Disaster risks are the results of complex spatiotemporal interactions between risk components, impacts and societal response. The complexities of these interactions increase when multi‐risk events occur in vulnerable contexts characterized by ethnic conflicts, unstable governments, and high levels of poverty, resulting in impacts that are larger than anticipated. Yet, only few multi‐risk studies explore human‐environment interactions, as most studies are hazard‐focused, consider only a single‐type of multi‐risk interaction, and rarely account for spatiotemporal dynamics of risk components. Here, we developed a step‐wise, bottom‐up approach, in which a range of qualitative and semi‐quantitative methods was used iteratively to reconstruct interactions and feedback loops between risk components and impacts of consecutive drought‐to‐flood events, and explore their spatiotemporal variations. Within this approach, we conceptualize disaster risk as a set of multiple (societal and physical) events interacting and evolving across space and time. The approach was applied to the 2017–2018 humanitarian crises in Kenya and Ethiopia, where extensive flooding followed a severe drought lasting 18–24 months. The events were also accompanied by government elections, crop pest outbreaks and ethnic conflicts. Results show that (a) the highly vulnerable Kenyan and Ethiopian contexts further aggravated drought and flood impacts; (b) heavy rainfall after drought led to both an increase and decrease of the drought impacts dependent on topographic and socio‐economic conditions; (c) societal response to one hazard may influence risk components of opposite hazards. A better understanding of the human‐water interactions that characterize multi‐risk events can support the development of effective monitoring systems and response strategies.

Abstract. The relation between drought severity, as expressed through widely used drought indices, and drought impacts is complex. In particular in water-limited regions where water scarcity is prevalent, the attribution of drought impacts is difficult. This study assesses the relation between reported drought impacts, drought indices, water scarcity, and aridity across several counties in Kenya. The monthly bulletins of the National Drought Management Authority in Kenya have been used to gather drought impact data. A Random Forest (RF) model was used to explore which set of drought indices best explains drought impacts on: pasture, livestock deaths, milk production, crop losses, food insecurity, trekking distance for water, and malnutrition. The findings of this study suggest a relation between drought severity and the frequency of drought impacts, whereby the latter also showed a relation with aridity, whilst water scarcity did not. The results of the RF model reveal that drought impacts can be explained by a range of drought indices across regions with different aridity. While the findings strongly depend on the availability of drought impact data and the socio-economic circumstances within a region, this study highlights the potential of linking drought indices with text-based impact reports. In doing so, however, spatial differences in aridity and water scarcity conditions have to be taken into account.

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

The absolute socio-economic damage from natural hazards has been increasing in recent decades in many parts of the World (Formetta & Feyen, 2019;IFRC, 2020b; IPCC, 2021), resulting in numerous humanitarian crises. These socio-economic impacts have often been attributed to a single hazard event (e.g., drought, flood, cyclone) combined with static exposure and vulnerability conditions (Ciurean et al., 2018). In reality, these impacts are often the result of complex dynamic interactions between societal and physical drivers tightly interlinked with the

<p>Floods and drought affect millions of people each year, but what if a riverine flood rapidly follows or occurs during a hydrological drought?</p> <p>The 2022 summer drought in Europe, for instance, was punctuated by flash floods, affecting societies, economies and the environment already impacted by the persistent drought. In the same summer, in Iran and Afghanistan, devastating riverine floods followed a severe drought, causing displacement and human losses. Although the abrupt transitions between opposite hydrological extremes can pose huge risks for societies, the processes behind and effects of drought-flood interactions remain largely unknown, as most studies address droughts and floods separately. This research provides the first global study of compound and consecutive drought-flood events, shedding light on the underlying hydrological interactions between opposite hydrological extremes.</p> <p>By analysing timeseries of hydro-meteorological and other biophysical variables for 8255 catchments globally, we reconstruct the propagation of droughts and floods through the hydrological cycle, thereby identifying and characterizing flood events that follow or compound with drought conditions. We use variable and fixed threshold-level approaches to detect extreme dry and wet conditions, and seasonality statistics to analyse the timing of riverine floods. Our results show that close succession between drought and flood occurs mainly during the transition between seasons: from winter to spring in mid-latitude areas and from dry to wet at the equator and polar regions. Although these events are rare, they have increased over time, especially in countries such as France and Germany, southern Brazil, and India. Furthermore, drought conditions often shift the flood timing, resulting in later winter floods in Europe, in the north-eastern coast of the United States and western Canada, and earlier summer floods in Central America and Northern Brazil.</p> <p>This study shows that although drought and flood events evolve from different hydrological processes and atmospheric dynamics, these hydrological extremes interact with the same hydrological system, resulting in system alterations that may modify flood dynamics.</p>

