Inverse Isotopic Altitude Effect of Surface Water across the Upper Reaches of the Jinsha River, China: A Consideration of Moisture Sources

Abstract

Characterization of the stable isotopic composition of modern surface water is essential to understand the complex hydrological processes in high-altitude mountainous areas. As one of the largest rivers draining from the Tibetan Plateau, there are large gaps in the knowledge of the isotopic variations of surface water across the Jinsha River. Here, we present new isotopic data from surface water along the upper reaches of the Jinsha River in June 2019. An inverse isotopic altitude effect was found with an average altitude gradient of 1.04‰/100 m for δ 2 H and 0.10‰/ 100 m for δ 18 O. The effect of the inflow of the tributary waters on the isotopic characteristics of the main stream was clearly observed. The history of the moisture transportation was evaluated at five major local hydrological stations along the main stream using the HYSPLIT model. A total of three moisture source classifications (southwest monsoon, recycled moisture, and westerly) were summarized, and a gradual increase in the percentage of the westerly was observed with increasing geographical altitude and latitude. A hypothesis was proposed that the moisture sources affected by different monsoons determine the isotopic characteristics of precipitation within the basin, and the rainfall runoff further affects the spatial distribution of surface water isotopes, responsible for the inverse isotopic altitude effect. These findings contribute to a better understanding of the spatial complexity of the isotopic altitude effect of surface water and the hydrological dynamics in the monsoon-influenced rivers around the Tibetan Plateau.