From Surface Drying to Hydrological Response: An Integrated Diagnosis of Droughts (Vaibhav Kumar)

“From Surface Drying to Hydrological Response: An Integrated Diagnosis of Flash Droughts across Europe” by Vaibhav Kumar The Open-Earth Monitor Global Workshop 2026 "Next-Generation EO & GeoAI Built on Openness and Distributed Data" 7-9 October 2026, Barcelona, Spain https://earthmonitor.org/global-workshop-2026/ 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.