Investigating the Mechanisms of Hydrological Impact due to Drought–Flood Abrupt Alternation in the Upper Hanjiang River Basin, China

Abstract Drought–flood abrupt alternation (DFAA) is defined as a compound extreme event characterized by the rapid transition between drought and flood conditions within a short period, leading to significant ecological and socioeconomic impacts. Currently, few studies have systematically investigated the interaction mechanism between meteorological and hydrological DFAA events using a robust index. Consequently, this study optimized the DFAA index (DFAI) using both subjective and objective methods and quantitatively assessed the correlation between meteorological DFAA and hydrological DFAA in the upper Hanjiang River basin (HJRB) through integrated statistical analysis and SWAT model simulations. The primary findings indicate that meteorological DFAA events exhibit distinct spatial heterogeneity, predominantly occurring in the eastern part of the basin, with flood-to-drought (FTD) event frequency surpassing drought-to-flood (DTF) events. There is a strong correlation between meteorological DFAA and hydrological DFAA, though their relationship is nonlinear across multiscales. Statistical analysis reveals that long-term hydrological events are more distinctly driven by meteorological factors, whereas short-term events are significantly influenced by human activities. Model simulations further show that the decrease in the frequency of hydrological DFAA events and the shift of their dominant manifestation type from DTF to FTD are attributed to changes in precipitation patterns, which have also intensified their extremity. Anthropogenic factors have exacerbated the magnitude of long-term hydrological DFAA events while diminishing short-term ones, partially mitigating their shift from DTF to FTD. This study offers insights into hydrometeorological risk and its implications for basin water resources management. Significance Statement Drought–flood abrupt alternation (DFAA) events are commonly defined as the rapid transition between drought and flood conditions within a short period, including both drought-to-flood (DTF) events and flood-to-drought (FTD) events. Generally, the characteristics of dry and wet transition during precipitation will directly affect relevant characteristics of runoff within a watershed. However, with global warming and increasing human activity, how will the correlation mechanisms between precipitation DFAA and runoff DFAA evolve? Through statistical analysis and model simulations, we found that there is a strong correlation between precipitation DFAA and runoff DFAA. The change in precipitation patterns reduces the frequency but increases the intensity of runoff DFAA events, while the coordinated operation of water conservancy projects in river basins can either alleviate or worsen these effects. Additionally, results indicate that there will be a higher frequency of future shifts from flooding to drought in both precipitation and runoff. This helps us gain a deeper understanding of the risks of hydrometeorological disasters and manage water resources in river basins more effectively.