Sources Of Moisture And Its Effect On Stable Isotopes Of Precipitation In The Shiyang River Basin

Atmospheric precipitation is an important part of the water cycle,and the convergence and condensation of moisture are necessary for the formation of precipitation,so it is important to clarify the sources of moisture to study regional hydrological changes.Stable isotopes of hydrogen（δ²H）and oxygen(δ¹⁸O)in precipitation are sensitive to changes in regional environment,and they can be natural tracers of the whole process of precipitation.The changes of the sources of moisture are important factors affecting the isotopes of precipitation.The Shiyang River basin is one of the typical inland river basins in arid area of the northwest China,for which the precipitation is an important item of water recharge.It is important to clarify the sources of moisture and its effect on the stable isotopes of precipitation for the understanding of the water cycle.In this study,the spatial and temporal characteristics of precipitation stable isotopes,moisture sources and the influence of moisture sources on precipitation isotopes were analyzed by using data of precipitation isotopes collected at 11precipitation sampling sites,National Centers for Environmental Prediction/National Center for Atmospheric Research（NCEP/NCAR）reanalysis meteorological data with the help of Hybrid Single-Particle Lagrangian Integrated Trajectory model（HYSPLIT）and concentration weighted trajectory（CWT）analysis in the Shiyang River basin.The following main conclusions were drawn:（1）Theδ²H andδ¹⁸O of precipitation showed an increasing trend from 2017 to2019,while the d-excess appeared the characteristic of“high-low-high”.Theδ²H andδ¹⁸O in precipitation showed significant seasonal changes with higher in summer and autumn,and lower in winter and spring,and the d-excess values showed seasonal changes with higher in spring and autumn,and lower in summer and winter.From the upstream area to the middle and downstream area,theδ²H andδ¹⁸O in precipitation showed a gradual increase trend,but the d-excess showed a gradual decrease trend.（2）Moisture of the Shiyang River basin mainly came from the continent,with a proportion of 72%.It was divided into four subgroups,including Central Asia（30%）,Mongolia-Siberia（24%）,Europe（11%）and inland China（7%）.The proportion of oceanic sources was 28%,that was divided into 5 subgroups including Atlantic Ocean（11%）,Pacific Ocean（7%）,Arctic Ocean（5%）,Mediterranean Sea（4%）and Indian Ocean（1%）.（3）In spring and winter,westerly moisture from the Atlantic Ocean,Europe,the Central Asian and the Mediterranean Sea was the key to determining the moisture content over the basin.In summer,monsoon moisture from the Pacific and Indian Oceans was the key to determining the moisture content over the basin.In autumn,northerly moisture from Mongolia-Siberia was the key to determining the moisture content over the basin.（4）Compared with continental sources,theδ¹⁸O in precipitation from oceanic sources was depleted.The significant depletion ofδ¹⁸O in precipitation whose moisture mainly comes from Atlantic Ocean（-13.9‰）and Indian Ocean was attributed to the increase of Rayleigh fractionation for longer transport distances.The enrichment ofδ¹⁸O in precipitation coming from continental sources was influenced by local surface evapotranspiration,such as precipitation,which is dominated by moisture from inland China（-3.9‰）,Central Asia（-7.7‰）,Europe（-7.8‰）and Mongolia-Siberia（-9.2‰）in summer and from Central Asia（-7.4‰）and Mongolia-Siberia（-9.2‰）in autumn.（5）In spring,δ¹⁸O was lower when the contribution of continent-sourced moisture from Central Asia,Mongolia-Siberia,and Europe was higher,and it was higher when the contribution of sea-sourced moisture from the Mediterranean Sea,Atlantic Ocean,and Arctic Ocean was higher.In summer,δ¹⁸O was lower when the contribution of distant moisture from the Indian Ocean,Pacific Ocean,Arctic Ocean and Atlantic Ocean was higher,and it was higher when the contribution of moisture from Europe,Central Asia,the Mediterranean Sea and inland China was higher.In autumn,δ¹⁸O was lower when the moisture contribution from higher latitude sources such as the Atlantic Ocean,Arctic Ocean,and Mongolia-Siberia was higher,and it was higher when the moisture contribution from lower latitude sources such as Central Asia,Europe,and the Mediterranean Sea was higher.In winter,δ¹⁸O was lower when the contribution of westerly moisture from the Atlantic Ocean and Central Asia was higher,and it was higher when the contribution of northerly moisture from Mongolia-Siberia was higher.