Enhanced groundwater depletion near borders in transboundary aquifers

Transboundary aquifers underlie a large share of global irrigation and urban water supply, yet their shared nature and associated governance challenges raise concerns about accelerated depletion. Here I provide the first global assessment of groundwater depletion in transboundary aquifers. Using long-term groundwater-level trends from more than 100,000 observation wells, I show that transboundary aquifers deplete significantly faster on average than matched domestic aquifers, and that depletion is systematically concentrated near international borders. Extending this analysis to the global population of transboundary aquifers using high-resolution data on irrigated cropland, I find that irrigation is likewise disproportionately concentrated near borders. However, these spatial patterns are largely explained by hydrogeography (the co-location of rivers, alluvial plains, and irrigation infrastructure) rather than by border-related competitive overuse. This suggests that transboundary groundwater stress is often driven by physical setting rather than strategic behavior, on average, a finding that is both informative and encouraging for the prospects of cooperative governance.