Warming-induced decrease in snowfall fraction shifts runoff seasonality over the Tibetan Plateau

Climate warming facilitates a transition from snowfall to rainfall, altering snowpack dynamics and affecting runoff seasonality. The Tibetan Plateau, located at the westerly-monsoon transition zone, exhibits varied impacts of snowmelt on runoff across different climatic regions. Using monthly runoff data from nine snow-affected river basins, runoff seasonality response to snowfall fraction changes is investigated during the historical (1998-2017) and future (2021-2060) periods. Distinct seasonal runoff responses to decreasing snowfall fraction are observed across basins dominated by different climate regimes, with a critical snowfall fraction threshold of 0.5. Specifically, the center timing of runoff mass in westerly-dominated basins shifts earlier, while delayed in monsoon-dominated basins. Seasonal runoff variability decreases with reduced snowfall fraction in westerly-dominated basins, but increases in monsoon-dominated basins, primarily due to asymmetric cold-warm season runoff responses. In westerly basins, declining snowfall reduces both warm and cold season runoff, dampening variability. In monsoon basins, warm-season runoff increases while cold-season runoff declines, amplifying seasonality. This shift is driven by snowfall fraction effects on snowpack dynamics. Below the threshold, rapid spring snowmelt occurs with sensible heat flux dominance. Above the threshold, persistent snowpack shifts energy partitioning toward latent heat flux, delaying and moderating peak runoff. Future projections indicate runoff timing will advance by 22-29 days with reduced variability in westerly-dominated basins, but delay by 18-20 days with increased variability in monsoon-dominated basins. Warming-induced earlier snowmelt will increase spring runoff, while rising evapotranspiration reduces summer and autumn water availability, exacerbating seasonal water shortages.