The variables used to estimate vulnerability fall into
three basic categories.
Resource Distribution
Resource distribution variables record the current
geographic distribution of significant environmental and human
resources. Numbers of native aquatic species are a resource of
value to society. High numbers of these species indicate areas
that could be a higher priority for protection over areas with
low numbers of species. Numbers of native species can also
tell us where stresses have been acting for some time (low
numbers of native species) or where stress has historically
been low (high numbers).
Here ReVA uses red to indicate lower resource value and
green to indicate higher resource value. |
Sensitivities
Sensitivity variables are conditions, or changes in
conditions, that modify a resource's response to stress,
either making it greater or less. For example, the percent of
forest cover that has been defoliated is an indication of how
susceptible to damage a given area of forest may be to
additional stress, and thus is considered to be a sensitivity
variable for forest condition in general. Forests that have
been heavily defoliated are more likely to experience high
levels of mortality when an additional stress, such as air
pollution, is also high. High levels of forest mortality can
in turn result in increased sediment loadings (through reduced
uptake of water by trees with higher runoff) and increased
nutrients (through decay of trees) in streams.
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In a final integration for vulnerability, a user may chose
to weight this factor more highly. Thinking of it in terms of
an if-then scenario may be helpful; for example, research and
experience may indicate that the stress on a forest jumps
significantly as percent defoliation rises above some
threshold level. In this case, the cumulative stress displayed
for areas above the threshold level can be amplified by some
appropriate factor. In time and with experience, the best
guesses for appropriate threshold and amplification factors
may be better understood. |
Stressor Distribution
Stressor variables indicate the distribution of activities
or conditions that combine to cause environmental degradation.
For example, acid rain stresses plants, contributing to poor
forest health. Nitrate (NO3) and Sulfate (SO4) are the major
pollutants contributing to acid rain, and thus estimates of
Nitrate wet deposition are commonly used as an indication of
acid rain. The degree to which Nitrate wet deposition becomes
a pollutant is dependent on the location and the
physiochemistry of the forest soils or waterbody upon which it
falls. Some important variables cannot be measured directly,
but their distribution can be estimated using well-established
models. In this example, the spatial distribution of wet
nitrate deposition is estimated from rainfall data (amount)
and proximity to sources of NOx air pollution (from fossil
fuel burning). Nitrate from air pollution contributes to
nutrient levels in streams, which affects aquatic habitat (in
particular aquatic plants) and native species'
condition.
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Nitrate wet deposition contributes to acid levels in
surface water and, with Sulfate, wet deposition can be
considered as an indication of acid rain. The degree to which
Nitrate wet deposition becomes a pollutant is dependent on the
location and the physiochemistry of the forest soils or
waterbody upon which it falls. |