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‰), δ2H (−101.8‰) and deuterium excess (d-excess; 7.6‰) 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.
Highlights
The Qinghai–Tibetan Plateau, known as the “Asian water tower“ in the Himalayas region, is the headwater source for the major Asian rivers [1,2,3]
We found that precipitation showed distinct changes in isotopic composition. δ18O and δ2H values were elevated in the pre- and post-monsoon seasons (April−May, September) and depressed in the monsoon season (June−August), which was directly related to precipitation (Figure 3)
We found that the moisture in the Nujiang headwaters was controlled alternately by the westerlies and the summer monsoon, with higher isotopic values in the non-monsoon season
Summary
The Qinghai–Tibetan Plateau, known as the “Asian water tower“ in the Himalayas region, is the headwater source for the major Asian rivers (e.g., the Yangtze, Yellow, Yarlung Zangbo and Nujiang) [1,2,3]. The hydrology of the plateau has been adversely affected by rapid warming periods and other anthropogenic activities [1,4]. These alterations have induced local climatological, hydrological and ecosystem changes in the headwater region [1,5,6,7,8]. The glacier coverage in this region has undergone a massive reduction, which caused increased precipitation and lake areas and higher river runoff [2,6,9] This region has been a research hot spot due to its unique geographic location and huge water circulation [1,10,11]. Due to lack of interest and knowledge, the mechanisms of these climatological and hydrological changes remain unclear, for the Nujiang catchment located in the core area of the plateau
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