Stable isotopic compositions and controlling factors of precipitation and atmospheric vapour in the headwaters of the Shule River

Stable isotopic compositions of precipitation (delta 18Op, delta 2Hp and d-excessp) and atmospheric vapour (delta 18Ov, delta 2Hv and d-excessv) with high spatial-temporal resolution are crucial in revealing hydrologic cycle. Based on the variation characteristics of delta 18Op/delta 18Ov, delta 2Hp/delta 2Hv and d-excessp/d-excessv in the headwaters of the Shule River (HSR) on hourly and daily scales from June to September 2018, this study analysed the relationships between delta 18Op/delta 2Hp and delta 18Ov/delta 2Hv combined with the equilibrium fractionation model, as well as delta 18Op/delta 18Ov and meteorological factors. The slopes of local meteoric water line (LMWL) and the delta 2Hv-delta 18Ov fitting equation were similar (7.96 and 7.94) with both intercepts exceeding 10, reflecting the great contribution of recycling moisture. The values of delta 18Ov/delta 2Hv were lower than delta 18Op/delta 2Hp but with consistent variation patterns throughout the period. The equilibrium simulation results suggested that precipitation and atmospheric vapour almost approached isotopic equilibrium state, especially during monsoon intrusion period. Affected by monsoon intrusion, the slopes and intercepts of the LMWLs and the delta 2Hv-delta 18Ov fitting equations were smaller than those during non-monsoon period and d-excess and delta 18O were negatively correlated. Relative humidity had significant negative correlations with delta 18Op and delta 18Ov in the whole period, however, the positive correlations between delta 18Op/delta 18Ov and temperature were observed during non-monsoon and monsoon intrusion period, respectively. Our results demonstrated that precipitation and atmospheric vapour isotopic compositions exhibited consistency under the influence of diverse moisture sources, while more complex relationships were found between delta 18Op/delta 18Ov and meteorological factors. This research provided evidence for using the isotopic compositions of atmospheric vapour to indicate moisture sources, and can improve understanding of the water cycle and eco-hydrological process from the perspective of the interaction between water and gas phases of the inland river basin in northwest China. Local meteorological factors are not sufficient to explain the variations in precipitation and atmospheric vapour isotopic compositions. The significant relationships between relative humidity and delta 18O of precipitation and atmospheric vapour might be attributed to the complex moisture sources in the study period.image