Spatiotemporal Variations of Water Stable Isotope Compositions in Nujiang Headwaters, Qinghai-Tibetan Plateau

The variations of the stable isotope compositions in water provide critical information on hydroclimatic mechanisms. The climatological and hydrological processes in the Nujiang headwaters in the central Qinghai-Tibetan Plateau are extremely complex and are controlled by alternating continental/local recycled and maritime moisture. However, previous studies have only derived limited data from different types of water in the Nujiang headwaters. Therefore, aiming to understand the sources of stable oxygen (delta O-18) and hydrogen (delta H-2) isotopes' compositional variability and how these are related to hydroclimatic processes, we measured delta O-18 and delta H-2 values from surface waters, snow and precipitation across the Nujiang headwaters from April to September 2018. We found higher delta O-18 (-13.7 parts per thousand), delta H-2 (-101.8 parts per thousand) and deuterium excess (d-excess; 7.6 parts per thousand) values in the non-monsoon season and lower values in the summer monsoon season. Our findings indicated that the delta O-18 and delta H-2 compositions were significantly affected by different moisture sources in this region. The slope (6.66) and intercept (-14.90) of the surface water line (SWL: delta H-2 = 6.66 delta O-18 - 14.90,R-2= 0.98) were lower than those of the local meteoric water line (LMWL: delta H-2 = 9.50 delta O-18 + 41.80,R-2= 0.99) and global meteoric water line (GMWL), indicating that precipitation was the primary water vapor source for surface water, and evaporation was the dominant hydrological process for the Nujiang headwaters. In general, delta O-18 and delta H-2 tended to be negatively correlated with precipitation and air temperature. In addition, delta O-18 and delta H-2 values in the Nagqu River were inversely correlated with the intensity of discharge, highlighting a precipitation-driven isotope-discharge pattern. Our findings provide a theoretical basis for the hydroclimatic mechanisms occurring in the Nujiang headwaters and further augment our understanding of the southern-middle-northern hydroclimate in the Qinghai-Tibetan Plateau.