Stable isotope composition of precipitation at different elevations in the monsoon marginal zone

The Qilian Mountains in western China are an important ecological and security barrier and are the primary water source for inland river basins. In this study, we collected precipitation samples and meteorological data from 4 sampling sites at different elevations on the northern slope of the Qilian Mountains from October 2016 to October 2017. The purpose was to analyse spatial and temporal variations of the isotope composition of precipitation and to evaluate the influence of temperature, elevation, monsoon circulation and continuous rainout processes on the measured values. The results show that the isotopic composition exhibit obvious seasonal variations; the higher δ18O values occur in summer and autumn, and the lower values occur in winter and spring, while the lower d-excess values occur in spring and summer and the higher d-excess values occur in autumn and winter. Stable isotope values varied with elevation. δ18O and δD values decreased with increasing elevation, and d-excess increased with increasing elevation. The average annual elevation gradient for δ18O was −0.26‰/100 m, δD was −1.77‰/100 m, and d-excess was 3‰/100 m, and the elevation gradient in summer was higher than in winter. The slope and intercept of LMWL increased with increasing elevation. The elevation gradients for slopes and intercepts are 0.07/100 m and 1/100 m, respectively. The temperature effect is most significant below 0 °C. Local moisture recycling and sub-cloud evaporation also play an important role. Moisture sources are mainly controlled by westerly circulation. Influenced by moisture from the Asian monsoon, the δ18O and δD values of precipitation show a weak “precipitation effect” in July and August, and the values gradually become more negative with continued rainout. This study improves knowledge of the isotope evolution of precipitation in the Qilian mountains, and lays the foundation for further research on isotope hydrology in cold and arid regions.