This study focused on the hydrological and runoff formation processes of river water by using stable isotope tracing in the source regions of the Yangtze River during different ablation episodes in 2016 and the ablation period from 2016 to 2018. The effects of altitude on stable isotope characteristics for the river in the glacier permafrost area were greater than for the main stream and the permafrost area during the ablation period in 2016. There was a significant negative correlation (at the 0.01 level) between precipitation and δ¹⁸O, while a significant positive correlation was evident between precipitation and d-excess. More interestingly, significant negative correlations appeared between δ¹⁸O and temperature, relative humidity, and evaporation. A mixed segmentation model for end-members was used to determine the proportion of the contributions of different water sources to the target water body. The proportions of precipitation, supra-permafrost water, and glacier and snow meltwater for the main stream were 41.70 %, 40.88 %, and 17.42 %, respectively. The proportions of precipitation, supra-permafrost water, and glacier and snow meltwater were 33.63 %, 42.21 %, and 24.16 % for the river in the glacier permafrost area and 20.79 %, 69.54 %, and 9.67 %, respectively, for that in the permafrost area. The supra-permafrost water was relatively stable during the different ablation periods, becoming the main source of runoff in the alpine region, except for precipitation, during the ablation period.

Mountains are sensitive to climate change, and while amplified warming at high elevations is widely observed and fairly well understood, changes in the water cycles of mountain regions remain poorly quantified. Due to low temperatures at higher elevations, these changes involve multiphase water transformation (MWT). Through analyzing extensive data from global mountain regions, we determined that under the accelerating warming and lengthening ablation period since the 1990s, the strengthening solid–liquid transformation can be confirmed for 45 glacier basins or single glaciers. This is marked by an increase of 21.5 km2/10a in glacier area retreat rate, 387.65 mm for average negative glacier mass balance, and 60 m/10a for average glacier length retreat rate (of 414 glaciers) from the study period before the 1990s until the period after the 1990s. The accelerating liquid–solid transformation was indicated by an increase of 31.2 d/10a for the delaying trend of complete freeze time, an increase of 4.3 d/10a for the advancing trend of complete melting time, and an increase of 3.9 d/10a for the decreasing trend of ice cover duration for 22 lakes from the period before the 1990s until the period after the 1990s. The accelerating liquid–gas transformation can be confirmed by an increase of 1 and 0.69 mm/d/10a in the variation trend of actual evaporation and bare-soil evaporation from 1980–1990 to 1990–2017, respectively. Snow sublimation decreased by 0.69 mm/d/10a during 1980–1990, followed by a statistically significant increase of 1.66 mm/d/10a during 1990–2017, further confirming the accelerating solid–gas transformation. The accelerating gas–solid transformation can be reflected by an increase of 0.3 d/10a for the decreasing trend of frost days from 1960–1990 to 1990–2017. The moisture recycling ratio decreased by –0.042 %/10a during 1980–1990 and then increased by 0.443 %/10a during 1990–2017, with the corresponding average values of 12.3% and 13.6%, respectively, which indicates an accelerating gas–liquid transformation. Approximately 59 rivers displayed an increase of 108.60 m3/s/10a for the runoff variation trend from the period before the 1990s until the period after the 1990s. In addition, the trends for lake number and lake area in the Tibetan Plateau increased 3.86 and 5.75 times, respectively, from 1976–1995 to 1995–2019. This acceleration can significantly change the spatiotemporal pattern of water resources and increase the frequency and intensity of disaster events, such as glacial lake outbursts, flooding, and waterlogging. Consequently, most mountain regions will require strong adaptation efforts to sustain water, food, and ecological security.