Characteristics Of Hydrogen And Oxygen Stable Isotopes In Precipitation And Its Moisture Sources In The Northeastern Tibetan Plateau

Stable isotopes of hydrogen and oxygen in water serve as excellent tracers for understanding water circulation processes and are key indicators in interpreting regional climate change and sustainable development.Particularly,in the northeastern Tibetan Plateau,these isotopes provide insights into the interaction between plateau and westerly weather systems.This area,being a climate transition zone with multiple moisture sources,is extremely sensitive to climate change.A comprehensive study of the characteristics of hydrogen and oxygen stable isotopes,and the moisture sources of atmospheric precipitation,in the context of global change,is crucial for understanding the spatial and temporal distribution of regional precipitation,and for promoting sustainable water resource management.This study analyzed 980 atmospheric precipitation samples(δ²H andδ¹⁸O)collected from 2012 to 2021 at the Gangcha meteorological station.Using isotope tracer technology and the HYSPLIT model,we uncovered the variations in hydrogen and oxygen stable isotopes and moisture sources over the northeastern Tibetan Plateau.Additionally,we discussed the features of deuterium excess（d）in atmospheric precipitation and the impact of isotope geography.We analyzed isotope change patterns and moisture transport processes in typical sustained precipitation events,categorizing and discussing isotope characteristics,meteoric water lines,and moisture sources in heavy and weak precipitation events.Key findings include:（1）Theδ²H andδ¹⁸O values of atmospheric precipitation over the northeastern Tibetan Plateau ranged from-222.63‰to 48.96‰and-30.54‰to 9.70‰,respectively,averaging-39.19‰and-6.17‰.Seasonal variation showed enrichment and significant fluctuation in summer and autumn,depletion in winter and spring,with the lowest value appearing in winter.The slope and intercept of local and seasonal meteoric water lines were lower than the global meteoric water line,indicating a significant influence of secondary evaporation on the precipitation process.The average d-value was 10.15‰,higher in summer and autumn,and lower in winter and spring.Annually,regional atmospheric precipitation’s hydrogen and oxygen stable isotopes had significant temperature and precipitation effects.On a seasonal scale,the temperature effect was notable in spring,the precipitation effect in summer,and both were significant in autumn.The winter results were not statistically significant due to a small sample size.（2）The primary moisture sources in the northeastern Tibetan Plateau were the westerlies,accounting for 66%,including 37%from the northwest,25%near-west wind,and 4%far-west wind.18%came from the east monsoon and local recycling,and 16%from the southwest monsoon.Spring saw a relatively simple moisture source path,dominated by westerlies.Summer’s source path was more complex,mainly from near-source westerlies,local recycling,and southeast monsoon.In autumn,the proportion of westerly moisture decreased,while north moisture and southwest monsoon increased.Winter precipitation,though infrequent,was mainly influenced by the continental air mass carried by the west wind and a few marine air masses from the Indian Ocean.Based on backward moisture source tracing and isotopic characteristics,monsoon precipitation had smallδ²H andδ¹⁸O variation ranges.In contrast,westerly precipitation had larger ranges,and the average isotope value was higher than monsoon precipitation.Northern China precipitation had smallδ²H andδ¹⁸O variation ranges,but the mean value was high.（3）During the sustained precipitation event from August 22 to 27,2019,δ²H andδ¹⁸O depleted then enriched,temperature increased then decreased,relative humidity decreased continuously,and d-values showed fluctuating trends.The meteoric water line’s slope and intercept were high,and the secondary evaporation effect was not prominent during this sustained precipitation event.Based on the backward trajectory of moisture sources,we divided the continuous precipitation process into three stages:initial,middle,and final.The initial and final stages were primarily influenced by continental air masses,while the middle stage,characterized by increasing precipitation,was mainly controlled by oceanic air masses,aligning with the Rayleigh fractional model.（4）For the heavy precipitation events（78 occurrences）,the meteoric water line equation wasδ²H=7.83δ¹⁸O+14.39,and the average d-value was 15.76‰.The secondary evaporation effect had a minimal impact on hydrogen and oxygen stable isotopes during these events.Cluster analysis of the backward trajectory showed that the majority of the moisture（56%）originated from the north,27%from the near-source westerlies,11%from the southwest monsoon,and the least（7%）from the far-source westerlies.For the weak precipitation events（773 occurrences）,the meteoric water line equation wasδ²H=7.44δ¹⁸O+5.69,with a mean d-value as low as 8.89‰.These events were more susceptible to secondary evaporation under clouds.The clustering analysis of the backward trajectory for weak precipitation events showed that combined northern moisture accounted for 50%,near-source westerly moisture for 28%,southwest monsoon moisture for 15%,and far-source westerly moisture for just 7%.The combined contribution of precipitation from westerly and local moisture exceeded 75%.Theδ²H andδ¹⁸O variation range in weak precipitation events was typically larger than in heavy precipitation events,with a mean d-value less than 10%,indicating that the moisture source of weak precipitation events was primarily from westerly winds and local moisture,with a weak influence from the monsoon.Except for the southeast monsoon moisture originating from oceanic air masses,the other four moisture sources originated mainly from continental air masses,with transport paths over land,all carrying small amounts of moisture.