Pulwarty, R. S., 2003: Transboundary river flow changes. In Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, T. D. Potter and B. R. Colman (Eds.), Wiley, 865-884.
Water is a "fugitive" resource in the sense that it flows naturally from one place to another, from one reserve to another (e.g., groundwater to surface), and from one physical state (solid, liquid, and gas) to another. Thus transboundary can mean many things including transitions from wet to arid zones, from upstream to downstream, from one country or province to the next, etc. The Convention on the Protection and Use of Transboundary Watercourses and International Lakes (1992) defines "transboundary waters" to mean "any surface or ground waters which mark, cross, or are located on the boundaries between two or more states." Whenever transboundary waters flow directly to the sea, these transboundary waters end at a straight line across their respective mouths between points on the low water line of their banks. Groundwater resources are also frequently shared by two or more countries. Emerging issues in water resources emanate from three categories of problems: (1) transboundary water availability; (2) transboundary groundwater allocation, management, and conservation, and (3) transboundary water quality (Caldwell, 1993). This chapter is primarily concerned with surface water in large river basins crossing international boundaries.
Watersheds can be defined by crossing a physical line (or bench), dividing surfaces from which waters flow in different directions to different outlets. The term 'river basin' is here used to denote channel length and catchment area. 'Catchment area' refers to a drainage area in which surface water flows to a common outlet channel. 'Watersheds' can also be crossed conceptually, such as in differentiating between upstream and downstream emphasis in managing water and sediment flows, water quality and quality relationships, changes from soil type to landscape type as a basis for study and administration, trade-offs between centralized versus decentralized emphasis in management, and concern for impacts on the environment and on basins of origin (White, 1997).
Shared watersheds constitute about 47% of the global land area and are inhabited by about 40% fo the world's population. Worldwide there are more than 214 major transboundary river basins, of which 74% are shared between only 2 riparian states (Kaufman et al., 1997). Thirteen basins are shared by 5 or more countries, 4 of these (the Congo, Danube, Nile, and Niger) are shared by 7 riparians; 4 (Chad, Volta, Ganges-Brahmaputra, Mekong) are shared by 6 countries, and 2 (La Plata, Elbe) are shared by 5 countries. Nine basins have 4 riparians each, and 30 major basins are shared by 3 countries. Of the 9 international water bodies shared by 6 or more countries, 5 are in Africa. The largest river basin in Europe, the Danube, is one-seventh the size of the Amazon Basin, but its waters are shared among 17 countries. On the smaller scale, there are over 400 surface waters between Finland and Russia alone, 150 crossing the Netherlands including the Rhine, and over 150 in the Ganges-Brahmaputra-Meghna system.
Much recent attention has been focused on highly visible water-related problems in the Middle East, the Indian Subcontinent, and the Aral Sea Basin. The UN Food and Agriculture Organization (FAO) has identified more than 3600 treaties relating to non-navigational water use dating between the years 1805 and 1984. Since 1945, approximately 300 treaties have been negotiated dealing with water management allocations of international basins (Kaufman et al., 1997). But, one-third of the major international rivers have no international agreement, and fewer than 30 have any cooperative institutional arrangements.
Rivers have constituted boundaries long before the rise of the modern nation state. Interestingly, the notion of "rival," derived from the Latin "rivalis," was originally used to describe people living on opposite banks of a river. Among the most important factors conditioning transboundary conflicts are the historical patterns of use and the needs of new "arrivals" and changes in values over time. Several countries now rely on significant amounts of surface waters originating outside their national borders (Fig. 1). Changing social factors in particular have made water-related resource conflicts within countries common and, in situations where water is shared between two or more countries, increasingly unavoidable (Kaufman et al., 1997).
Frederick (1996), Dinar (1997), and others have highlighted several factors underlying most international disputes involving river flows, including: the variability and uncertainty of supply, interdependencies among users, increasing water scarcity, overallocation and rising costs, the increasing vulnerability of water quality and aquatic ecosystems to human activities, ways and means of supplying safe water facilities, and the mobilization of financial resources for water development and management. Many of these issues derive from concerns in water resources management in general. How these concerns are met is strongly shaped by the choice of the spatial unit within which studies and management actions are conducted, by the way problems have been defined and changed over time, and by who benefits from defining problems in a particular way.
The primary concern in an international context is the need for international cooperation in the development of institutional and human resources for the efficient and equitable management of transboundary waters under variable and changing environments. In the following discussion the scales of human activities and interactions with large river basins are put in the context of streamflow changes on the time scales of century, decadal, seasonal, and extreme events.