Study On Water Vapor Sources And Sub-cloud Secondary Evaporation Effect Of Precipitation In Liaocheng Based On Hydrogen And Oxygen Isotopes

As a basic supply source of regional surface water,groundwater,mountain glacier,snow and other water bodies,precipitation is one of the indispensable links in the water cycle system.As natural tracers of water cycle,hydrogen and oxygen stable isotopes D and¹⁸O are helpful for people to understand the process of geochemistry and hydrological cycle deeply.Taking Liaocheng as the research area,this paper analyzes the temporal variation characteristics of hydrogen and oxygen stable isotopes of precipitation in Liaocheng and discusses its influencing factors based on the measured data of precipitation isotopes and the corresponding meteorological data.Combined with the HYSPLIT backward trajectory model,the water vapor source of precipitation was tracked,and the influencing factors of sub-cloud secondary evaporation and the quantitative calculation of sub-cloud secondary evaporation rate were explored.The results showed that:(1)The changes of?D and?¹⁸O in precipitation in Liaocheng have a good synchronicity with time,and the seasonal changes of?D and?¹⁸O are obvious.The variation of?D and?¹⁸O in winter and spring is more significant than that in summer and autumn,which may be mainly affected by temperature and water vapor sources.The local meteoric water line is?D=7.85?¹⁸O+9.79(R²=0.921,p<0.01),and the intercept and slope are slightly lower than the global precipitation line,which indicates that the precipitation in Liaocheng is affected by evaporation to a certain extent.The slope and intercept of the local meteoric water line in spring,summer and autumn are all lower than that of the global meteoric water line,while the slope and intercept of the local meteoric water line in winter are obviously higher than that of the global meteoric water line.The stable isotopes of precipitation have a significant temperature effect in winter,and the rainfall effect is reflected in the annual scale,summer and autumn.The?¹⁸O of precipitation is negatively correlated with water vapor pressure.The fluctuation of excessive deuterium in meteoric precipitation is great,which indicates that there are great differences in the water vapor source of precipitation and the climate environment at the time of precipitation occurrence.The weighted average value of excess deuterium is slightly higher than the global average,indicating that the main water vapor in the precipitation in Liaocheng is continental water vapor.(2)In the whole year,the water vapor sources of the precipitation in Liaocheng can be divided into five types:ocean type,inland and westerly type,local circulation type,inland and ocean mixed type,and inland and local mixed type.The?¹⁸O and?D values of precipitation formed by water vapor from westerly and inland areas are higher,while the?¹⁸O and?D values of precipitation formed by water vapor from ocean are lower.The seasonal variation of water vapor source of precipitation is obvious.The?¹⁸O value of spring precipitation is generally high because of the dominant continental air mass in spring.The main source of water vapor in summer precipitation is oceanic water vapor,and the?¹⁸O value in precipitation is mainly affected by water vapor transport distance and precipitation effect.Water vapor formed by surface water evaporation is an important source of water vapor in autumn precipitation,leading to high?¹⁸O value in autumn precipitation.In winter,the water vapor transport distance is longer,and the temperature is lower in winter,and the evaporation is weaker,so the?¹⁸O value in precipitation is relatively low.(3)The influencing factors of sub-cloud secondary evaporation in Liaocheng include temperature,relative humidity and rainfall,among which temperature has the most significant influence on sub-cloud secondary evaporation.When the temperature is higher,the precipitation is smaller or the relative humidity is lower,the slope of the local meteoric water line and d-excess value are usually lower,and the?¹⁸O value is usually higher,that is,the sub-cloud secondary evaporation is more intense.The sub-cloud secondary evaporation rate ranged from 0.24%to 5.19%,with significant seasonal differences,showing that autumn>spring>summer>winter.