<p>Future climate projections show a strengthening of the hydrological cycle with more droughts and floods expected in many regions of the world. This means a higher likelihood of cascading drought-to-flood disasters such as the Millennium Drought – Brisbane flooding in Australia or the California drought – Oroville spillway collapse in the US. Droughts allow ample time for impacts and adaptation, which influence hazard, exposure, and vulnerability of a subsequent flood. When we treat the flood risk as independent from the drought this might lead to large underestimations of future risk.</p><p>Here, we present the PerfectSTORM project (‘STOrylines of futuRe extreMes’). In this project we will study drought-to-flood events to provide the understanding needed to prevent major disasters in the future. We will use a mixed-methods approach based on a combination of qualitative and quantitative storylines of past and future drought-to-flood risk in case studies and extrapolation of this rich case study information to the global scale. Qualitative storylines will be collected with narrative interviews and mental simulation workshops and will be analysed to develop timelines and causal loop diagrams. Quantitative storylines will be developed from timeseries of hydrological and social data that will be analysed to distinguish interrelated drivers and modelled with system dynamics modelling. These storylines will then be combined in an iterative way using innovative data visualisation as a basis for co-creating management solutions.</p><p>To generalise our case study understanding, a range of global datasets will be analysed to find global types and hotspots of drought-to-flood events. This information will be combined with the system dynamics model developed in the case studies and a global multi-dimensional possibility space will be developed. This will allow us to explore positive pathways for future management of drought-to-flood events in different parts of the world. The PerfectSTORM project will provide in-depth understanding of the hydrosocial feedbacks and dynamic vulnerability of cascading hazards.</p>

<p>Droughts are long-lasting and have a range of cascading impacts on society. These impacts and their responses can influence the further development of the drought itself, but also continue into the period after the drought ended. Especially if society is hit by a next hydrological extreme event, heavy rainfall resulting in flooding, the effects of this may be increased or decreased by the preceding drought and its impacts and responses. We here present a review and a global assessment of cases of these events, based on scientific literature, NGO and governmental reports, and newspaper articles, to study the diversity of how drought affects flood risk. We find that the balance between the positive and negative effects of extreme rainfall after a long dry period is mostly dependent on the underlying vulnerability and the effect of specific responses, and is different for different countries, and for different sectors and groups in society. Based on our initial analysis of the collection of case studies, we see some emerging patterns. For example, in Europe, the USA and Australia, the highly managed water system with hard infrastructure and early-warning systems makes that in most cases the rainfall after drought are managed and adverse effects mitigated, but also lock-ins exist that can make feedbacks of either inaction or maladaptation result in increased economic losses. In Africa and Latin-America, with a fragile governance system, less hard infrastructure, and a more exposed population, extreme rainfall after drought brings relief and replenishment of water resources, but also increased impacts, conflict and displacement. Here, we hypothesise that impacts are unequally distributed in society, because of issues of power, access to land and water resources, inadequate soft infrastructures, etc. We will test this hypothesis with an in-depth qualitative study of local stakeholder knowledge of these human-water processes in selected case studies. The typology of drought-to-flood events that we developed can serve as a starting point for further research on the complexity of these cascading events.</p>

<p>Humanitarian crises often result from a combination of multiple physical and societal processes, rather than independently from a single driver. The combination of processes leads to “compound events”, whose socio-economic impacts could be larger than those expected by analysing each driver individually. In recent years, the Horn of Africa has been increasingly exposed to compound events. Frequent extreme wet and dry conditions often compound with its fragile context characterized by internal ethnic conflicts, unstable governments, and high levels of poverty, resulting in impacts usually larger than anticipated. An improved understanding of the drivers and their interactions can help to reduce future risks associated with compound events.</p><p>Here, we conducted a retrospective analysis of the humanitarian crises that occurred in Kenya and Ethiopia in 2017-2018. In this period, a severe drought that occurred over the span of around 18/24 months, was followed by extensive flooding during the 2018 March-May rainy season. The impacts and their related drivers were explored, first through a review of the literature, and then through a survey and semi-structured interviews with several stakeholders from national agencies, civil societies, and NGOs. The approach resulted in a participatory co-creation of causal loop diagrams used as qualitative mental maps of the perceived drivers and interactions. These were then used as a basis for the semi-quantitative analysis of driver-interactions, modelling the impacts of immediate and long-term effects of the compound events.</p><p>The analysis disentangles the spatial-temporal feedback of drought and flood events, and their interconnections with societal forces. We found both negative and positive feedback on the food security level of the Kenyan and Ethiopian population. For instance, the flood initially exacerbated food insecurity caused by the long drought, but in the long term, it helped alleviate related water shortages. The results show the importance of taking drought response actions that first do not increase the risk related to subsequent floods (e.g., encouraging the allocation of people in lowland areas), but also that can boost the positive impacts of above-average rainfall on drought effects. Moreover, we investigated potential early warning signs and explored the impacts of several measures, identifying windows of opportunity for interventions.</p>