[Environmental Significance of the Stable Isotopes in Precipitation at Different Altitudes in the Tuolai River Basin].

Precipitation samples and meteorological data were collected simultaneously during individual precipitation events at Tuole station (3367 m a.s.l.) and Jiayuguan station (1658 m a.s.l.) in the Tuolai River Basin. A study of temporal variation, Local Meteoric Water Lines, and altitude change on precipitation stable isotopes was conducted. The relationships between precipitation stable isotopes and temperature, precipitation, average vapor pressure, and relative humidity were determined in order to explore the environmental significance of the stable isotopes at different altitudes in the middle reaches of the Qilian Mountains. The analysis indicated that the stable isotopes of the precipitation in Tuole and Jiayuguan station were characterized by pronounced seasonal variation, with Tuole having higher δO values in summer and autumn and lower δO values in spring and winter, while Jiayuguan displays higher δO values in spring and lower in other seasons. The -excess was correlated negatively with δO, and the correlation coefficients between δO and -excess decreased with increasing altitude due to weakening sub-cloud evaporation. The slope and intercept of the Local Meteoric Water Lines from Jiayuguan to Tuole rose significantly, showing an increasing trend from low altitude to high altitude. For the precipitation events above 10℃, δO of Tuole was positively correlated with the temperature, but the Jiayuguan results indicated the opposite. Sub-cloud evaporation weakened with high precipitation events in Jiayuguan. δO an-excess were positively correlated with the average vapor pressure, which declined from Tuole to Jiayuguan. Since the water vapor pressure and saturated water vapor pressure increased, it was difficult to form precipitation with decreasing altitude. The local strong sub-cloud evaporation caused δO and δD was positive at low altitude, while the effect of moisture recycling is obvious, such that δO and δD are negative in high altitude areas. There is no significant positive correlation between the δO and the relative humidity of the precipitation in Jiayuguan, while Tuole displays an opposite pattern. The results of the study will provide a scientific basis for further study of precipitation isotopes in the Tuolai River basin.