Identification and Evolution Characteristics of Drought–Flood Abrupt Alternation Events from 1951 to 2020 Using a Daily SWAP Index in Henan Province, China

Drought–flood abrupt alternation (DFAA) has attracted increasing attention because of its severe compound impacts. This study used a daily SWAP index calculated by the precipitation data from 17 meteorological stations in Henan Province from June to September during the period of 1951–2020 to identify and analyze the spatiotemporal evolution of DFAA events. The results show that a drought duration of 10 d, together with a transition interval and a flood duration of 7 d, has a relatively good applicability for identifying DFAA events in Henan Province. The identified DFAA events were generally consistent with historical disaster records. DFAA events were characterized by slight decreasing trends in frequency and duration, with no obvious trend in intensity. The mean annual frequency, mean intensity, and mean duration of drought-to-flood (DTF) events were 2.19 events, 1.09, and 66.33 d, respectively, whereas those of flood-to-drought (FTD) events were 1.36 events, 0.36, and 73.82 d, respectively. Spatially, the distributions of DTF and FTD events exhibit distinct differences in their characteristics of frequency, intensity, and duration. Although the identification results obtained are based on precipitation as a single meteorological factor, the findings may provide a scientific basis for improving the understanding of DFAA evolution in the short term and enhancing regional disaster risk management in Henan Province, China.