Spatiotemporal Variation Of Stable Hydrogen And Oxygen Isotopes In Precipitation On The Guanzhong Area

As an environmental tracer,hydrogen and oxygen stable isotopes in precipitation are very sensitive to environmental changes,and their isotopic compositions record the process of precipitation,which is an effective means to study atmospheric circulation,climate and environmental changes,and extreme events.It is also an important background information for the use of water stable isotope technology,which is of great significance for the study of water cycle mechanism,ecological hydrology and geochemical processes.In view of the lack of spatial width and time length of relevant studies in Guanzhong area,this study intends to take the precipitation in Guanzhong area as the main research object.Through large-scale and long-term continuous observation of stable hydrogen and oxygen isotopes of precipitation in Guanzhong area and hydro-meteorological information data of the same period,combined with the HSPLIT model,to carry out systematic studies on hydrogen and oxygen stable isotopes of local atmospheric precipitation,and clarify the spatiotemporal variation of hydrogen and oxygen stable isotope composition of precipitation under multi-scale and multi-scenario conditions.The main conclusions are as follows:（1）The spatial variability of hydrogen and oxygen stable isotopes of precipitation in Guanzhong region is limited,and the spatial variation coefficient（CV）at annual,seasonal and monthly scales is less than 10%,which indicates a weak variation.In contrast,the coefficient of variation in time is greater than 10%,which belongs to moderate variation.Limited space coverage,small topographic relief,and the similar vapor source of the same period are the main reasons for the small spatial variability.The temporal variability is mainly related to the ebb and flow of the Asian monsoon and the seasonal vapor source conversion.The results provide a scientific basis for the subsequent research in Guanzhong area.（2）The annual variation of hydrogen and oxygen stable isotopes in precipitation over the Changwu was mainly controlled by the Marine vapor in the rainy season,with little variation and depleted values(δ²H:-60.97±3.66‰,δ¹⁸O:-8.76±0.51‰),which reflected the background information of the Marine vapor source and vapor migration process.In the dry season,stable isotopes of hydrogen and oxygen in precipitation are not only controlled by inland water vapor masses,but also sensitive to the response of sub-cloud secondary evaporation caused by local meteorological environment during the precipitation process,and their isotopic composition variability is large and relatively enriched(δ²H:-44.33±3.61‰,δ¹⁸O:-7.44±0.45‰).The inter-annual variation of hydrogen and oxygen stable isotopes of precipitation is mainly controlled by the global scale water-vapor cycle（ENSO）.In years with frequent La Nina events,hydrogen and oxygen stable isotopes of precipitation are relatively depleted.In years with frequent El Nino events,stable isotopic compositions of hydrogen and oxygen in precipitation are relatively enriched.（3）The characteristics of hydrogen and oxygen stable isotopes in precipitation are various at different time scales,and special attention should be paid to the selection of methods when using stable isotope data of precipitation at different time scales to establish local meteoric water line（LMWL）.The LMWL from the ordinary least squares regression（OLSR）,major axis regression（MA）and reduced major axis regression（RMA）methods are significantly different based on evently data,monthly data and annual precipitation data,respectively.Only when OLSR,MA and RMA considering precipitation weighting are used,the LMWL established by these scale data is relatively close.This suggests that special attention should be paid to the selection of precipitation stable isotope data scale and regression method when LMWL is established in areas with temporal heterogeneity of precipitation and stable isotopic characteristics.For regions where stable isotope observations of precipitation are difficult and data are limited,it is recommended to use the precipitation weighted major axis regression（PWRMA）or reduced major axis regression（PWMA）methods to establish representative LMWL.（4）The stable isotopes corresponding to extreme-high autumn precipitation year（HAP;2021）,general autumn precipitation year（GAP;2015-2017,2019-2020）and extreme-low autumn precipitation year（LAP;2018）exhibit significant differences,with the LAP>GAP>HAP trend forδ²H andδ¹⁸O values,and the d-excess value and the slope and intercept of meteoric water line of autumn rain exhibiting an opposite variation trend.Although the meteorological environment factors have a certain influence on the stable isotope composition of hydrogen and oxygen in precipitation,which is represented by the precipitation effect in ordinary precipitation years and the mixed signal of secondary evaporation under clouds and inland water vapor in extremely low rainfall years,the main reason controlling the extreme variation of autumn rain is the El Nino-Southern Oscillation and the Indian Ocean dipole event.They can explain 99%and 93%of autumn rain isotopic variation respectively.These coupling phenomena affect the distance and intensity of transport of warm and humid air from low latitudes to high latitudes,and thus determine the autumn rainfall and stable isotope composition of hydrogen and oxygen.This thesis is helpful to understand the precipitation process and water vapor source in Guanzhong region,and provide accurate and reliable background information for related regional hydrological studies,so as to promote the application and development of hydrogen and oxygen stable isotope hydrologic tracer technology,and provide important scientific and technological support for sustainable development and utilization of water resources and high-quality development of ecological environment in Guanzhong region.