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Examples of Variables Used to Estimate Vulnerability

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).

Native aquatic species

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.

Forest cover and defoliation for watersheds

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.

Nitrogen deposition

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.

 
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