Runoff dominated by supra-permafrost water in the source region of the Yangtze river using environmental isotopes

Accelerating multiphase water transformation affects runoff processes and components greatly and have changed the spatiotemporal patterns of water resources in the Third Polar Region. The source region of the Yangtze River is one location where accelerated warming has resulted in the gradual extension of the ablation period since 1990. This has caused the acceleration of multiphase water transformation, characterized by increases in the rate of glacial retreat, maximum freezing depth, and annual actual evapotranspiration and by decreased snowfall. In response, the total runoff increased by 53% at the Tuotuohe national hydrological station (TTH) and 6% at the Zhimenda national hydrological station (ZMD) during the periods 1961–1990 and 1991–2017, respectively. Under these conditions, runoff components were being determined based on stable isotope tracing. Substantial seasonal differences in δ18O (δD) among precipitation, river water, supra-permafrost water, and glaciers snow meltwater indicate that the runoff has been replenished by multiple components, and that these first infiltrate the ground, becoming part of the groundwater, and then recharge river water. Supra-permafrost water rather than precipitation now dominates river water. Based on the end-member mixing analysis model, supra-permafrost water, precipitation, and glaciers snow meltwater accounted for 51%, 26%, and 23% of river water at the TTH station from June 2016 to May 2018; the corresponding values at the ZMD station were 49%, 34%, and 17%. Additionally, there are also differences in the seasonal contributions of runoff components to river water. Seasonal variations in the freezing and thawing of the active layer directly trigger the runoff process. Future research should be focused on determining the mechanisms underlying the dynamics of precipitation-supra-permafrost water-runoff, which will aid the assessment of the impacts of an unstable Asian Water Tower on water resources.