Population at Water Risk in World’s International River Basins

Slightly over half of the world’s human population lives in a river system shared by two or more countries. This transboundary aspect – caused by the utterly differing geographies of administrative country borders and river basins – adds to the intricacy of the global and continental-scale assessment of water-related risks. Whereas such assessments have started to evolve towards the inclusion of multiple hazards and stressors, vulnerabilities, exposures, and consequent risks, they have thus far been largely immune to the transboundary aspects of hydrology and water resources management. At the same time, the research on transboundary waters has its strongholds in matters such as risks related to conflicts or potential sources of conflicts, transboundary water agreements, and their diplomatic aspects, and other aspects related to water diplomacy, typically aiming at reducing political risks related to potential tensions and their mitigation between riparian countries. Bridges between these two strong research traditions are needed as international river systems are not immune to conventional water risks such as those related to hydrometeorology, contamination, or infrastructure deficiencies. We analyze spatially the exposure of the human population to ten major such water risks (due to interannual and seasonal variability; overuse; groundwater; coastal eutrophication; riverine and coastal floods; droughts, and water and sanitation services) in the major 310 international river systems of the planet. Our study approach (risk = stressor/hazard x exposure x vulnerability) aligns with that of the United Nations Sendai Framework and Intergovernmental Panel on Climate Change. Our results indicate that the lack of appropriate sanitation had globally the largest headcount, followed by riverine floods and lack of appropriate water supply. Each risk shows a specific pattern across the river systems, though. The largest human population at water risk was by far in the Ganges-Brahmaputra-Meghna system, followed by the Indus, Nile, Niger, Congo/Zaire, Rann of Kutsch, and Lake Chad Basin. Yet, many of these river systems have limited transboundary cooperation arrangements. The analysis outlines the importance of the transboundary aspect of water risks and their improved quantification in the pursuit of building up international cooperation and security through environmental management policies.