Effects Of Moisture Sources And Weather System On Precipitation Isotope In Minxian,Gansu Province

The stable isotopes ¹⁸O and ²H（D）,as components of water molecules,are good vehicles for water cycle studies.Global warming has strongly modified the water cycle processes at different scales.Precipitation as the most important link in the water cycle process,carrying out the study of precipitation isotopes can help to deeply understand the water cycle process.Gansu Province is located at the intersection of three integrated natural regions in China,and its unique geographical location has a profound impact on many aspects of the natural characteristics of Gansu Province.Long-term studies on precipitation isotopes in Gansu Province have mostly focused on its central and western regions,and the study of precipitation isotopes in southern Gansu Province needs to be deepened.Therefore,this study systematically analyzed the basic characteristics of precipitation isotopes in Minxian based on 188 precipitation event samples collected in Minxian from April 2019 to October 2021,and explored the influence of water vapor sources and weather systems on precipitation isotopes.The main conclusions of this study are as follows:（1）Precipitationδ¹⁸O,δ²H,and d-excess in Minxian show obvious seasonal variations,with precipitationδ¹⁸O andδ²H enriched in the summer half year and depleted in the winter half year,and d-excess values lower in the summer half year and higher in the winter half year.The local atmospheric precipitation line（LMWL）of Minxian isδ²H=7.72δ¹⁸O+12.84（R²=0.98,p<0.01,n=27）,whose slope is lower than the global atmospheric water line（GMWL）and the intercept is higher than the GMWL.Minxian precipitation isotopes show a strong temperature effect,but there are differences in the temperature effect under different temperature conditions;in addition,Minxian precipitation isotopes show a certain humidity effect,but the precipitation effect of the isotopes is not obvious.（2）The source of water vapor and water vapor transport time in Minxian vary greatly in different seasons.Water vapor in the summer half year mainly comes from the middle and lower reaches of the Yangtze River plain,the Sichuan basin and northern Xinjiang,etc.,and the water vapor transport time takes about 5 days on average;in the winter half year water vapor mainly comes from Xinjiang and Inner Mongolia,etc.,and the water vapor transport time takes about 3.5 days on average.The isotope values of precipitation caused by different water vapor sources are different.The isotopic values of precipitation caused by water vapor from the northwest and the south in the summer half year are more enriched,while the isotopic values of precipitation caused by water vapor from the northeast and the east are more depleted;the isotopic values of precipitation caused by water vapor from the west and the northeast in the winter half year are more depleted,while the isotopic values of precipitation caused by water vapor from the east are more enriched.In addition,precipitationδ¹⁸O shows a tendency to depletion with longer water vapor transport time,but this tendency is not obvious in the winter half year;precipitationδ¹⁸O shows a positive correlation with water vapor transport height,and this relationship is stronger in the winter half year.（3）There are seasonal variations in the potential evaporation source areas that generate local recycled vapor.During the summer half year,they are mainly concentrated in eastern Gansu Province,northeastern Sichuan Province,and Hanzhong,Shaanxi Province,and during the winter half year,they are mainly distributed in southeastern Gansu Province and some areas in southern Shaanxi Province.Comparing different methods to determine potential evaporation source areas,it is reasonable to use precipitation water vapor isotope values calculated by the summary of equilibrium fractionation and kinetic fractionation to bring the potential evaporation source areas determined by CWT（Concentration Weighted Trajectory）model.The contribution of local recycled vapor to precipitation in Minxian was further evaluated by superimposing the potential evaporation source area and the backward trajectory after the specific humidity correction,and the contribution of local recycled vapor was about 32.17%in the summer half year and26.05%in the winter half year.（4）The weather systems affecting precipitation in Minxian are mainly westerly trough,shear line and low vortex.The precipitation caused by different weather systems has different isotopic characteristics.Precipitation caused by low vortex and shear line systems has depleted isotope values and obvious temperature effects;precipitation caused by westerly trough systems has enriched isotope values and weak temperature effects,but there is a certain humidity effect.The differences in precipitation types due to different weather systems are the reason for the differences in precipitation isotope expressions.Precipitation due to low vortex and shear systems has a more convective nature,higher condensation height of water vapor,and lower temperature at condensation,so its isotopic composition is more depleted;precipitation due to westerly trough systems has a weaker convective nature,lower condensation height of water vapor,and higher temperature at condensation,so its isotopic composition is more enriched...