Eco-Hydrological Regime Shifts Triggered by Declining Snow To-Rainfall Ratios in Alpine Transition Zones

Abstract

Climate warming is accelerating the transition from solid to liquid precipitation in high-altitude regions, yet the physical mechanisms by which this phase shift alters intra-annual water partitioning and ecosystem availability remain poorly quantified. Using multi-decadal (2001–2020) hydro-climatic datasets and remote sensing observations across the Tibetan Plateau, we investigated the eco-hydrological dynamics within the critical 4000–5500 m elevation transition zone. We identified a sequential moisture retention mechanism where pre-monsoon snowmelt acts as a critical seasonal buffer, sustaining subsurface soil moisture and initiating spring vegetation green-up prior to the summer monsoon. However, our analysis reveals that advancing snowmelt onset is temporally decoupling this moisture supply from peak biological demand, inducing an early-season water deficit that significantly constrains vegetation growth. Crucially, we identified a hydrological equilibrium threshold at a multi-year mean snowmelt-to-rainfall (S/R) ratio of approximately 0.23, which demarcates the boundary where alpine ecosystems transition from a snow-sensitive to a rain-dominated state. This shift fundamentally diminishes the cryospheric buffering capacity due to the loss of slow-release snowpack infiltration. Furthermore, the reduction in autumn snowmelt compromises antecedent soil moisture memory—a stabilising function that rainfall fails to replicate due to increased rapid surface runoff. These findings demonstrate that declining S/R ratios inherently reduce the intra-annual reliability of seasonal water supplies, fundamentally altering the eco-hydrological stability of alpine environments.