Spatial Distribution Of Stable Isotopes Of Precipitation And Analysis Of Water Vapor Sources In The Loess Plateau

Precipitation is a vital link in the hydrological cycle.Its stable isotope of hydrogen and oxygen is very sensitive to environmental response,and it is an important carrier to study the regional water cycle.The Loess Plateau is an important base for grain production in China.The ecological environment is extremely fragile.To improve the regional ecological environment,the country has implemented a large-scale conversion of farmland to forests and grasslands in the Loess Plateau.Therefore,clarifying the stable isotope characteristics of precipitation in the Loess Plateau is of great significance for evaluating regional water cycle studies.In this paper,based on the precipitation?¹⁸O data from 45 precipitation stations on the Loess Plateau,combined with the BW isotope prediction model,a high-precision spatial distribution map of the stable oxygen isotope of precipitation in the Loess Plateau was generated.Then,according to the stable isotopes of hydrogen and oxygen,the fitting method of the local atmospheric precipitation lines in different climate regions of the Loess Plateau under the control of different air masses was determined.Finally,from the data of typical precipitation monitoring stations,the characteristics of the spatiotemporal variation of the stable isotope of precipitation in the Loess Plateau and its environmental effects were analyzed,and the process of precipitation water vapor transport in the Loess Plateau was clarified.The main results are as follows:?1?Based on the ?¹⁸O data of 46 precipitation monitoring stations on the Loess Plateau and the BW regression model,the quantitative relationship between the stable isotope?¹⁸O of precipitation and latitude and altitude is obtained by stepwise regression:d¹⁸O?28?-0.009Lat²+0.427Lat-0.001Alt-11.321.In the Guanzhong Plain area in the south of the Loess Plateau and the Linfen area in the southeast of the Loess Plateau,which are greatly affected by the monsoon and relatively low in altitude,the residuals of the model are very small?between-0.7‰and-0.4‰?,indicating that the regional isotope model can simulate the actual isotope value more accurately.The large residual error of the model in the western region?-2.4‰?-1.6‰?shows that the precipitation?18O in this region is affected by other local factors in addition to the latitude effect and altitude.Similarly,the precipitation isotope of the Loess Plateau gradually depleted from southeast to northwest.Due to the large number of lakes in the Yinchuan Plain area,the precipitation?18O is more abundant than in the surrounding area.The precipitation of?¹⁸O in Yuzhong and Xining areas in the west is extremely depleted.The main reason is that these two areas have high altitudes.When the precipitation water vapor passes through high mountains or plateaus,the water vapor arriving at the monitoring station is very poor.In summary,the?¹⁸O is enriched in the low-altitude plain area,the isotope latitude effect and altitude effect are obvious in high latitude and high altitude areas,and the isotope is depleted.This study improves the precision of the spatial distribution of precipitation isotopes in the Loess Plateau and provides a new platform for understanding the spatial variability of precipitation isotopes in the Loess Plateau.?2?Influenced by the intensity of secondary evaporation under the cloud,the slope and intercept of the LMWL in the semi-humid area are larger than those in the semi-arid area and arid area.Compared with the precipitation event,the monthly precipitation data of each climate zone using OLSR increases the slope and intercept of the LMWL,indicating that the even-based data weighted by rainfall to monthly precipitation data weakens the impact of small precipitation on the LMWL.When the LMWL was established based on event-based or monthly precipitation data,there was no significant difference between the slope of OLSR and the slope of PWRMA in the semi-humid area?P=563.4 mm yr-1?.In the semi-arid area?P=451.9 mm yr-1?,the OLSR slope and the PWRMA slope of the secondary precipitation data are significantly different,while the OLSR slope and the PWRMA slope of the monthly precipitation data are not significantly different.In the arid region?P=166.3 mm yr-1?,the OLSR slope of the event-based and monthly precipitation data are significantly different from the PWRMA results.The above results indicate that whether to use rainfall weighting should be considered according to climatic conditions when establishing LMWL.The LMWL obtained by OLSR based on event-based data in the semi-humid area of the Loess Plateau is:d²H?28?7.5d¹⁸O+7.5,and in semi-arid region based on monthly precipitation data by OLSR is:d²H?28?7.4d¹⁸O+7.4,and the LMWL obtained by PWRMA for precipitation event data in arid regions is:d²H?28?7.3d¹⁸O+4.9.This study provides reliable LMWL for the study of isotope ecological hydrology on the Loess Plateau,and at the same time clarifies the effect of sub-precipitation rainfall on the establishment of LMWL in different climate regions.?3?There are obvious seasonal changes in the stable isotopes of hydrogen and oxygen in precipitation.The isotope composition of semi-humid areas is depleted during the rainy season?May to October?and enriched during the dry season?November to April?,while the semi-arid and arid regions are enriched during the rainy season and depleted during the dry season.Comprehensive analysis of?¹⁸O,d-excess,and ¹⁷O-excess confirmed that the precipitation in study area is jointly contributed by the warm and humid air mass from the southeast monsoon and the dry and cold air mass from the westerly wind.Among them,approximate 55%?79%of precipitation water vapor in the semi-humid area comes from the ocean,mainly falling in June to August,and about 21%?45%of the water vapor comes from inland and local evaporation,mainly from October to April.The water vapor sources of precipitation in May and September are complex and may intermittently originate from ocean water vapor and inland water vapor.About 26%to 52%of precipitation water vapor comes from the ocean in semi-arid areas,mainly in June,and 48%to 74%of water vapor comes from inland and local evaporation,mainly concentrated from August to May.In July,the source of precipitation water vapor is complex and may be jointly supplied by ocean water vapor and inland water vapor.About 17%to 29%of precipitation water vapor in arid areas originated from the ocean,mainly concentrated in August,and 71%to 83%of water vapor originated from inland and local evaporation,mainly concentrated from September to May.The source of water vapor from June to July is complex.The reason may be that the precipitation water vapor in these months is jointly supplied by the water vapor of the ocean and the inland air mass.