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Determining the Magnitude of Impacts

The magnitude of impacts depends on the degree and extent to which the project changes the environment.

Impact magnitude depends on the degree and extent to which the project changes the environment and usually varies according to project phase. In general, most project impacts are associated with construction and operations. Site characterization has fewer impacts and decommissioning and reclamation impacts are relatively short-lived and should result in an improvement in conditions relative to the operational period.

Factors to consider in determining impact magnitude include but are not limited to:

  • Area of Influence: The impact magnitude is often directly related to the size of the area affected. An example would be the acres of land disturbed.

  • Overlap Between Area of Influence and Resource of Interest: The impact magnitude is often directly related to the area of overlap between resources and the overall area of influence for the project. An example would be the overlap between mule deer winter range and the project.

  • Deviation from Current or Baseline Conditions: For projects that affect air or water quality, how much would concentrations of contaminants increase? For projects that result in a large influx of workers, what is the current capacity of housing, schools, and other support services?

  • Project Duration: Magnitude is often directly proportional to the lifespan of the project. A project that operates for 5 years as opposed to 20 years is likely to have much less impact.

  • Sensitivity of the Resources: Some species appear to be very sensitive to disturbance (e.g., sage grouse and bald eagles), whereas others are fairly tolerant of disturbance (e.g., many plant species adapt to disturbance).

  • Project Timing: Project activities that occur during periods of sensitivity (e.g., nesting season, high-precipitation periods) have greater impacts than at other times.

Quantifying Impacts

Impact magnitude is usually quantified on the basis of the factors listed above. Simple calculations (e.g., summing area of influence, calculating areas of overlap between resources and area of influence, determining project duration) or complex mathematical models (e.g., modeling changes in water concentrations or air emission dispersions) are used to quantify impacts.

Often, the magnitude of impacts cannot be readily quantified. This can result from incomplete data or a poor understanding of the effect the project will have on resources. In these cases, professional judgment is usually used to describe in qualitative terms what the magnitude of impacts would be.

When determining impact magnitude, conservative assumptions are often used. Examples of conservative assumptions include assuming that all plants and animals in the project footprint will be killed, that the project will result in a fairly high decrease in property values, and that emission control features will be some percentage below design levels. Sometimes a “worst-case” situation is assumed. The purpose of using conservative assumptions is to ensure that impacts are not underestimated. Conservative assumptions can sometimes result in unrealistically high estimates of impact magnitude. The use of professional judgment in identifying assumptions is essential to ensure that the results are conservative but not unrealistic.