<strong class="journal-contentHeaderColor">Abstract.</strong> Global change is altering hydrologic regimes worldwide, including large basins that play a central role in the sustainability of human societies and ecosystems. The basin water budget is a fundamental framework for understanding these basins' sensitivity and future dynamics under changing forcings. In this budget, studies often treat atmospheric processes as external to the basin and assume that atmosphere-related water storage changes are negligible in the long term. These assumptions are potentially misleading in large basins with strong land-atmosphere feedbacks, including terrestrial moisture recycling, which is critical for global water distribution. Here we introduce the Land-Atmosphere Reservoir (LAR) concept to include atmospheric processes as a critical component of the basin water budget and use it to study long-term changes in the water storage of some of the world's largest basins. Our results show significant LAR water storage trends over the last four decades, with a marked latitudinal contrast: while tropical basins have been accumulating water, temperate basins have been drying. If continued, these trends will disrupt the discharge regime and compromise the sustainability of these basins with widespread impacts.