Since the National Environmental Policy Act (NEPA) became law in the United States, much effort has been expended by agency personnel, consultants and academics in devising methods to aid preparation of environmental impact statements (EISs). Most of this work has emanated from the USA, but with the introduction of EIA procedures into more and more countries, the ingenuity devoted to developing EIA methods has increased correspondingly. EIA methods are formulated throughout the world although the USA is still the main source.
For the purposes of this discussion a ‘recent’ method is one which has appeared in the literature since 1978. This literature consists of articles in journals, EISs, unpublished conference papers and items from the ‘grey literature’ which contain so much of the thinking and writing devoted to EIA. Pre-1978 methodological developments have been widely reviewed in the literature and comprehensive descriptions can be found in, for example, Clark et al. (1980), Canter (1983), Bisset (1984a) and Wathern (1984).
The term ‘method’ deserves some elucidation. A distinction must be made between methods and techniques used in EIA. EIA techniques are concerned with predicting future states of specific environmental parameters such as noise levels. In any single EIA study a number of techniques may be used. Together, they provide data which are then collated, arranged, presented and sometimes interpreted according to the organizational principles of the EIA method being used. EIA methods have been described alternatively as methodologies, technologies, approaches, manuals, guidelines and even procedures in the literature.
This chapter cannot contain a description or discussion of every method which has been put forward since 1978 nor can it ignore totally the pre-1978 literature, upon which many of the more recent developments are based. Rather it will try to identify common themes or trends which appear to characterize the methods developed since 1978. Attention will be paid to recent and current thinking which has resulted in the development of a particular type of method or its variants. The types of method described here are index approaches, systems diagrams, simulation modelling and the ‘sound ecological principles’ approach.