Integrated water scarcity index reveals increased exposures of populations and areas to water scarcity

Freshwater resources are fundamental to supporting humanity, and measures of water scarcity have been critical for identifying where human water requirements and water availability are imbalanced. Traditional metrics for water scarcity primarily focus on actual blue water withdrawal, while the contribution of rain-fed water requirements (RWR) and water quality – dimensions with important implications for multiple societal sectors – to overall water scarcity remains unclear. Here we address this gap by explicitly merging the three dimensions of water scarcity into an integrated index (iWSI). Specifically, combining a process-based crop water model with spatially detailed information on water pollution and sector-specific withdrawals, we first develop global gridded (30 arcminute) estimates of iWSI and its individual dimensions (blue water, RWR, and water quality) averaged over the period 2001–2010. We then perform a quantitative comparison of water scarcity indices that consider different combinations of the three water scarcity dimensions, together or in isolation, and estimate their water withdrawals and associated global land area and population under conditions of monthly and annual water scarcity. We find that the global land area and population under water scarcity increases by 126% (119–133%) and 53% (49–57%) using this integrated index relative to assessments focusing only on blue water. These effects are most pronounced for populations in Africa and Asia. Examining seasonal water scarcity, we estimate that 4.4 billion people are exposed to integrated water scarcity at least one month per year – 31% more people than under blue water scarcity alone. Our research highlights that water scarcity challenges are more widespread than previously understood. As such, our findings underscore the need for actions to bring human pressure on freshwater resources into balance with both water quantity and quality, addressing previously overlooked blindspots in global water sustainability.