Study region: The Yellow River Basin (YRB), the second-largest river basin in China, spans three major geomorphological units: the Qinghai-Tibet Plateau, the Loess Plateau, and the North China Plain. Elevation across the basin ranges from near sea level to approximately 6000 m.
Study focus: This study evaluates elevation related differences and spatiotemporal agreement between the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Using meteorological records from 1970 to 2020, we combined drought event identification, trend analysis, agreement assessment, conditional probability analysis, and Structural Equation Modeling to examine how thermal-hydrological coupling (the interaction between precipitation supply and atmospheric evaporative demand) affects drought index performance.
New Hydrological Insights for the Region: SPI showed greater sensitivity to short term droughts at 1-3 month timescales in low elevation areas, whereas SPEI showed stronger long term stability at 6-12 month timescales with increasing elevation. The strongest agreement between the two indices occurred in mid elevation transition zones. In high elevation source regions above 4000 m, these regions showed strong asymmetry, Cp(SPI|SPEI) was 0.53, whereas Cp(SPEI|SPI) reached 0.71 at 12 month scale. Temperature driven evapotranspiration became the dominant control on drought development, leading SPI to underestimate water stress systematically. These findings provide a mechanistic basis for selecting drought indices according to elevation under continued climate warming.