Global hydrological drought diverges from atmospheric drying

Atmospheric evaporative demand is increasing with climate warming and can intensify drought, yet how strongly long-term atmospheric drying propagates into realized river-flow deficits remains poorly quantified. Here using streamflow observations from approximately 19,000 catchments worldwide, we provide a global observational assessment of hydrological drought and compare its evolution with atmospheric drought diagnosed from precipitation and evaporative demand. We show that atmospheric drought trends exhibit a broad spatial predominance towards drying over the past four decades, whereas streamflow drought trends are substantially more heterogeneous and show highly spatially variable change at the global scale. This heterogeneity is consistent with nonlinear drought propagation that can attenuate, delay, reshape or amplify atmospheric drought signals before they emerge in streamflow. Our results provide global observational evidence that long-term atmospheric drying is not transmitted uniformly or proportionally into river flow but can be weakened in many catchments and reinforced in others, highlighting the need to account explicitly for hydrological propagation when assessing water-security risks under climate warming. Atmospheric drought has tended towards drying in recent decades, but this signal is expressed unevenly in river flow. Observations from approximately 19,000 catchments show that hydrological drought responses can be weakened, reshaped or amplified as atmospheric drought propagates through catchments, underscoring the need for drought assessments tailored to the water-related impacts of interest.