Increased Future Streamflow Drought-to-Flood Transitions across Brazilian Catchments

Too much and too little water can trigger cascading, multisectoral impacts through floods and droughts, respectively. Although these natural disasters are traditionally analyzed in isolation, their temporal sequencing can produce compounding impacts that exceed the impact of single, independent events. As climate change has already increased the magnitude and frequency of both droughts and floods worldwide, future climatic conditions are also expected to substantially alter the frequency, timing, and characteristics of drought-to-flood transitions.

A few existing studies on these consecutive shifts focus on climate change impacts at a global scale, while the limited number of regional-scale analyses of streamflow drought-to-flood transitions often do not assess how climate change may reshape these transitions at subseasonal and seasonal timescales. Thereby, we shed light on anticipated impacts of climate change on streamflow drought-to-flood transitions across 505 catchments in Brazil by comparing the baseline period (1980-2010) with a near-future period (2015-2040) and a distant-future period (2071-2100) under a medium-emission (SSP2-4.5) and high-emission (SSP5-8.5) scenario derived from 10 bias corrected climate change models. We distinguish between rapid transitions (14 days between extremes) and seasonal transitions (90 days between extremes).

Our results indicate that both rapid and seasonal drought-to-flood transitions in Brazil are projected to more than double by the end of the century under both emission pathways. Notably, approximately 80% of investigated catchments are expected to first experience rapid drought-to-flood transitions in the future, i.e., this share of catchments did not experience such events during the baseline period. This emergence of previously unobserved transitions underscores the urgency of proactive water management to mitigate potential multisectoral impacts of sequential, contrasting extremes. Relative changes substantially increase from the near to distant future under both climate change pathways, with the most pronounced increases occurring in the most populated (e.g., São Paulo in Southeast Brazil) and agricultural-relevant regions (e.g., areas in the Cerrado biome in Mid-West Brazil). Beyond changes in frequency, transition times are also expected to shift. Rapid transitions exhibit increased variability and longer transition times approaching the 14-day threshold, whereas seasonal transitions remain predominantly distributed between 30 and 60 days.

The projected increase in drought-to-flood transition frequency poses significant challenges for water resources management, particularly for systems designed to cope with hydrological extremes independently. Rapid transitions may undermine reservoir operation rules, drought contingency plans, and early warning systems that implicitly assume longer recovery periods between extremes. This is especially critical in densely populated areas, where abrupt shifts from drought to flood can simultaneously strain water storage, allocation, and flood control objectives, amplifying risks to urban water supply and other water ecosystem services. Anticipating these transitions is paramount for adaptive management strategies that incorporate compound-event risk into operational decision-making and adaptation planning.