Asymmetry Analysis and Hazard Assessment of Drought–Flood Abrupt Alternation Events in the Yellow River Basin

Drought–flood abrupt alternation (DFAA) is a typical compound hydroclimatic extreme process and has important implications for regional water resources regulation, agricultural production, and ecological stability. However, existing studies have mainly focused on event identification and frequency variation, while lacking a systematic investigation of the directional differences between drought-to-flood (DF) and flood-to-drought (FD) events in terms of process structure, cumulative effects, and spatial hazard patterns. Based on daily precipitation data from 1960 to 2024, this study identified DFAA events in the Yellow River Basin by combining the standardized weighted average precipitation (SWAP) index with run theory, and analyzed the asymmetric characteristics of DF and FD events from the perspectives of event frequency, phase duration, abrupt-transition characteristics, cumulative severity, and integrated hazard. The results show that: (1) the frequency of DFAA events in the Yellow River Basin exhibited pronounced spatial heterogeneity, with an overall pattern of being higher in the middle reaches and lower in the upper and lower reaches. The frequency of DF events was generally higher than that of FD events, and their spatial distribution was also more continuous. No significant long-term trend was detected in the annual frequency, although clear interdecadal variability was observed, characterized by a transition from relatively low-frequency periods to medium- and high-frequency periods. (2) DF and FD events exhibited stable asymmetry in process structure. The abrupt-transition duration of DF events was mainly concentrated within 1–2 days, whereas that of FD events was mainly concentrated within 3–5 days. The two event types had comparable pre-transition durations, but DF events tended to shift more rapidly and were followed by a longer-lasting flood phase. (3) The differences between the two event types in terms of instantaneous intensity were relatively limited, whereas clearer divergence was observed in cumulative severity, with DF events showing greater overall severity than FD events. This indicates that the directional difference is manifested primarily in cumulative process effects rather than in the magnitude at a single moment. (4) The comprehensive hazard index (CHI) revealed that the northern and central-eastern parts of the middle reaches of the Yellow River Basin were the main hotspots of DFAA hazard. Among them, high-hazard areas of DF events were more extensive, whereas FD hazards were characterized more by localized intensification. These findings indicate that within the identification framework adopted here, DFAA in the Yellow River Basin is characterized not only by rapid dry–wet transitions, but also by clear directional differences between DF and FD in process structure and hazard pattern. This study can provide a scientific reference for the monitoring, early warning, and zonal hazard prevention of DFAA in the basin.