What is a drought-to-flood transition? Pitfalls and recommendations for defining consecutive hydrological extreme events

Abstract. Research into rapid transitions between hydrological drought and flood is growing in popularity, in part due to media-reported catastrophic impacts from recent events. Droughts and floods are typically studied as events that are independent from one another, and thus, a clear definition and assessment of the methods used to define consecutive drought-to-flood transition events does not yet exist. In this paper, we use a series of eight catchments that have experienced real-world impacts from drought-to-flood transitions as case studies to assess the suitability of, and differences between, different event selection methods applied to observational data. We demonstrate that different threshold level methods can result in the selection of different drought and flood events. When combined this can influence the number, seasonality and characteristics of detected drought-to-flood transitions. The time period used to define the maximum interval between drought and flood also influences whether transitions are detected. We show that the probability of a transition occurring within a set time window could vary substantially between different methodologies. The differences in detected events are especially apparent in highly seasonal flow regimes. We also show that previously applied methodologies likely fail to detect transition events that have been broadly impactful in the historical record. Further, we qualitatively assess the streamflow time series of the case study catchments, and outline a number of potential pitfalls in the event detection process. Finally, we make recommendations regarding methodological choices in the context of potential impacts of interest, and outline some priorities for future methodological development and research into transitions.