Intra-event Variability Of Stable Hydrogen And Oxygen Isotopes In Precipitation And The Relationship With Water Vapor Isotopes In The Upper Reaches Of The Urumqi River

Precipitation is an important part of natural water cycle.As important tracers of water cycle,stable hydrogen and oxygen isotopes in precipitation are sensitive to environmental changes.The variability of precipitation isotopes at different time scales contains different environmental information.Especially,the intra-event variability of stable isotopes in precipitation is helpful to understand the source and sink of water vapor for each precipitation event,which is a new way to explore the mechanism of isotopic variability.The simultaneous monitoring of stable isotopes in intra-event precipitation samples and water vapor is also useful to understand the isotopic fractionation in water cycle.In this paper,the 30-min based records of stable hydrogen and oxygen isotopes in precipitation and water vapor were collected from June to July2019 in the upper reaches of the Urumqi River in the Tianshan Mountains,Xinjiang;and then the intra-event variability of precipitation isotopes as well as the relationship between precipitation isotopes and water vapor isotopes were investigated.?1?During the sampling period,the variation range of?¹⁸O in intra-event precipitation samples is between-15.26‰and 4.16‰,and the average is-4.99‰;the variation range of?²H is between-108.46‰and 15.66‰,and the average is-32.85‰.No matter what time scale?event or intra-event scales?is applied,temperature is always the dominant meteorological factor influencing precipitation isotopes;however,the correlation coefficient between isotopic ratio and temperature at intra-event scale is slightly less than that at event scale.The backward trajectory analysis shows that the air mass may dominantly originate from the western paths.According to the backward trajectories before and after the precipitation events,the modification of vapor paths should be considered as a factor influencing precipitation isotopes.Cases of the Central Asian vortex is selected as a typical synoptic system in the study region;when the Central Asian vortex occurs,the rainfall amount is large,and the precipitation is isotopically depleted;as the Central Asian vortex weakens,the precipitation intensity decreases,and the falling process of raindrops is increasingly affected by sub-cloud secondary evaporation,leading to a gradually increasing?¹⁸O in precipitation.The intra-event variability of precipitation isotopes can be divided into five types:decline type,L-shaped type,rising type,V-shaped type and inverted V-shaped type.The different isotopic types are related to the transport path of water vapor and the meteorological elements when precipitation occurs.?2?During the sampling period,the maximum?¹⁸O in water vapor usually occurs in the morning,and the minimum appears in the afternoon;while the trend of d-excess is opposite.Under different weathers?sunny and rainy?,the water vapor isotopes show different diurnal variations.In sunny days,the diurnal trend in?¹⁸O is opposite to that in d-excess,which is associate to the diurnal variation in air temperature and relative humidity;in contrast,the meteorological controls in rainy days are weakened to some extent.Based on the simultaneous stable isotopes in precipitation and water vapor,there is a large difference between measured and estimated?¹⁸O in water vapor when the intra-event isotopic records and an equilibrium assumption are applied;however,the estimated stable isotopes in water vapor can be much improved under an assumption of non-equilibrium fractionation that adjusted using local evaporation line.The sub-cloud secondary evaporation is an important factor related to the departure of stable isotope in water vapor from the equilibrium condition.