Research On The Source Of Water Vapor In The 4 Northwestern Provinces Based On Stable Isotope Of Precipitation

Hydrogen-oxygen stable isotope is an important component of natural water.Although the content is very low,it can respond to the changes of natural environment sensitively,and contains historical information of water cycle evolution.The analysis of stable isotope composition and ratio of precipitation,as the fundamental source of terrestrial water resources,is helpful to distinguish the water vapor sources,reflect the process of atmospheric water vapor cycle.At the same time,it can indicate regional characteristic and the climate to a certain degree.The study area is located in the northwest of China except Qinghai Province,which is far away from the ocean and the climate is dominated by drought.Therefore,the precipitation has become an important source of water resources in this region.In this paper,mainly analyzes the correlation between ?D and ?18O in precipitation and the spatial and temporal distribution of precipitation ?18O and d values,and analyzes environmental effects of stable isotopes in precipitation at 10 stations(8 from GNIP and 2 from field sampling)in Northwest China.And the variation of water vapor sources in the study area is discussed,which is of positive significance to understand the hydrological cycle in the study area.The slope and intercept of the Local Meteoric Water Line(LMWL)in each province of the study area were lower than them of the Global Meteoric Water Line.It indicates that the climate of the study area is relatively dry,and the precipitation has experienced a certain degree of evaporation.By comparing the precipitation line equations in different periods,the intercept and slope(-0.07,6.80)of the atmospheric precipitation line in the summer monsoon period was significantly lower than that of annual mean(3.37,7.27),indicating that the secondary evaporation was stronger during the summer monsoon.The intercept and slope(6.07,7.46)of the atmospheric precipitation line during the winter monsoon period was higer,mainly because the temperature during the winter wind period in the study area is lower,which restrains the evaporation to some extent.The ?18O value in precipitation at 10 stations was enriched in summer and autumn,poor in winter and spring,which is consistent with the seasonal variation of temperature,indicating that the change of precipitation ?18O is closely related to temperature in the study area.The spatial variation of precipitation ?18O in the whole study area is chaotic.However,it was found from the discussion of shannang-ningsia region and xinjiang region that the spatial trend of the Shaanxi-Gansu-Ningxia region decreases from northwest to southeast,and that of the Xinjiang region decreases from Hetian to Urumqi.Analyzing the spatial variation of ?18O and d in different periods,we can see that during the summer monsoon,the ?18O and d values decreased from south to north in Xinjiang,while the ?18O value increased but d values decreased from south to north and from east to west of Shaanxi-Gansu-Ningxia region.During the winter monsoon,the ?18O value decreased from east to west in whole Northwest region,while the d value variation is different.The temperature effect is significant in all the four provinces in Northwest China,but there no precipitation effect existed.It is indicated that the temperature effect is the main factor controlling the variation of precipitation ?18O in the study area.The precipitation effect in the study area was analyzed seasonally,and the results showed that the precipitation effect was significant in summer.It shows that the water vapor source of summer precipitation in Northwest China is affected by the oceanic monsoon to some extent,and the effect of summer precipitation in the study area cannot be ignored.The quantitative relation of elevation effect is-0.05‰/100m in Northwest China.The secondary evapora-tive effect of northwestern China was quantitatively discussed by using the water droplet evaporation model.The evaporation ratio in the summer monsoon was 4.49%,which was higher than 3.65%in the winter monsoon.There are differences in evaporation rates at different sites during different monsoon periods,which is closely related to the geographic location and climatic conditions of each station.Various meteorological factors will affect the below-cloud secondary evaporation,However,the influence degree of each factor on secondary evaporation varies in different ranges.With the increase of temperature,the decrease of relative humidity and the decrease of water vapor pressure,the degree of evaporation experienced by precipitation in the study area is intensified.The influence of precipitation on secondary evaporation is different in different ranges.The water vapor sources in the Shaanxi-Gansu-Ningxia region during the summer half year mainly include water vapor from the southern Bay of Bengal,southeast monsoon water vapor and the westerly water vapor.The temperature in summer half year is higher and drier,and a certain amount of local reevaporation water vapor is mixed in,which makes the value of ?18O higher in the summer half year.Water vapor in this westerly zone is the primary source of water vapor in the winter half year,so the precipitation 1lVO during the winter half year is depleted.The main sources of water vapor in Xinjiang are water vapor in the northwest of Siberia,the water vapor transport from the Bay of Bengal to southern Xinjiang and the westerly vapor transport in the Atlantic Ocean,mainly with westerly water vapor transport.This paper quantifies the effect of below-cloud secondary evaporation in the study area,discusses the basis of the effect of it,and proposes the sensitive factor of the secondary evaporation effect,in order to provide relevant information for the study of the water cycle model in northwestern China,and provide basic data for integrated water resources management in this area.