Despite environmental impact assessments' relatively short history, a concensus of the need for minimal information regarding a) impact identification, b) predictions, c) interpretation, d) tradeoff determinations and preferences, and e) communication (Munn, 1975). However, most schemes generally have proved to be less than satisfactory in collecting, interpreting, and displaying the required information.
A majority of assessment methods have centered on or have emphasized one of four general strategies: a) checklists, b) matrices, c) networks, and d) map overlays. In each, the focus of the method is on identification of potential impacts; weighting the relative values of these impacts has received only secondary emphasis.
The checklist is common to many of the methods and makes use of a catalogue of impacts that might be expected from different activities. By itself, this is valuable but it says little about the extent or relative importance of the impacts. An example of the checklist is that developed by Batelle (see Dee et al, 1973) which divides potential impacts into four major categories: a) ecology, b) environmental pollution, c) esthetics, and d) human interest. These are subdivided into 18 components and 78 parameters. Although the method emphasizes quantitative impact, the methodology for assigning weights to the various impact parameters and converting them to a common base (environmental quality units) through specific graphs and value functions is a bit unwieldly. Other checklist methodologies include that of Adkins and Burke (1971) for evaluating social, economic, and environmental impacts of highway routing and construction, and the "Georgia methodology" which incorporates 56 environmental components specific to the evaluation of highway project alternatives (Institute of Ecology, 1971).
Matrices combine a list of possible impacts with different project activities that might be associated with or bring about such consequences. The intent is to be more explicit in discerning the specific actions that impact on specific environmental characteristics. Matrices go beyond a simple listing to deal, at least in an initial way, with cause and effect relationships.
The now classic approach of Leopold et al. (1971) utilizes the matrix approach to identify 100 project activities and 88 environmental characteristics or conditions which may be impacted. As presented by Leopold, heavy emphasis is placed on ecological and physical-chemical impacts whereas social and economic impacts and secondary impacts are not evaluated.
Network proposals attempt to deal more fully with cause and effect relationships. As in other methods, impacts are listed but indications of how these are brought about are presented by means of flow charts. Sorensen (1970) and Sorensen and Pepper (1973) use examples of this approach. Its advantage lies in its capabilities to trace pathways that allow identification of both primary and secondary impacts.
Map overlays, in the main, attempt to find areas of lesser conflict among resource uses and environmentally important values. This is accomplished by overlaying a series of maps which indicate various environmental characteristics, such as vegetation types, water courses, cultural and historic sites, and wildlife habitat. One of the pioneers in the development of this approach was McHarg (1968, 1969). Its advantage is that it can be used as a "first cut" method to identify alternative project sites for later, more detailed impact analysis. However, it is difficult to establish the relative importance of and interrelationships among resource uses.
In all of these methods, a major problem is how to assign values of significance to forecasted changes; things must be deemed of greater or lesser importance in accord with some scale and this, almost by definition, will vary with the evaluator. This, in turn, creates problems of comparability and reproducibility between and perhaps even within alternatives. Similarly, the impacts will normally be expressed in a variety of measurement units, and reaching a common numerary is a major problem. Consequently, the resultant indices are inherently arbitrary since they depend on the largely subjective weights used. Likewise, there are disadvantages in substituting a single number for an array of information that could more usefully resolve the conflicts (Lord and Warner, 1973). It is also abundantly clear that the current procedures and methods are not adequate to give a fair balancing of economic, technical, and environmental values in project planning nor do they exert an appropriate influence throughout the planning process. It is also evident that the current procedures can be burdensome and costly; that little clear focus is provided for data collection; and that often only vague criteria are given for comparison and weighting. While methods are still evolving, it now appears that, contrary to much of the current work in the area, the search should not be simply for a more intricate, ingenious, and longer list of the types of impacts that may be encountered as a result of initiation of a project. A checklist by itself is not sufficient to do more than provide the enumeration of impacts and encourages simply the listing of the predicted demise of environmental resources. Although a useful first step, it does nothing to ensure that such enumerations are explicitly taken into account in the evaluation of a project's desirability and, unfortunately, it very often encourages consignment to virtual obscurity.
Despite these problems, experience has indicated that environmental values do receive more explicit and proper attention when an environmental impact evaluation is made.
From this short review of environmental impact assessment methods, several conclusions on methodology can be made.
- Methods can discriminate minimal information needs.
- They provide comprehensive lists of kinds of impacts and potential consequences.
- They may elicit reaction and articulation of values from most affected parties, including the public.
- They assume the existence of good data base - thus they do not suggest data collection or field survey methods.
- They assume that the proposed action does not adequately take into account the environment impacts.
- They do not develop information for use in economic analysis.
- They make an artificial distinction between socio-economic costs and benefits and changes in environmental conditions.
- They do not adequately show linkages.
It is clear that the various methods in existence must be closely scrutinized and adapted if they are to be effectively used elsewhere in the world. Also, and perhaps more importantly, it is clear that the impact assessment concept as currently practiced is not entirely suited for adaptation 10 early stages of river basin planning.