Evolutionary Characteristics and Transmission Processes of Meteorological and Hydrological Droughts in the Middle and Upper Reaches of the Yellow River Basin

Understanding drought evolution and the propagation from meteorological to hydrological drought is essential for regional water security and water resources management. This study examines the middle and upper reaches of the Yellow River Basin using the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Runoff Index (SRI) derived from hydro-meteorological records from 1967 to 2023. Drought evolution and propagation across multiple timescales are analyzed using correlation and wavelet-based approaches.The results demonstrate pronounced differences between meteorological and hydrological droughts. Meteorological droughts occur more frequently with shorter duration and lower intensity, whereas hydrological droughts are less frequent but exhibit stronger persistence and greater severity, reflecting the cumulative response of hydrological systems to sustained meteorological water deficits. At the basin scale, dry and wet conditions were relatively balanced before 1995, while droughts became dominant and Intensified thereafter. Hydrological droughts show a clear lagged response to meteorological droughts, primarily controlled by cumulative meteorological drought effects at an approximate 7-month timescale. Significant spatial heterogeneity characterizes drought propagation, with more dispersed response timescales in upstream regions and increasingly concentrated responses downstream. At medium- to long-term timescales (approximately 16–64 months), stable and persistent coherence between meteorological and hydrological droughts highlights the dominant role of basin-scale water storage and regulation processes.These findings support improved drought monitoring and adaptive water resources management in the Yellow River Basin.