Stable Isotopes In Precipitation Of The Qilian Mountains And Their Significance In Sub-cloud Evaporation

Stable isotope,as an important part of water,although content is low,it is very sensitive to environmental changes and record the information of water circulation,have been widely used in the study of hydrology,climatology,ecology and other fields.tmospheric precipitation is an important link in the process of water circulation on earth.The study of stable hydrogen and oxygen isotopes in precipitation is helpful to understand the process of water circulation in depth,The study of stable hydrogen and oxygen isotopes in precipitation is also an important foundation and basic parameters for the application of Isotopic Technology in various disciplines.As an important ecological security barrier in the west of China,Qilian Mountains is the core water source area of inland river basins.In this paper,we selected the precipitation samples and meteorological data in October 2016 to October 2017 at 4different elevation sampling sites in the north slope of Qilian Mountains,in order to analyze the spatio-temporal variation of isotope composition in precipitation,and the influence of temperature,elevation,monsoon circulation on stable isotope variations are discussed.We use the Stewart raindrop model to analyze the sub-cloud evaporation effect in the study area.At the same time,the relationship between water vapor source and stable isotope variation is analyzed by the model of water vapor transport field and backward trajectory calculated from reanalysis data.This study can improve the cognition of precipitation isotope evolution in Qilian mountains,and lay a foundation for further research on isotope hydrology in cold and arid region.The results of this paper are as follows:?1?In Xiying River Basin,isotopes composition of precipitation showed a obvious seasonal change with higher?¹⁸O values in summer half year and lower?¹⁸O values in winter half year.meanwhile,d-excess showed a opposite tend of seasonal change.The LMWL of Xiying River Basin is:?D=7.97?¹⁸O+15.96.The slope is lower than GWML.The slopes and intercepts of LMWL have an increasing trend from low to high elevations,and there are significant changes of local climate at different elevations in this study area.?2?The temperature effect of the stable isotope is most significant at the temperature below 0?.The values of?¹⁸O and?D decreased with elevation increase,while d-excess increase,the elevation gradients for annual average?¹⁸O are-0.26‰/100 m,?D are-1.77‰/100 m and d-excess are 3‰/100m,and the elevation gradients in summer half year is higher than that in winter half year.In Xiying River Basin,there is no obvious“precipitation amount effect”for all precipitation events at four sampling sites,but?¹⁸O indicated a weak“precipitation amount effect”in July and August of 2017,this is due to the influence of Asian monsoon.?3?Sub-cloud evaporation effects make the d-excess value of precipitation depleted.There is a linear correlation between the residual ratio of raindrop evaporation and the variation of d-excess,that is,the d-excess of precipitation decreases by about 0.99%with the increase of 1%of raindrop evaporation.Moreover,when the residual evaporation ratio of raindrops is higher than 60%,the correlation between them is more significant,and the slope between them decreases to 0.81.?4?Affected by Xiying Reservoir,the intensity of water vapor recycling increased,the temperature decreased,the relative humidity increased and the height of cloud bottom decreased,which weakened the Sub-cloud evaporation effect around the Xiying Wugou.The loss of raindrops in the process of falling is less,and the residual evaporation of raindrops is higher.The sudden increase of the residual evaporation ratio of raindrops in the downstream is due to the difference of the underlying surface properties.The influence of temperature and relative humidity on the evaporation residual ratio and d-excess variation are more obvious,the influence of precipitation is not obvious.In the study of sub-cloud evaporation effect in Alpine mountainous areas,setting the diameter of raindrops as a constant may lead to incorrect conclusion.?5?In Xiying River Basin,the moisture sources in winter,spring and autumn mainly comes from westerly circulation.In summer,it is also affected by moisture carried by Asian monsoon.there are some precipitation events with extremely negative?¹⁸O and d-excess values in winter,it is probably influenced by polar air masses.During continuous rainout,?¹⁸O and?D values decreased rapidly in a short time.Although there was a wide range of stable isotopic values,it still accords with Rayleigh fractionation effect.In the process of rainout,the recharge of different water vapor sources will make the stable isotope values phase change.