Copula-based joint distribution analysis of the compound drought-fire weather conditions during the 2025 spring large-scale wildfires in Japan

Study region Japan, focusing on five large-scale wildfires at four locations and representative cities across all prefectures. Study focus This study developed a daily-scale copula framework to characterize compound drought–fire weather conditions associated with five large-scale wildfires in Japan in 2025. Drought was represented by the 3-month Standardized Precipitation Index (SPI3) and soil moisture (SM), and fire weather by effective humidity (EH) and wind speed. The analysis evaluated dependence and compound severity for drought–fire weather variable pairs, quantified pre-fire changes in EH and SM using seven-day rolling slopes, and mapped compound severity at representative locations across all prefectures on key wildfire dates. New hydrological insights for the region Drought–fire weather pairs generally showed weak daily-scale dependence. Several combinations reached severe or extreme levels during both Ofunato wildfires and the Otsuki wildfire, reflecting moisture deficits coinciding with low EH or strong winds. Okayama and Imabari showed compound anomalies of varying severity and rapid prefire declines in EH and SM. On 26 February, severe compound conditions occurred at many representative locations along the Pacific side, indicating that geographically separated wildfires occurred within a broader regional background of concurrent moisture deficits and adverse fire weather conditions. Severe compound conditions during wildfire periods and rapid pre-fire drying may represent complementary aspects of the hydrometeorological conditions associated with large-scale wildfires in Japan.