Observed Streamflow Record Shows Streamflow Drought Onset during Hot–Dry Compounding Increases the Likelihood of Intense Droughts

Streamflow droughts, i.e., below-average river discharge for an extended period, pose significant challenges to the regional water-food-energy nexus. While several assessments have so far analyzed space-time trends in streamflow droughts, mostly driven by the delayed arrival of the monsoon or large-scale climate variability, the spatiotemporal trends in compound streamflow drought characteristics, considering sequential and concurrent multiple anomalous weather and climatic stressors, have not been assessed at the continental scale. We analyze streamflow records from over 250 sites worldwide at a centennial scale (1901–2023) and demonstrate that, globally, the overall frequency of streamflow droughts has increased significantly over time, with a rate of rise for uncompounded streamflow droughts is approximately 5 events/year over the analysis period. While the compound streamflow drought frequency has shown a relatively weaker significant increase in frequency (~0.5 events/year) than the uncompounded streamflow droughts, spatially a significant spatial clustering of compound drought is observed across the arid (44%), followed by sub-humid (23%) climate regimes. Meanwhile, approximately over half (~56%) of catchments show at least a two-fold increase in streamflow drought deficit volume (severity) when drought onset is compounded by hot and dry compounding events, described by lower-than-normal precipitation deficit followed by higher-than-normal potential evapotranspiration within ±2 months of drought initiation, compared to uncompounded streamflow droughts. A higher likelihood of compound droughts is observed during the boreal summer season, spanning from June to August across the Northern Hemisphere, while an intense drought likelihood is apparent during the austral summer season, varying from December to February in the Southern Hemisphere. The results of this study underscore the importance of considering multi-hazard investigation of hydrological droughts for improving drought preparedness within the short- to long-term planning horizons.