A fourth category of impact prediction approaches in environmental impact studies is environmental indices (Ott 1978). An environmental index is a mathematical or descriptive presentation of information factors used for classifying environmental quality and sensitivity and predicting the impacts of a proposed project or activity. The basic concept for impact prediction is to anticipate and quantify the change in the environmental index as a result of the project or activity, and then to consider the difference in the index from the with- and without-project conditions as a measure of impact. Numerous environmental indices have been developed for air quality, water quality, noise, visual quality, and quality of life (a socioeconomic index which can include a large number of specific factors). One widely used type of index is based on habitat considerations and uses the Habitat Evaluation Procedures developed by the U.S. Fish and Wildlife Service (1980) or a Habitat Evaluation System developed by the U.S. Army Corps of Engineers (1980). This system is based on the development of a numerical index to describe habitat quality and size. A key advantage of index approaches for impact prediction is that these approaches can be related to available information and they provide a systematic basis for considering the consequences of a project or activity.
The fifth category of impact prediction approaches is experimental methods. These methods encompass the conduction of laboratory experiments to develop factors or coefficients for mathematical models, and the conduction of large-scale field experiments to measure changes in environmental features as a result of system perturbations. In addition, physical models are used to examine impacts related to hydrodynamics and ecological changes within microcosms of environmental settings. Experimental methods are primarily useful in dealing with physical-chemical components and biological features of the environmental setting.
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