Spatiotemporal variations of water stable isotope compositions in Nujiang Headwaters, Qinghai-Tibetan Plateau

Abstract

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 (δ18O) and hydrogen (δ2H) isotopes' compositional variability and how these are related to hydroclimatic processes, we measured δ18O and δ2H values from surface waters, snow and precipitation across the Nujiang headwaters from April to September 2018. We found higher δ18O (-13.7%ο), δ2H (-101.8%ο) and deuterium excess (d-excess; 7.6%ο) values in the non-monsoon season and lower values in the summer monsoon season. Our findings indicated that the δ18O and δ2H 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: δ2H = 6.66 δ18O-14.90, R2 = 0.98) were lower than those of the local meteoric water line (LMWL: δ2H = 9.50 δ18O + 41.80, R2 = 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, δ18O and δ2H tended to be negatively correlated with precipitation and air temperature. In addition, δ18O and δ2H 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.