MULTICRITERION DECISION ANALYSIS FOR CONFLICT RESOLUTION IN TRANSBOUNDARY RIVER BASINS by J. GANOULIS, El. Kolokytha and Y. Mylopoulos Division of Hydraulics and Environmental Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece e-mail: iganouli@civil.auth.gr ABSTRACT Sharing international waters in transboundary river basins may lead to potential conflict, political tensions and even, in some extreme cases, as the media often predicts, to armed confrontations. The reasons behind these situations may be of an internal nature, or caused by external or international forces. Internal issues include temporal fluctuations of available water resources, conflicts in water use between different sectors (drinking water, agriculture, industry), mismatch between water supply and water needs and other institutional, legal and economic factors. External or international issues include socio-economic, cultural or political differences, and historical and geopolitical issues. All these reasons may result in countries setting different objectives and having different management plans for their shared national water resources. Integrated water resources management in transboundary regions involves alternative options for water storage and use (reservoirs, river diversion and irrigation systems), for wastewater treatment and disposal (biological oxidation, nitrification-denitrification, use of pipe outfalls in different locations and use of lagoons or aquifer recharge), different states of nature (climatic conditions, type of soils, irrigated crops, irrigation systems and socio-economic environments) and various preferences or objectives (economic, social, environmental, aesthetics, etc.). Alternative options may carry environmental consequences and risks to ecosystems and human health, such as microbiological and toxic contamination or eutrophication. The impacts on the environment and their prevention should be weighted against the economic benefits and social consequences. Multicriterion Decision Analysis (MCDA) is proposed in this paper as a decision support methodology for managing transboundary risks related to different criteria and objectives set by different countries. For this purpose, three alternative methods are proposed in order to facilitate negotiations and the final decisions. All are based on the combined use of modelling and a decision support method called Composite Programming (CP). In the first method, each country proceeds separately and evaluates alternatives according to its own objectives. In the second approach the different objectives used by the two countries are first traded-off and then alternatives are ranked according to the composite objectives. The third method is based on the aggregation of the countries' different alternatives in order to obtain a consensus between the two partners. The case of the international river Nestos/Mesta, flowing between Greece and Bulgaria, provides an illustration of this methodology in practice. KEYWORDS: Transboundary Water Management, Multicriterion Decision Analysis, The Nestos/Mesta River