Sun et al. (2026) Copula-based joint distribution analysis of the compound drought-fire weather conditions during the 2025 spring large-scale wildfires in Japan

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-09-29
• Authors: Chenling Sun, Yoshiya Touge, Ke Shi, Kenji Tanaka
• DOI: 10.1016/j.ejrh.2026.104020

Research Groups

• Graduate School of Engineering, Kyoto University
• Institute for Advanced Academic Research, Chiba University
• China Institute of Water Resources and Hydropower Research
• Disaster Prevention Research Institute, Kyoto University

Short Summary

This study develops a daily-scale copula framework to characterize compound drought–fire weather conditions associated with five large-scale wildfires in Japan in 2025. The analysis evaluates dependence and compound severity for drought–fire weather variable pairs, quantifies pre-fire changes in effective humidity (EH) and soil moisture (SM), and maps compound severity at representative locations across all prefectures on key wildfire dates.

Objective

• Investigate the joint behavior of hydrometeorological variables during large-scale wildfires in Japan
• Characterize the dependence structures among drought conditions (SPI3 and SM) and fire weather variables (EH and WIND)
• Evaluate the compound hydrometeorological conditions at the wildfire locations during the February–March 2025 events

Study Configuration

• Spatial Scale: Nationwide, with a focus on four study locations: Ofunato, Otsuki, Okayama, and Imabari
• Temporal Scale: Daily-scale analysis for the period of February–March 2025

Methodology and Data

• Models used: Copula functions (Gaussian, Student-t, Frank, Clayton, Gumbel, Joe, and independence copulas)
• Data sources: Automated Meteorological Data Acquisition System (AMeDAS), Japanese 55-year Reanalysis for Three-hourly Quantities (JRA-3Q) dataset, land surface model (SiBUC)

Main Results

• The daily distributions of drought indicators (SPI3 and SM) and fire weather variables (EH and WIND) showed pronounced deficits in precipitation and soil moisture at all four locations during most of the study period.
• The marginal probabilities of drought variables and fire weather variables indicated that Ofunato and Otsuki experienced more severe drought conditions than Okayama and Imabari.
• The copula-based joint probabilities revealed that compound drought–fire weather conditions were more frequent in February 2025, particularly at Ofunato and Otsuki.

Contributions

• This study provides a quantitative framework for comparing the hydrometeorological environments associated with historically rare large-scale wildfires in a humid temperate region.
• The copula-based approach enables the characterization of dependence among hydrometeorological variables and the assessment of compound severity during large-scale wildfire events.

Funding

• This research was funded by [insert funding projects, programs, and reference codes].

Citation

@article{Sun2026Copulabased,
  author = {Sun, Chenling and Touge, Yoshiya and Shi, Ke and Tanaka, Kenji},
  title = {Copula-based joint distribution analysis of the compound drought-fire weather conditions during the 2025 spring large-scale wildfires in Japan},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.104020},
  url = {https://doi.org/10.1016/j.ejrh.2026.104020}
}