Isotopic Composition Characteristics And Water Vapor Sources Of Atmospheric Precipitation In The Central Loess Plateau

Precipitation is a key link in the water cycle process,determining the total amount and distribution characteristics of regional water resources.Therefore,understanding the source of precipitation water vapor is the fundamental work for hydrological process analysis and water resource management.The water resources on the Loess Plateau are inherently scarce,and in recent years,various hydrological variations have emerged that characterize the reduction of water resources.In this context,it is particularly necessary to clarify the source of precipitation water vapor for more efficient and sustainable water resource management.This article takes Dingbian County,Xifeng District,Changwu County,and Yangling District in the central part of the Loess Plateau as the research area.Using measured precipitation isotope data and meteorological data（temperature,precipitation,relative humidity）,the seasonal and spatial variations of atmospheric precipitation isotope composition and their influencing factors are analyzed.The second-order variables d-excess,¹⁷O-excess,and HYSPLIT backward trajectory model of hydrogen and oxygen stable isotopes are combined to trace the source of water vapor in atmospheric precipitation,Furthermore,the Stewart raindrop evaporation model and two-phase mixture model were used to quantify the intensity of secondary evaporation under clouds and the proportion of local recycled water vapor.The main results achieved are as follows:（1）There are significant seasonal variations and regional differences in stable isotopes of precipitation.In precipitationδ¹⁸O、δ²H the range of variation is-22.8‰to 3.1‰and-169.9‰to 25.6‰,respectively.The isotopic composition of precipitation shows seasonal variations,and precipitation at four locations the seasonal variation trend ofδ¹⁸O is similar,with the maximum value mostly appearing in May and the minimum value mostly appearing in August;δ¹⁸O exhibits a characteristic of enrichment during the dry season and dilution during the rainy season.There is spatial variation in isotopic composition and precipitation in Dingbian the most abundantδ¹⁸O,precipitation in Changwuδ¹⁸O is the most depleted,showing a trend of gradual enrichment of isotopes from south to north.The atmospheric waterline（LMWL）in the study area isδ²H=7.41δ¹⁸O+6.23.The slope of LMWL at different points of is also smaller than that of GMWL,with Changwu LMWL having the highest slope and intercept（7.86 and 16.28）,while the slope and intercept of the fixed edge waterline are the lowest（7.04 and 2.44）.（2）Precipitation isotopes are influenced by different environmental factors in different seasons and regions.Precipitation in the research areaδ¹⁸O did not show a temperature effect,but showed a significant rainfall effect.This may be related to the distribution pattern of precipitation,and in addition,there are many precipitation events in the region formed by convective water vapor,resulting in relatively depleted precipitation isotopes.Among the four points,the secondary evaporation under clouds in the dry season is stronger than that in the rainy season.The influence of temperature and precipitation on precipitation isotope composition gradually weakens from south to north,while the influence of relative humidity on precipitation isotope composition gradually strengthens from south to north.（3）The source of water vapor in atmospheric precipitation is relatively complex,with significant seasonal and regional differences.The variation range of d-excess values in atmospheric precipitation is relatively large,with an average value greater than 10‰,indicating that the source of water vapor is relatively complex and mainly formed by rapid evaporation of water bodies at low relative humidity.The gradual decrease of d-excess from south to north indicates differences in the dominant process.For example,Dingbian may be related to strong secondary evaporation under clouds,while Changwu is mainly related to the participation of a large amount of recycled water vapor.The d-excess is the largest in the dry season and the smallest in the rainy season.The dry season precipitation water vapor is mainly inland water vapor such as the westerly belt and the Arctic Ocean water vapor.The rainy season precipitation water vapor is influenced by the Indian Ocean water vapor brought by the southwest monsoon,the Western Pacific water vapor brought by the southeast monsoon,and the North Atlantic water vapor in the northwest direction.In addition to external water vapor,local recycled water vapor is also an important source of regional precipitation;The maximum proportion of recycled water vapor in Dingbian is 25%,and the proportion of recycled water vapor in dry season is greater than that in rainy season.The atmospheric drawdown line of the three oxygen isotopes indicates that,except for the Changwu water vapor source area,which was formed in a steady state with high relative humidity,and the precipitation isotopes were subjected to weak dynamic fractionation,the water vapor source areas of Dingbian,Xifeng,and Yangling were all formed in a non steady state with low relative humidity,and precipitation isotopes underwent strong dynamic fractionation.By analyzing the isotopic precipitation effect and temperature effect in the central part of the Loess Plateau,as well as tracing the source of water vapor,quantifying the intensity of local recirculating water vapor and secondary evaporation under clouds,the controlling factors and sources of precipitation isotopes in the region have been clarified,providing a reference basis for further understanding the water cycle process and water resource allocation in the northwest inland region.