Open-Earth-Monitor Global Workshop 2026

Abstract
Flash droughts—characterized by abrupt onset and rapid soil moisture depletion—are emerging as a consequential hydroclimatic extreme across Europe. Their fast evolution, strong sensitivity to atmospheric evaporative demand, and reinforcement through land–atmosphere coupling challenge traditional drought monitoring approaches that were largely developed to track slowly evolving deficits. Despite growing attention, continental-scale understanding of how flash droughts initiate, propagate, and vary across Europe’s diverse climate regimes remains limited.
Herein, we propose a framework for flash drought detection and characterization using three complementary soil moisture perspectives: ASCAT satellite observations, ERA5-Land reanalysis, and GloFAS hydrological model soil moisture. The analysis covers 2007–2024 at dekadal (10-day) resolution. Flash drought onset is diagnosed from rapid short-timescale declines in near-surface soil moisture percentiles derived from ASCAT and ERA5-Land, while GloFAS is used to assess whether—and where—these surface drying signals propagate into catchment-scale hydrological response. To ensure comparability across datasets with differing process representations and effective soil depths, all soil moisture variables are expressed in a common percentile space, which isolates anomalous moisture states relative to local climatology.
Using this unified framework, we quantify key flash drought characteristics—including frequency, mean duration, severity, and mean onset speed—across Europe and examine how these metrics vary across major climate regimes. The findings highlight pronounced regional heterogeneity and systematic cross-system contrasts. ASCAT captures the sharpest and most immediate surface drying signals, whereas ERA5-Land and GloFAS provide complementary insight into physically consistent drivers and the potential for downstream hydrological impacts. Overall, the results emphasize that flash drought diagnosis benefits from combining observation-informed onset detection with process-oriented evaluation of drivers and hydrological propagation. This multi-perspective approach offers a consistent basis for strengthening monitoring and supporting early-warning readiness under Europe’s intensifying hydroclimatic variability.

Keywords: Flash drought; Soil-moisture; ASCAT, ERA5-Land; GloFAS, Hydrological response, Land–atmosphere coupling, Europe.

Abstract
Flash droughts are increasingly recognized as a distinct class of hydroclimatic extremes marked by rapid soil-moisture depletion over only a few weeks. Because of their abrupt onset, these events can severely affect agricultural production, terrestrial ecosystems, and regional water resources before conventional drought indicators fully capture their development. Recent drought episodes across Europe have reinforced the need to better understand how flash drought behaviour may evolve under continued climate warming. Yet substantial uncertainty remains, partly because many previous assessments rely on meteorological indicators or single datasets that do not adequately represent the subsurface soil-moisture processes governing rapid drought emergence.
This study proposes to investigate the future evolution of flash drought characteristics across Europe using root-zone soil moisture simulations from a multi-model ensemble of global hydrological models, including WaterGAP, H08, and CWatM, forced by bias-adjusted CMIP6 climate projections. Daily soil-moisture outputs will be aggregated to pentad scale (5-day averages) to reduce high-volatility while preserving the rapid depletion signals associated with flash drought onset. Root-zone soil moisture will then be transformed into grid-specific climatological percentiles, enabling a temporally and spatially consistent identification of flash drought events across models and time periods. A key methodological contribution of this study is the integration of pentad-scale root-zone soil moisture percentiles with a multi-model hydrological ensemble, enabling a consistent and process-relevant assessment of future flash drought dynamics across Europe.
Using this framework, the study will assess potential changes in flash drought frequency, duration, severity, and onset speed under historical conditions and future projections for SSP1-2.6, SSP2-4.5, and SSP5-8.5. The analysis is intended to advance understanding of future rapid drought development and to support drought risk assessment, climate adaptation planning, and early-warning strategies across Europe.

Keywords: Flash drought; Root-zone soil moisture; Global hydrological models; CMIP6; Europe.