Spatiotemporal Characteristics of Hydroclimatic Extremes in the Weihe River Basin, China

Hydroclimatic extremes pose increasing challenges to water security and ecosystem stability, particularly in climate-sensitive transitional regions. Taking the Weihe River Basin in China as a representative monsoon-influenced basin, this study investigated the temporal evolution, spatial heterogeneity, severity, and transition characteristics of flood and drought events. An integrated framework combining the Soil and Water Assessment Tool (SWAT), the Standardized Runoff Index (SRI), the Standardized Soil Moisture Index (SSMI), run theory, three-dimensional drought-event identification, and drought–flood abrupt alternation (DFAA) analysis was developed to reconstruct and characterize historical hydroclimatic extremes. The results showed pronounced spatial heterogeneity in flood characteristics. Floods occurred more frequently in the downstream reaches of the Weihe River mainstream but were generally shorter and less severe, whereas the upstream mainstream and the middle reaches of the Jinghe and Beiluohe Rivers experienced fewer but longer-lasting and more severe flood events. Flood characteristics also varied markedly among representative years, with the 2003 event exhibiting particularly strong spatial differentiation in duration and severity. The mean frequencies of mild, moderate, severe, and extreme droughts were 0.16, 0.11, 0.05, and 0.05, respectively. Droughts occurred more frequently in the northern and western parts of the basin, while severe drought-event centers were mainly concentrated in the central region. The representative 2008–2009 drought originated in the western basin, expanded and intensified toward the central region, subsequently shifted westward while weakening, and finally dissipated in the southern basin. In addition, flood-to-drought events occurred more frequently than drought-to-flood events, whereas drought-to-flood events exhibited slightly greater transition intensity; the southwestern mountainous areas and downstream reaches were identified as major DFAA hotspots. These findings reveal pronounced spatial heterogeneity and directional asymmetry in hydroclimatic extremes and provide a scientific basis for integrated flood–drought monitoring and water-resources management in monsoon-influenced transitional basins.