributes listed has some effect on the ultimate resiliency of the subwatersheds, they do
not have equal effects. For example, the amount of precipitation or the amount of water withdrawn from a
subwatershed is likely more important than a primary productivity increase from calcareous geology.
Consequently, a simple method of scaling the relative influence of the attributes was developed.
Since physically removing water from the stream (water uses) has the most direct effect on aquatic
systems, each attribute was weighted relative to that, with values ranging from 0.25 (1/4 the effect) to 1
(similar effect). The assigned weights are as follows: geochemistry of parent geology (0.25), extent of
122 Assessing the Vulnerability of Watersheds to Climate Change