Coupling Relationship of Extreme Soil Moisture, Precipitation, and Flood Events in the Yangtze River Basin Under the Event Coincidence Framework

Abstract

In the context of global environmental change, the frequency of extreme events is increasing, accompanied by a continuous increase in flood risk. Despite the established close interconnections among soil moisture, precipitation, and floods under climate change, the lack of systematic studies on their spatiotemporal variations in the Yangtze River Basin (YRB) hinders a comprehensive understanding of flood formation mechanisms and impedes effective emergency management efforts. Therefore, in this study, we quantified the soil moisture-precipitation (SM-P) correlation in the YRB from 1961 to 2022 under climate change by employing precursor coincidence, and examined both the predictive role of SM-P coincidence in forecasting floods and the influence of floods on SM-P coincidence through Event Coincidence Analysis (ECA). The results indicated that (1) there was a significant seasonal SM-P coincidence in the YRB, with the strongest SM-P coincidence occurring in summer when the precursor coincidence rate was > 0.50. (2) The predictive role of SM-P coincidence on floods was the highest in summer (> 0.30), whereas the impact of floods on SM-P coincidence was most significant during both summer and fall, with most regions showing conditional trigger coincidence rates > 0.60. (3) The interactions between SM-P and floods were strengthened (the coincidence increased by approximately 100%) before and after mutation. Over a longer temporal tolerance window ($∆T$ = 7), SM-P coincidence was more pronounced, whereas the coupling between SM-P and floods was more significant in a shorter conditional temporal tolerance window ($∆T_{\text{cond}}$ = 0), highlighting both an enhanced effect and significant spatial heterogeneity. Our findings offer a scientific basis for understanding and predicting floods, as well as for managing and mitigating flood risks in the flood-prone YRB.