Katsora et al. (2025) Flash Drought Assessment: Insights from a Selection of Mediterranean Islands, Greece

Identification

• Journal: Hydrology
• Year: 2025
• Date: 2025-11-18
• Authors: Chrysoula Katsora, Evangelos Leivadiotis, Nikoletta Papadopoulou, Isavela Ν. Monioudi, Ε. Κωστοπούλου, Petros Gaganis, Aris Psilovikos, Ourania Tzoraki
• DOI: 10.3390/hydrology12110308

Research Groups

• Coastal Morphodynamics, Coastal Management and Marine Geology Lab, Department of Marine Sciences (DMR), University of the Aegean, Mitilini, Greece
• Laboratory of Ecohydraulics & Inland Water Management, Department of Ichthyology & Aquatic Environment, University of Thessaly, Nea Ionia, Greece
• Department of Geography, University of the Aegean, Mitilini, Greece
• Department of Environment, University of the Aegean, Mitilini, Greece

Short Summary

This study assesses historical flash drought (FD) events in the Northeastern Aegean islands, Greece, using ERA5 soil moisture data and the Standardized Precipitation Evapotranspiration Index (SPEI) to characterize their spatio-temporal distribution and identify vulnerable "hotspot" areas. The findings reveal significant spatial and seasonal variability in FD characteristics, with northern islands generally experiencing higher frequency and longer durations, while southern islands show greater severity.

Objective

• To identify and define historical flash drought (FD) events in the Northeastern Aegean islands (Chios, Lemnos, Lesvos, Samos, Ikaria) using ERA5 soil moisture data.
• To produce a comprehensive regional overview of FD risk across the Northeastern Aegean Islands, including frequency, duration, decline rate, magnitude, intensity, recovery rate, and recovery duration.
• To provide a more localized and accurate interpretation of past FD events on Lesvos Island using the Standardized Precipitation Evapotranspiration Index (SPEI) derived from local meteorological station data.

Study Configuration

• Spatial Scale: Northeastern Aegean islands, Greece (specifically Chios, Lemnos, Lesvos, Samos, Ikaria, Agios Efstratios, Fournoi, Oinousses, Psara). Data mapped onto a 0.1° × 0.1° grid with a native resolution of approximately 9 km. Localized assessment for Lesvos Island.
• Temporal Scale: Hourly soil moisture data from 1990 to 2024 (accessed between January and March 2025). SPEI analysis for Lesvos from 2008 to 2022/2023. Analysis conducted using pentad (5-day) averages.

Methodology and Data

• Models used:
  • Soil Moisture (SM) Percentile Method for FD identification (using 5-day average root-zone soil moisture, 0–100 cm).
  • Standardized Precipitation Evapotranspiration Index (SPEI) for localized assessment (Thornthwaite method for Potential Evapotranspiration, log-logistic probability distribution).
  • Inverse Distance Weighting (IDW) interpolation for spatial analysis and visualization.
  • Natural Breaks (Jenks) classification method for map output.
• Data sources:
  • ERA5 Land reanalysis dataset (Copernicus Climate Change Service, ECMWF) for volumetric soil moisture (m3·m−3) in three layers: 0–7 cm, 7–28 cm, and 28–100 cm.
  • Local meteorological stations on Lesvos (Agia Paraskevi, Akrasi, Sigri) for precipitation and average temperature data (for SPEI calculations).

Main Results

• Spatial Variability: Significant variability in the spatial, seasonal, and temporal distribution of FD events across the Northeastern Aegean islands.
• Vulnerable Areas (SM Method):
  • Northern islands (Lemnos and Lesvos) exhibit the highest frequency, longest durations (up to 50 days), and most rapid onset (decline rates up to 0.0062 m3·m−3 per pentad).
  • Southern island group (Samos–Ikaria) shows the highest magnitudes (overall severity, up to 0.0040 m3·m−3) and intensities (up to 0.0007 m3·m−3 per pentad), despite fewer events.
  • Northeastern Lesvos, northeastern Lemnos, northeastern Chios, and the southeastern part of Samos are identified as "hotspot" areas for frequent, rapidly declining, and severe FD events.
• Seasonal Variability (SM Method):
  • Samos–Ikaria recorded no FD events in spring; all islands experienced events in summer and autumn.
  • FDs are generally longest in autumn, followed by spring, and shortest in summer.
  • The highest overall magnitudes and intensities for all regions are observed during autumn.
  • The highest overall recovery rates occur in autumn, indicating a rapid end to FD events in this season.
  • The longest recovery duration for all islands is observed in summer, followed by spring and autumn.
• Correlation Analysis: Lesvos and Lemnos exhibit systematic, coupled drought behavior with high inter-variable coherence, while Chios and Samos–Ikaria show more variable and event-driven relationships, underscoring the dominant role of local conditions.
• Localized SPEI Results (Lesvos): Sigri (lowest altitude) experienced the most frequent and severe FD events (in terms of magnitude and intensity), particularly during summer and autumn. Agia Paraskevi showed moderate frequency with higher magnitudes in summer, and Akrasi had similar frequency but generally lower magnitudes and higher occurrence in spring. No statistically significant linear trend in FD frequency or intensity was observed, but interannual variability and significant peaks suggest climatic influence.

Contributions

• Provides the first systematic assessment of fine-scale characteristics, spatial heterogeneity, and seasonality of flash droughts in the Northeastern Aegean Islands, addressing a critical regional knowledge gap.
• Demonstrates the value of combining large-scale satellite-derived soil moisture data (ERA5) for comprehensive regional coverage with localized meteorological station data (SPEI) for accurate, fine-scale interpretation in data-scarce, complex environments.
• Identifies specific geographical "hotspot" areas vulnerable to flash droughts, establishing a crucial baseline for future climate change risk assessment and targeted adaptive management strategies.
• Develops a quantitative framework and transferable protocol for FD risk assessment applicable to other climatically sensitive island and coastal areas with similar geological complexity and sparse observational networks (e.g., broader Mediterranean and Black Sea regions).
• Underscores the importance of continuous monitoring and advanced modeling techniques as essential first steps toward developing predictive FD models and early warning systems.

Funding

This research received no external funding.

Citation

@article{Katsora2025Flash,
  author = {Katsora, Chrysoula and Leivadiotis, Evangelos and Papadopoulou, Nikoletta and Monioudi, Isavela Ν. and Κωστοπούλου, Ε. and Gaganis, Petros and Psilovikos, Aris and Tzoraki, Ourania},
  title = {Flash Drought Assessment: Insights from a Selection of Mediterranean Islands, Greece},
  journal = {Hydrology},
  year = {2025},
  doi = {10.3390/hydrology12110308},
  url = {https://doi.org/10.3390/hydrology12110308}
}