Hydrological response regimes of compound hydroclimatic events across China revealed by behavior-space regionalization

Abstract. Compound hydroclimatic events can trigger diverse hydrological responses, yet most large-scale assessments still characterize them primarily through event type, frequency, or intensity. Such event-centered descriptions may obscure the fact that the same compound anomaly can propagate through different hydrological pathways depending on antecedent conditions, soil-moisture storage, runoff response, seasonality, and regional hydroclimatic setting. Here we develop a behavior-space regionalization framework to diagnose hydrological response regimes of compound hydroclimatic events across China. Monthly standardized indices of thermal anomaly, meteorological moisture anomaly, soil moisture, and runoff are used to identify four compound-event types, including warm-wet, warm-dry, cold-wet, and cold-dry events. We then construct a multidimensional behavior-feature space that integrates event occurrence, conditional severity, persistence, seasonality, antecedent state, soil-moisture–runoff propagation, transition behavior, and threshold sensitivity. Based on this feature space, eight hydrological response regimes are identified and further grouped into three response pathways: amplification, propagation, and decoupling. The eight regimes show distinct behavior fingerprints, event-type coupling, antecedent-memory sensitivity, and spatial organization across major basins and climate zones. Seasonal high-intensity, soil-moisture propagation, and weak-response regimes account for the largest national fractions, but no single regime dominates China’s compound-event response structure. Spatial transition analysis further reveals pronounced regime mixing along hydroclimatic transition belts, plateau-edge regions, and basin-climate boundaries. These findings show that compound hydroclimatic risk across China is organized by multiple hydrological response pathways rather than by a single gradient of event frequency or severity. The proposed framework provides a response-centered basis for regional hydroclimatic risk assessment and mechanism-specific monitoring.