Shift regime of multiphase water in the Three-River-Source Region

A significant hydrological feature of the cryosphere is that water bodies exist in multiphase forms. Abrupt shifts in multiphase water indicate a break in the stability of the hydrological cycle and may threaten sustainable water resources supply. However, the phenomenon of abrupt shifts in alpine regions and their underlying driving mechanisms remain poorly understood. This study investigated the spatiotemporal patterns, gradual accumulation–abrupt characteristics, and driving mechanisms of multiphase water in the Three-River-Source Region (TRSR). The results revealed that solid water decreased, while liquid and gaseous water increased from 2002 to 2022. A total of 32.25% of the TRSR experienced abrupt shifts, primarily in the Yangtze and Yellow River source regions. The critical thresholds for solid, liquid, and gaseous water were identified as 2.54×10⁴ m³，1.60×10⁷ m³, and 3.21×10⁵ m³, respectively. Abrupt shifts were most likely to occur when the volumes of solid, liquid, and gaseous water reached their respective thresholds. Climate was the primary driver of gradual and abrupt changes, while vegetation significantly moderated solid water ablation and enhanced liquid water accumulation in regions with abrupt shifts. Specific environmental conditions, such as leaf area index (0.11–1.00), annual rainfall (199.26–649.76 mm), and the maximum temperature (0.83–9.80℃), were found to increase the likelihood of triggering abrupt shifts. This study proposed a more accurate method to depict the shift regime of multiphase water, providing critical insights for water resources management and risk governance in alpine regions.