From Drought to Flood: uncovering the dynamics of risk

Riverine floods and droughts affect millions each year, yet their interactions remain insufficiently understood. While typically studied in isolation, their co-occurrence or succession can significantly alter hydrological processes and societal resilience. This thesis addresses this gap by examining drought-flood dynamics across global and local scales. It first analyses large-scale hydrological patterns, then explores socio-hydrological interactions through case studies, and finally assesses implications for risk management in vulnerable contexts. Chapters 2 and 3 focus on large-scale hydrological processes. Chapter 2 identifies flood events occurring after or during droughts using hydro-meteorological and biophysical data from 8,255 catchments worldwide. These events account for approximately 24% of global floods, with hotspots in the northeastern United States and Canada, southwestern Brazil, parts of Europe, and India’s west coast. Droughts can significantly influence seasonal timing of riverine flooding, delaying floods by over two months under severe conditions, and, less frequently, leading to earlier floods in snow-dominated regions. Floods following droughts tend to have similar severity to standalone floods, whereas those during drought are generally less severe, suggesting a dampening effect. However, in snow and warm-temperate climates, severe droughts can coincide with more extreme floods, indicating potential amplification. Chapter 3 investigates how drought influences catchment response to precipitation. Using multivariate statistical analysis, it finds that drought conditions reduce catchment response compared to normal conditions. Hydrological and soil moisture droughts have the strongest dampening effects, while vegetation stress may slightly enhance streamflow response. The study also identifies non-stationary precipitation-streamflow relationships, with abrupt shifts in catchment behaviour often occurring during droughts. These shifts include both increases and decreases in response, linked to prolonged soil moisture droughts and extreme meteorological droughts, respectively. Chapters 4 and 5 extend the analysis to socio-hydrological dynamics, emphasizing vulnerability and adaptive capacity. Using a mixed-methods approach combining literature review, time series analysis, surveys, and interviews, they examine how communities experience and manage consecutive and compound drought-flood events. Chapter 4 examines the 2017-2018 drought-to-flood sequence in Kenya and Ethiopia. Impacts were shaped not only by climatic extremes but also by socio-political and ecological conditions such as conflict, instability, pest outbreaks, and weak institutions. Consecutive drought and flood events do not always worsen impacts; in some cases, floods after drought provide benefits, such as replenishing water resources or reducing crop pests. However, outcomes vary widely. The study also finds that drought responses can increase vulnerability to subsequent floods, underscoring the need for integrated risk management. Chapter 5 analyses hydrological anomalies in the Brazilian and Peruvian Amazon between 1981 and 2023. Extreme droughts and floods are occurring closer together in time and have mixed effects on riverine livelihoods, depending on livelihood type, proximity to rivers, and topography. Even moderate anomalies can be highly disruptive if they occur during critical periods like planting, harvesting, or fish migration. Chapter 6 addresses risk management implications through a case study of the Limpopo River basin. Floods play a key role in aquifer recharge and sustaining water availability during droughts. However, adaptation measures such as sand dams and injection wells often face challenges related to cost, maintenance, technical capacity, and community uptake. Technical solutions alone are insufficient without addressing underlying vulnerabilities. Strengthening links between communities and governance systems, along with improved transboundary water management, is key to enhancing resilience. Overall, this thesis contributes to a growing understanding of drought-flood interactions, particularly in fragile contexts. It demonstrates the importance of integrating these interactions into disaster risk management and supports a broader shift toward multi-risk approaches that consider cascading hazards, feedbacks, and complex system dynamics.