Three-Dimensional Assessment of Drought in Global River Basins

Understanding the spatiotemporal variability of drought is critical for assessing its impacts on water resources and terrestrial ecosystems. Despite extensive drought research, basin-scale drought characteristics and their long-term changes have rarely been explored globally within a three-dimensional identification framework, particularly from a multi-index perspective. In this study, drought events across 59 major global river basins during 1979–2020 were identified using the standardized precipitation evapotranspiration index (SPEI) combined with a three-dimensional clustering approach. To assess the robustness of drought characterization, results derived from SPEI were further compared with those based on the standardized precipitation index (SPI). Overall, most river basins did not exhibit statistically significant long-term trends in drought occurrence. Spatially, drought events detected by both indices were largely concentrated along river corridors, highlighting the close coupling between drought evolution and basin hydrological structure. Basin size strongly modulates drought behavior: larger basins tend to experience longer-lasting and more severe droughts, whereas smaller basins are characterized by more frequent but weaker events. Temporal analysis revealed pronounced periodicity in drought variability, especially in small- and medium-sized basins, while drought-affected area and severity consistently increased with event duration. Comparative analysis between SPEI and SPI revealed broadly consistent spatial patterns but notable regional differences in drought frequency and severity. In low- and mid-latitude regions, including South America, Central Africa, and parts of Asia, SPEI identified more extensive and persistent drought events than SPI, suggesting a stronger sensitivity of drought characteristics to temperature-related effects. In contrast, high-latitude and temperate basins generally showed similar drought responses across the two indices. Relationships among drought area, severity, intensity, and duration exhibited comparable behaviors for both indices, with drought area and severity tending to increase over time, while drought intensity showed a gradual decline in most basins. Furthermore, atmospheric circulation was found to exert a stronger influence on drought variability in coastal basins than in inland regions. These findings provide new insights into basin-scale drought dynamics and their controlling mechanisms under a three-dimensional perspective.