Declining Runoff Efficiency and Storage Reorganisation in the Source Region of the Yellow River Under Climate Warming, 1974–2025

ABSTRACT The source region of the Yellow River (SRYR) has become warmer and wetter, but annual runoff has not increased proportionally, suggesting that a comparatively stable water yield may conceal a reorganisation of runoff generation. A 52‐year hydroclimatic record (1974–2025) was analysed by integrating change‐point diagnostics, cumulative precipitation‐runoff analysis, water‐resource accounting, satellite‐era storage and cryospheric indicators, seasonal runoff coefficients and Eckhardt baseflow separation. The slope of the cumulative precipitation‐runoff relation decreased by 13.3% around 1989. The runoff coefficient shifted in 1997, preceding the dominant runoff‐volume shift in 1999. Relative to 1974–1989, precipitation during 2013–2025 increased by 20.0%, whereas runoff increased by only 2.3% and the runoff coefficient decreased by 14.9%. The accounting‐based groundwater‐resource ratio increased by 36.5%. Summer and autumn runoff coefficients decreased by 21.6% and 21.1%, respectively, whereas the winter coefficient increased by 29.9% and the baseflow index rose from 0.614 to 0.694. Concurrent increases in terrestrial water‐storage anomaly, soil moisture, model‐derived active‐layer thickness and evapotranspiration, together with fewer cold‐season snow‐cover days, were consistent with greater warm‐season retention and consumption and enhanced delayed release. The temporal sequence and convergence of these diagnostics indicate that recent wetting restored runoff volume without restoring the earlier high‐efficiency runoff regime. Annual runoff alone therefore underrepresents changes in storage dependence, seasonal water availability and climate sensitivity in warming alpine headwaters.