Subsurface snowmelt buffers river response to warming in the Tibetan Plateau

Warming across the Tibetan Plateau is reorganizing snow dynamics and the resulting runoff regime, with increasingly direct implications for regional freshwater security. However, the distinct roles of rapid surface versus delayed subsurface snowmelt in regulating river flow remain poorly understood. Here, we apply a dual-phase tracking framework to decipher surface and subsurface snowmelt contributions to runoff across ten headwater basins from 1968-2018. Results show that surface snowmelt delivers a short spring pulse, contributing over 25% of total runoff during the melt peak, whereas subsurface snowmelt provides sustained recharge year-round and primarily constitutes winter discharge. Over the past five decades, the centroid timing of surface snowmelt advances significantly at ~6.8 days per decade across ~93% of grid cells, while subsurface snowmelt timing remains nearly stationary. Consequently, the timing of total runoff shifts only modestly (0–3 days per decade), demonstrating strong timing buffering by subsurface pathways that retain and gradually release meltwater. This buffering softens late-season low-flow declines in some basins, yet basin dependence limits its capacity against warming-driven drying. These results underscore the critical role of subsurface meltwater in regulating river flow timing in a warming climate, with implications for water resource management in cryospheric regions.