Characteristics of Flash Droughts during the Growing Season in the Songhua River Basin, Northeast China

Flash droughts severely threaten food security and disrupt ecosystem functions within a short period. A clear understanding of the occurrence characteristics and risks of flash droughts in the Songhua River basin, a major grain-producing region and ecological barrier in China, is imperative for providing scientific disaster prevention and mitigation measures. Flash droughts are always featured with rapid soil moisture depletion. Thus, based on the long-term (1950–2023) ERA5-Land 1°× 1°gridded soil moisture reanalysis data, this study systematically identified flash drought events during the growing season in the Songhua River basin, and revealed their spatiotemporal distribution patterns. (1) The frequency of flash droughts in the eastern part of the basin is higher than that in the western, with a high occurrence rate in humid areas, where severe droughts dominate the severity. The high-frequency areas of flash droughts are concentrated in the western foothills of the Changbai Mountains (up to 45 events per grid point over 74 yr). Since 2000, the flash- drought affected area has expanded at a rate of 1.8 grids per decade, and the probability of experiencing two flash droughts in one growing season has doubled compared with the earlier period. (2) The average duration of flash droughts in the basin is 34 days, and the start date shows a significant advancing trend of 4.9 days per decade. The spatial difference in flash drought severity is significant, with the highest severity observed in the southeastern foothills of the Lesser Khingan Mountains. (3) We established a Gumbel Copula joint distribution model and found that flash droughts dominate risk escalation: exceeding a severity of 182.9 (a 20-yr return period) defines a high-risk event, irrespective of duration. The “severity–duration–return period” flash-drought risk-classification system established in this study can provide scientific support for precise early warning, region-specific prevention and control, and emergency dispatch of flash droughts in the basin.