Daniel Viviroli, Matti Kummu, Michel Meybeck, Yoshihide Wada

Mountain areas provide disproportionally high runoff in many parts of the world, but their importance for lowland water resources and food production has not been clarified so far. Here we quantify for the first time the extent to which lowland inhabitants potentially depend on runoff contributions from mountain areas (39% of land mass). We show that ~1.4 billion people (23% of world’s lowland population) are projected to depend critically on runoff contributions from mountains by mid-21st century under a ‘middle of the road’ scenario, compared to ~0.2 B (8%) in the 1960s. This striking rise is mainly due to increased local water consumption in the lowlands, whereas changes in mountain and lowland runoff play a minor role only. We furthermore show that one third of global lowland area equipped for irrigation is currently located in regions that both depend heavily on runoff contributions from mountains and make unsustainable use of local blue water resources, a figure that is likely to rise to well over 50% in the coming decades. Our findings imply that mountain areas should receive particular attention in water resources management and underscore the protection they deserve in efforts towards sustainable development.