Structural evolution and event-chain risk of extreme events in China's largest freshwater lake: Event chain quantification based on continuous-state sequences

Study region Poyang Lake Basin (PLB) Study focus Climate change is intensifying hydrological hazards in lakes, yet the evolution of the internal structure of flood–drought event chains and the drivers of this evolution remain insufficiently quantified. This study introduced Continuous Hydrological State Sequences (CHSS) to define single and compound event chains and used complexity and persistence capacity to characterize their hierarchical internal structures. Inspired by cumulative-accounting concepts in structural fatigue analysis, we developed a Multi-scale Continuous Hydrological Transition Index (MCHTI) to quantify two internal process dimensions, duration-normalized exposure and transition perturbation, thereby revealing structural evolution and examining its statistical associations with climate variability and river–lake interactions. New hydrological insights for the region The results showed that: (i) compound chains, although accounting for only 8.2% of all chains, contributed 21.0% of anomalous days, and their transition complexity was 2–4 times that of single chains; (ii) after 2003, monthly event-chain complexity and chain length increased by 42.8% and 50.4%, respectively; (iii) each of the two screening rules independently identified 48 high-MCHTI event-chain records, with event clustering and evolutionary signatures shifting from flood dominance toward increasing drought and compound representation; and (iv) climate variables were more strongly associated with duration-normalized exposure, whereas internal structural transitions showed stronger statistical associations with river–lake interactions.