How downstream sub-basins depend on upstream inflows to avoid scarcity: typology and global analysis of transboundary rivers

Abstract. Countries sharing river basins are often dependent upon water originating outside their boundaries; meaning that without that upstream water, water scarcity may occur, with flow-on implications for water use and management. We develop a formalisation of this concept using water stress and shortage as indicators of water scarcity, and including both persistent and occasional scarcity. Dependency occurs if water from upstream is needed to avoid either persistent or occasional water scarcity. This can be diagnosed by comparing different types of water availability on which a sub-basin relies, starting with reliable local runoff (available even in a dry year), followed by less reliable local water (available in the wet year), reliable dry year inflows from possible upstream area, and finally less reliable wet year inflows from upstream. At the same time, possible upstream water withdrawals reduce available water downstream, influencing the latter two water availabilities. In this paper, we further present a typology describing how scarcity and dependency evolve in transboundary river basins, and use this typology for a global analysis of transboundary river basins at the scale of sub-basin areas (SBAs). Four groups of SBAs are identified that experience scarcity and dependency differently depending on their i) location in the basin, and ii) hydro-climate characteristics, specifically the level of reliable support provided by natural upstream inflows. Each group has its own set of transitions in scarcity and dependency category, driven by changes in local water demand and/or upstream withdrawals. Our results show that almost one billion people (33 % of the total transboundary population) live in SBAs that are dependent on upstream water to avoid stress because of their own water use, while 500 million people (17 % of the total transboundary population) live in SBAs dependent on upstream water to avoid possible shortage. The identification of groups and their transitions enables discussion of the pathways SBAs might take in future, potentially contributing to further refined analysis of inter and intrabasin hydro-political power relations and strategic planning of management practices in transboundary basins.