Onset seasonality controls compound streamflow drought risk at a global scale

Streamflow droughts threaten water, food, and energy security, often triggering socioeconomic crises. Compound streamflow droughts arise when low streamflow coincides with or is preceded by precipitation deficits and followed by elevated evaporative demand within ±2 months of onset. Yet the role of onset seasonality in shaping drought hazards remains largely unexplored. Using 374 global daily streamflow records (1901 − 2023), we demonstrate that over 70% of catchments show significant links between onset seasonality and drought severity. Since the 1900s, modest-to-extreme drought frequency has markedly increased (0.17 ± 0.14 to 5.2 ± 0.52 events/decade), with more than half beginning during March–May. Compound droughts display wider uncertainty ranges than uncompounded extremes, indicating more variable and less predictable evolution, and show pronounced clustering in arid (~44%) and sub-humid (23%) regimes, with hotspots in northwestern North America, central Europe, eastern South America, and northeastern Australia. Drought severity is amplified two-fold or more in ~56% of catchments under compound conditions when conditioned on onset seasonality, likely driven by land-atmosphere coupling. These patterns reveal hemispheric contrasts and pronounced seasonal clustering: ~37% of Northern Hemisphere sites experience drought onsets during June–August (boreal summer), whereas in the Southern Hemisphere, onsets are strongly concentrated during the extended austral summer (December–March). Additionally, 45% of Northern Hemisphere sites portray a southeastward clustering of elevated drought hazard, while 60% of Southern Hemisphere sites show a northwestward synchronization. Ignoring onset seasonality substantially underestimate compound drought risk, its propagation and associated impacts.