European runoff drought event types: From historical classification to projected future changes

Abstract. European drought events have intensified in recent decades, raising concerns about water resources, agriculture, and ecosystem health. Most existing continental-scale drought assessments provide limited attribution of drought changes to the processes that generate runoff drought, which constrains understanding of how these processes will evolve in a warming climate. In this study, we provide the first continental-scale assessment of future changes in runoff drought generation processes across Europe, focusing on rainfall deficit, rain-to-snow and wet-to-dry season transitions, and snow-related processes. We used a continental-scale hydrological model in combination with observed meteorological data and climate simulations (GCMs) under a middle-of-the-road emission scenario (i.e., RCP4.5, ≈2 °C global warming by 2100). Unlike widely used aggregated drought index-based studies, our mechanism-specific classification approach distinguishes the physical drivers of drought events by classifying runoff droughts into seven event types based on their severity, duration, and frequency and leverages spatial clustering analysis (Getis–Ord G^∗_i method) to assess the climate responses. Our analysis reveals considerable regional differences in drought mechanisms and their responses. Historical observations (1971–2000) highlight that Mediterranean Europe experiences the most severe drought conditions, dominated by rainfall deficit processes and wet-to-dry season transitions, with deficits exceeding (> 4 mm day⁻¹), prolonged durations (> 165 days), and high event frequencies. Future projections (2070–2099) indicate further drought intensification in the Mediterranean driven by increasing rainfall deficit and wet-to-dry transition events, with runoff deficit increases by 2–6 mm day⁻¹ and duration extensions exceeding 200 days, while Northern and Western-Central Europe show predominantly decreasing drought severity due to declining cold-snow season droughts under warming conditions. Importantly, drought related to temperature-driven processes, especially those triggered by rain-to-snow transitions, exhibit the most pronounced projected changes. These findings demonstrate that different drought mechanisms respond distinctly to climate forcing. By attributing projected drought changes to specific generation processes, our results enable region-specific interpretation of drought hazard, which is crucial for effective water-resources planning across Europe.