Evaporation Processes in the Upper River Water of the Three Gorges Reservoir: Evidence from Triple Oxygen Isotopes

The triple oxygen isotopes supplement the information of kinetic fractionation processes that cannot be recorded by traditional hydrogen and oxygen isotopic analysis methods. To improve the understanding of the signatures of the water cycle in the Three Gorges Reservoir, the spatial variation of the isotopic composition and hydrological processes were investigated by measuring stable isotopes (δ¹⁸O, δ¹⁷O, and δ²H) of the river water in this research. The results showed that the isotopic compositions of the upper (Yibin–Luzhou) and lower reaches (Fengdu–Wushan) of the river underwent kinetic fractionation, while the middle reaches (Chongqing) were dominated by equilibrium fractionation. According to the correlation analysis, the evaporation processes of the river were associated with temperature and wind speed, and the isotopic compositions of river water were mainly controlled by the quantity effect of precipitation. The d-excess responded well to climate change, while the ¹⁷O-excess was more sensitive to altitude and mixing processes. This study revealed the information of triple oxygen isotopes in flood season and improved the understanding of the evaporation processes in the Three Gorges Reservoir, which provides a theoretical basis for further understanding the hydrological processes of the river–reservoir system.