Identification and Spatiotemporal Evolution of Drought–Flood Abrupt Alternation Events in the Yellow River Basin Based on Standardized Precipitation Evapotranspiration Index (SPEI)

This study proposes a quantitative identification method for drought–flood abrupt alternation (DFAA) events in the Yellow River Basin (YRB) based on the daily standardized precipitation evapotranspiration index (SPEI) data from 1982 to 2021 and analyzes their spatiotemporal evolution characteristics. The results show that the proposed identification method has good applicability and agrees well with historical records. Grid-scale DFAA events showed an overall slowly increasing trend in occurrence frequency. The mean occurrence frequency, mean duration, and mean intensity were 0.67 events, 30.57 d, and 1.45, respectively. The mean occurrence frequency had a pattern of being higher in the middle and lower reaches and lower in the upper reaches, whereas the mean intensity had a pattern of being higher in the west than in the east and higher in the south than in the north. A total of 16 DFAA events were identified in the YRB, with a mean annual occurrence frequency of 0.4 events per year and an increasing trend across decades. The mean total duration of these events was 31.81 d, and the intensity ranged from 0.96 to 1.79. DFAA events were generally less frequent in the upper reaches and more frequent in the middle and lower reaches and the inland-drainage area. For the level-II water resource subregions, Hekouzhen–Longmen (Subregion IV), Sanmenxia–Huayuankou (Subregion VI), the area below Huayuankou (Subregion VII), and the inland-drainage area (Subregion VIII) had higher occurrence frequencies and larger fluctuations in duration. These findings could provide a scientific reference for flood control, drought relief, and disaster risk management in the YRB.