Plateau river research: spatial-temporal variations of δ18O and δd in the water of the Yarlung Tsangpo River and their controlling factors.

Stable isotope technology has been widely used to trace sources and evolution of water bodies, relationships between different water bodies and pollution sources. Based on δ ¹⁸O and δD data from the Yarlung Tsangpo River in 2017, this paper analyzes the composition characteristics of δ ¹⁸O and δ D in the river water during the low-flow, high-flow and normal-flow seasons of the entire Yarlung Tsangpo River and further reveals the spatial evolution and influencing factors. The results show that the values of δ ¹⁸O, δD and d-excess were different in time and space. The δ ¹⁸O, δD and d-excess values of the Yarlung Tsangpo River in the low-flow season were significantly higher than in the high-flow season. This was mainly due to weak evaporation enrichment and precipitation supply effects on the river water during the wet season. From the perspective of season change, the d-excess in the low-season is significantly higher than in the high-flow season and the normal-flow season; from the perspective of spatial change, the d-excess shows the same spatial variation trend as the δ ¹⁸O, which first decreases and then increases. Based on the investigation of δ18O and δD in the main and Branch rivers of the Yarlung Tsangpo River Basin, it was found that the upper reaches of the basin were mainly supplied by snowmelt, the lower reaches were mainly supplied by rainfall. • Altitude effect: The composition of δD and δ ¹⁸O isotopes was gradually enriched with light isotopes as the altitude increased; thus, δD and δ ¹⁸O gradually decreased. The height effect was related to the distribution of temperature and terrain: when the altitude increased, the terrain became higher, the temperature decreased, the isotope fractionation coefficient increased, and the values of δD and δ ¹⁸O in atmospheric precipitation were lower, and it was rich in light isotopes. Obviously, as the altitude increased, δD and δ ¹⁸O became lower and show a positive correlation. The δ ¹⁸O value of the Yarlung Tsangpo River demonstrated a significant height effect. • By analyzing the characteristics of δD and δ ¹⁸O composition in the water of the Yarlung Tsangpo River during the low and high water periods, it was found that δD, δ ¹⁸O and d-excess values in the water of the Yarlung Tsangpo River were significantly different in time and space, and δD, δ ¹⁸O and d-excess values in the low-flow season were significantly higher than those in the high-flow season, which was mainly due to the weak evaporation enrichment effect and precipitation supply in the high water period. The δ ¹⁸O value increased gradually from the upstream to the downstream, which was related to the inflow of tributaries and lake water from different reaches of the Yarlung Tsangpo River. • There is a positive proportional relationship between δ ¹⁸O content and rainfall in the Yarlung Tsangpo River Basin. Due to the different water vapor sources and water vapor cycle patterns between the north and the south, the uplift of the plateau and the Himalayas caused the decrease of δ ¹⁸O in the river water of the plateau. The measured data show that the δ ¹⁸O value in the water body of the Yarlung Tsangpo River gradually increases from upstream to downstream, which may also be related to the cross contribution of tributaries and lakes along the river. [ABSTRACT FROM AUTHOR]