Recycled Moisture Ratio In The Kashgar Oasis Of Xinjiang Based On Stable Hydrogen And Oxygen Isotopes

As an important part of water cycle,precipitation is the result of water vapor transport and phase change.The local recycled moisture(including surface evaporation and plant transpiration)is considered as the vital component of precipitation sources,and plays an important role in the terrestrial hydrological cycle,especially in the widely distributed oases across the arid region of northwest China.Quantitative assessment of the proportional contribution of recycled moisture to local precipitation(i.e.,recycled moisture ratio)in the arid region of northwest China is useful to understand the hydro-ecological processes and land-air interactions in an arid setting,and is also helpful to investigate the anthropogenic impact of oasis development on the regional water cycle.Therefore,the Kashgar River Delta Oasis(Kashgar Oasis,for short)in the western Tarim Basin of Xinjiang was selected as the study region,and the basic characteristics of stable hydrogen and oxygen isotopes in precipitation collected from April 2018 to June 2020 were analyzed;the moisture recycling in this typical oasis was assessed using a three-component isotopic mixing model,and the influences of different algorithms on the calculated recycled moisture ratio were analyzed.The results of the study showed that:(1)The annual mean values of precipitation?¹⁸O at each station in the study region ranged from-13.56‰to-1.46‰,and the?²H values ranged from-98.90‰to-4.79‰,with enrichment in summer months and depletion in winter months.The seasonal variation of precipitation isotope values can be fitted using a cosine curve.The slope of meteoric water line obtained in this study is lower than that of the global meteoric water line,both on the event-based scale and on the monthly scale;the slope of meteoric water line is usually lower in the summer months and higher in the winter months.In addition to the temperature effect,the precipitation?¹⁸O values also correlate with the altitude with a linear gradient of-0.35‰/100m.Generally,the precipitation isotope values are connected to the meridional circulation factor,while the correlation with the latitudinal circulation factor is much weak.According to the simulations of an isotope-equipped general circulation model,in the upwind region adjacent to the west(east of 60°E),the precipitation?¹⁸O values exhibit a strong positive correlation with the meridional(southerly)wind speed,and the linkage with the latitudinal(westerly)wind speed is relatively weak;while near 30°N,the latitudinal(westerly)wind speed shows a significant positive correlation with the precipitation?¹⁸O values.(2)Different algorithms for isotopic values of each water vapor source were compared,and the isotopic values of these water vapor fluxes(i.e.,precipitation,advection,surface evaporation,and plant transpiration)required in the three-component mixing model were determined.In the study area,the isotopic non-equilibrium fractionation method adjusted using the slope of local evaporation lines and meteorological parameters(e.g.,air temperature and relative humidity)can be used to calculate the isotopic values in precipitating water vapor.The single station was selected to calculate the advected vapor isotopic values from the upwind region,and the weighted average isotopic values in precipitation can be used as the transpiration vapor isotopic values.The isotopic values of surface evaporation vapor were calculated based on the Craig-Gordon model using precipitation as evaporation front water.(3)The average local recycled moisture ratio in the Kashgar Oasis from May to September ranged between 20.8%and 33.8%;the average contribution from transpiration ranged from 12.3%to 32.3%,and the contribution from surface evaporation ranged from 1.1%to 8.6%.The proportional contribution of plant transpiration to precipitation is greater than that of surface evaporation.The local recycled moisture ratio presents obvious seasonal variations,and the contribution of transpiration and surface evaporation both show a decreasing and then increasing trend from May to September.Comparing the impacts of different algorithms on the local recycled moisture ratio,the ratio calibrated using the sub-cloud evaporation setting is lower than that calibrated using the non-equilibrium fractionation assumption.