Preface 
This research report describes MEXSES, a rule-based expert system for environmental impact assessment at a screening level, implemented for the analysis of water resources development projects in the Lower Mekong Basin. The system was developed and implemented under contract to the Mekong Secretariat, Bangkok, Thailand.
The system provides an example of a qualitative and logic-based technique for the analysis of complex environmental assessment problems. Rather than attempting to predict, in any absolute sense, environmental impacts and to evaluate them, we have organized the available information in a consistent and plausible framework that links project characteristics and environmental features to expected impacts in a simple logical format of IF ... THEN rules. This assessment is based on mainly qualitative descriptions of the relevant variables and descriptors of development projects and the environment. It uses logic and rules derived from expert opinions, rather than algorithms and numerical models, to arrive at conclusions, and thereby an assessment of expected environmental impacts.
MEXSES represents an experimental approach to the analysis of complex systems. Recognizing that the uncertainties are tremendous, our main objective is to construct a plausible and consistent framework for thinking about the problem, a tool to organize and analyze the available information in a specific institutional and planning-oriented context.
Rather than numerical precision of questionable origin, we seek to identify basic patterns and trends and approximate classifications, and to challenge our own mental models (or any numerical ones for that matter), in a dialogue with the computer. The expert system is primarily an attempt to model our understanding, our perception of the problem, rather than ``reality''. It is also designed as a collection and repository of expertise and relevant information, compiled from more than one expert and numerous other sources of information, in a format that can directly, and in fact automatically, be brought to bear on the difficult task of environmental impact assessment.
So as to organize the relevant information, MEXSES provides specific check lists for development projects such as dams and reservoirs, hydropower and irrigation projects, or fisheries, aquaculture and navigation development. The checklists are based on the Environmental Guidelines Series of the Asian Development Bank.
The system uses hierarchical checklists, organized by problem classes, following the logic of project planning and implementation and a qualitative assessment procedure based on rules and descriptors, which allow the analyst to assess the individual subproblems identified in the checklists in terms of their expected environmental impacts. On the basis of this list of estimated impacts, a summary assessment of a given project can be performed in terms of a few aggregated criteria covering environmental as well as socio-economic aspects of river basin development.
The knowledge representation uses a simple syntax for rules and decision tables, that operate on descriptors of project features, environmental characteristics and impacts. The descriptor definitions are implemented in a frame-like, object-oriented language that includes the descriptor name, the list of symbols and associated numerical ranges for allowable descriptor values (most descriptors can have both numerical or symbolic values concurrently), references to rules that can be used to derive a descriptor value from other descriptors, instructions for an ask function to obtain the value from the user interactively, and the linkage to a hypertext system of help and explanation, background information, definitions of concepts and a glossary of terms.
The inference engine uses look-ahead pre-processing for the dynamic pruning of the inference tree. It offers both forward and backward chaining functions for standard assessment and an alternative hypothesis testing feature, respectively. Different modes of interaction, all based on a fully menu-driven graphical user interface implemented in X Windows, offer alternative levels of verbosity with the optional display, and selection by the user, of rules, as well as an integrated hypertext system of help and explanatory text functions.
In addition to the expert system proper, the software system includes a project data base as well as an integrated geographical information system (GIS) for the management of spatial environmental data.
The report makes a brief review of environmental impact assessment methods and tools. It discusses expert systems technology, with emphasis on environmental applications. The Lower Mekong Basin and its specific environmental problems, as well as the Mekong Secretariat's environmental policy are examined. Subsequently, the software system is described from a user's perspective, followed by a detailed description of the methodology employed and its implementation. In the final chapter, a number of issues around the successful application of such a system are discussed, including a number of suggested improvements and extensions to the current operational prototype.
EIAxpert: An Expert System for screening-level EIA
