Sudden Shift From Drought to Flood May Be Infrequent in Debris-Flow Events in Southwest China: Evidence From Antecedent Hydrometeorological Fingerprints

Debris flows are one of the most destructive mass-movement processes. Previous studies have shown that debris-flow occurrence may be related to antecedent higher temperatures or droughts, yet a comprehensive and objective regional understanding is still lacking. Here we integrated meteorological reanalysis with remotely-sensed soil moisture products to investigate the hydrometeorological conditions preceding debris flows in Sichuan Province, Southwest China, during 2008–2018. Antecedent meteorological settings were classified into four characteristic patterns: “higher temperature with more rain”, “higher temperature with less rain”, “lower temperature with more rain”, and “lower temperature with less rain”. Approximately 80% of events occurred after anomalously high temperatures relative to climatology, highlighting a robust warming signal prior to debris-flow initiation. Although spatial heterogeneity existed, most events were concentrated in the mountainous areas surrounding the western Sichuan Basin. Soil was predominantly wet before events (96.6%), while only a minority of events were preceded by drought. These drought-related cases included both flash droughts (63%) and slowly-evolving droughts (37%), also clustered in the western Sichuan Basin. The results demonstrate that antecedent high temperatures and droughts can both facilitate debris-flow occurrence, and they provide new insights into how changing climate regimes may reshape debris-flow hazards in mountain regions worldwide.