Interannual Variability of Snowmelt Runoff and Its Climatic Controls across Major River Basins of the Tibetan Plateau

The Qinghai–Tibet Plateau (QTP) is one of the most climate-sensitive regions in the world. Changes in snowmelt water resources are critical for sustaining the “Asian Water Tower” and downstream water security. However, due to the complexity of cryospheric processes and uncertainties in models, data, and temporal scales, a consistent understanding of snowmelt runoff evolution and its climatic drivers across multiple basins is still limited. This study applies the VIC-CAS hydrological model to simulate snowmelt runoff in 15 major watersheds on the QTP during 1970–2020. We analyze the spatiotemporal variations of snowmelt runoff, total runoff, and snowmelt contribution ratios, and examine their responses to climate change within a unified framework. Results show strong spatiotemporal heterogeneity in snowmelt water resources across the plateau. On average, snowmelt contributes 24.8% of total runoff and exhibits clear seasonality, with peak contributions in June–July. Afterward, runoff generation gradually shifts from snowmelt dominance to combined glacier melt and rainfall. Monsoon-dominated basins show strong runoff seasonality, while westerly-controlled basins exhibit more uniform intra-annual distributions. Total runoff displays a weak and non-significant decreasing trend, with transition years mainly between 1980 and 1995 and a delayed pattern from east to west. In contrast, the snowmelt contribution ratio decreases significantly at a rate of about 1.7% per decade, with later transition years, especially in monsoon-influenced basins. Process-based analyses further indicate that snowmelt runoff initiation and center-of-mass dates advance significantly across all basins, accompanied by prolonged runoff duration. Snowmelt runoff exhibits a clear elevation dependence, with a threshold near ~4,000 m a.s.l., below which runoff decreases and above which it increases; this threshold shifts downward in glacier-rich basins. Overall, precipitation anomalies emerge as the dominant driver of interannual snowmelt runoff variability by controlling runoff magnitude, while rising air temperature primarily regulates the timing and phase of snowmelt runoff generation. Cryospheric elements, including glaciers and permafrost, further modulate basin-scale hydrological responses by exerting buffering and amplifying effects.