Ha et al. (2025) Geographic Extent of Flash Droughts Across the Seven Climatic Sub-regions of Vietnam

Identification

• Journal: VNU Journal of Science Earth and Environmental Sciences
• Year: 2025
• Date: 2025-11-14
• Authors: Pham Thanh Ha, Phan Van Tan
• DOI: 10.25073/2588-1094/vnuees.5394

Research Groups

• Faculty of Meteorology Hydrology and Oceanography, VNU University of Science, Hanoi, Vietnam

Short Summary

This study analyzed the spatio-temporal characteristics and geographic extent of flash droughts across seven climatic sub-regions of Vietnam using ERA5 soil moisture data at three depths. It revealed significant regional and depth-dependent variations, with shallow soil layers being most sensitive, and identified specific northern and southern regions where flash droughts frequently occur during the rainy season, often on a broad spatial scale.

Objective

• To analyze the spatio-temporal characteristics of flash droughts across seven climatic sub-regions of Vietnam.
• To assess the sensitivity of flash drought detection to different soil depths (7 cm, 28 cm, and 100 cm).
• To investigate the geographic extent (local versus widespread) of flash droughts in these regions to inform future forecasting and early warning models.

Study Configuration

• Spatial Scale: Seven climatic sub-regions of Vietnam (Northwest (R1), Northeast (R2), Red River Delta (R3), North Central Coast (R4), South Central Coast (R5), Central Highlands (R6), Southern Vietnam (R7)).
• Temporal Scale: 60-year period from 1961 to 2020, using daily soil moisture data processed into 5-day (pentad) averages.

Methodology and Data

• Models used: Flash drought identification criteria based on soil moisture percentiles:
  • Soil moisture drops from above the 40th percentile to below the 20th percentile.
  • Average reduction rate of at least 5 percentiles per 5-day period (pentad).
  • Event ends when soil moisture recovers to the 20th percentile.
  • Minimum duration of 3 pentads (15 days).
  • Widespread event definition: At least 50% of grid points within a region simultaneously record a flash drought.
• Data sources: ERA5 reanalysis soil moisture data from the European Centre for Medium-Range Weather Forecasts (ECMWF), at a horizontal resolution of 0.25 degrees longitude/latitude. Data was used for three soil depths: 0-7 cm (D7), 0-28 cm (D28), and 0-100 cm (D100).

Main Results

• The shallowest soil layer (D7) exhibited the highest sensitivity to short-term dry conditions, showing the highest frequency and duration of flash droughts across Vietnam, particularly in regions R1, R2, R4, R5, and R7 (averaging 2-3 events per year and over 50 days of drought per year).
• Sensitivity to flash droughts generally decreased with increasing soil depth (D7 > D28 > D100).
• Regions R1, R2, R4, and R7 consistently showed high flash drought frequency and duration across all three soil depth criteria.
• Flash droughts primarily occurred during the rainy season: June-August (JJA) in the northern regions (R1-R3) and September-November (SON) in the southern regions (R4-R7).
• Flash droughts tended to occur on a broad spatial scale (affecting more than 50% of the regional area) in sub-regions R1-R3 and R7, with approximately 60% of identified events being widespread based on D7 and D28 criteria.
• The number of widespread flash drought events was highest for D7 (around 1.5 events per year) and significantly lower for D28 and D100.

Contributions

• Provides the first comprehensive analysis of the spatio-temporal characteristics and geographic extent of flash droughts across all seven climatic sub-regions of Vietnam using multi-depth soil moisture data.
• Highlights the critical role of soil depth in flash drought detection, demonstrating that shallow soil layers are most vulnerable to rapid drying.
• Identifies specific regions (R1, R2, R4, R7) and seasons (JJA for northern regions, SON for southern regions) most susceptible to flash droughts, including widespread events.
• Offers a scientific foundation for selecting relevant variables and constructing appropriate input datasets for developing future flash drought forecasting and early warning models in Vietnam.

Funding

• Scientific task QG.25.48 of Vietnam National University, Hanoi (ĐHQGHN).

Citation

@article{Ha2025Geographic,
  author = {Ha, Pham Thanh and Tan, Phan Van},
  title = {Geographic Extent of Flash Droughts Across the Seven Climatic Sub-regions of Vietnam},
  journal = {VNU Journal of Science Earth and Environmental Sciences},
  year = {2025},
  doi = {10.25073/2588-1094/vnuees.5394},
  url = {https://doi.org/10.25073/2588-1094/vnuees.5394}
}

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