Using Stable Isotopes to Estimate Stream Water Age in the Qinghai-Tibet Plateau

Stream water age is an essential indicator of stream water renewal and pollutant transport rates in the Qinghai-Tibet Plateau (QTP). Recently, the concept of a young water fraction (F_yw , calculated using stable isotopic data, was proposed as a measure of stream water age. The relative age of older stream in streams has yet to be quantified, however. In this study, we proposed a method to calculate the relative proportions of middle-aged and old-age water in runoff. The results revealed that in the Nagqu watershed, an average of 23% of the runoff was < 51.6 days old, whereas an average of 55% of the runoff had ages ranging from 51.6 days to 1. The change in stream water age is significantly influenced by the runoff composition, evaporation, air temperature, and soil moisture at a depth of 20 cm. The amount of stored glacial ice, soil ice, snow cover, groundwater, and other old water in the QTP appears to be decreasing under the influence of climate change. The results of this study are helpful for the study of runoff composition and stream water age, as well as their responses to climate change.