Study On Precipitation Process In Monsoon Edge Region Based On Stable Isotope Technology

Atmospheric precipitation is an essential link in the water cycle,and the study of stable isotopes helps to understand the water cycle process.The Binggou River Basin of the eastern part of Qilian Mountains is located in the monsoon marginal zone,and the precipitation process is complex and changeable.Therefore,in view of this,this study established precipitation isotope monitoring points in the Binggou River Basin.According to the precipitation isotope data of January^December2018 and the reanalysis data of 2018 provided by NCEP,the basic variation characteristics of precipitation stable isotopes in the basin were systematically analyzed,and the water vapor transport process and the secondary evaporation under the cloud were studied by combining the backward trajectory model and the raindrop evaporation model.The results show that:?1?The variation of?D and?¹⁸O values in the daily scale descending water of Binggou River basin is very synchronous,which has obvious characteristics of high winter and low summer,and the coefficient of variation shows that the stable isotope fluctuation in summer and autumn is larger than that in spring and winter.the d-excess value of most precipitation in summer half year is less than 10‰;,while in winter half year,the d-excess value of most precipitation is more than 10‰.Both local evaporation and water vapor affect the d-excess value distribution in the study area.The atmospheric precipitation isotopes in the study area are mainly affected by the temperature effect.Although the precipitation effect in the study area is not reflected in the whole year,it makes the warm season show a weak precipitation effect under the influence of monsoon water vapor.?2?The meteoric water line of the Binggou River basin at event scale is?D=7.79?¹⁸O+10.47.Under the influence of dry climate,the slope of the local meteoric water Line is lower than that of the global meteoric water Line.The obvious gap between the theory and the actual slope in each season of the meteoric water line in the Binggou River basin can explain to some extent that the precipitation process in the region has experienced the secondary evaporation under the cloud;The variable local climatic conditions,different precipitation forms and different water vapor sources are all the factors that affect the seasonal variation of meteoric water line in the study area.At the height of 300 hPa in the study area,the water vapor flux streamlines basically flow from west to east.?3?There are great differences in the circulation system which control the water vapor source of precipitation in different seasons,among which,the westerly zone is the water vapor source that dominates the perennial precipitation in the Binggou River basin,the local recirculation water vapor also occupies a large proportion,and the southwest monsoon water vapor and the northern polar water vapor also some contributions.In summer,the precipitation caused by westerly circulation is the highest isotope value,and the precipitation caused by monsoon circulation is the lowest;the isotopic value brought by water vapor from the north is higher than that from the inland and southern monsoon water vapor,which may be related to the temperature when the water vapor is formed.Evaporation water vapor from lakes,rivers and other water bodies in dry environment brings higher d-excess value precipitation to the study area;westerly water vapor transport brings lower isotope value precipitation to the study area in winter,while water vapor from the north brings higher isotope value precipitation to the study area.?4?The variation of raindrop diameter in the study area was between 0.4mm¹.2 mm,in which the proportion of precipitation events with raindrop diameter small equal to 1 mm in the total precipitation events was as high as 91.4%.The raindrop end velocity was distributed in the range 0.74¹3.87 m/s,of which the precipitation events between 2 and 6 m/s reached 32 times,accounting for 55.2%of all precipitation times.The maximum value of evaporation rate appears in summer half year,the evaporation residual ratio is smaller than the whole,the minimum value appears in winter half year,and the evaporation residual ratio is generally larger.The?d values at the monthly scale in the study area are relatively large in may-august,and the variation in the remaining months is very small.the appearance of evaporation residual ratio?f?and?d in precipitation is significantly linear,that is,at the evaporation residual ratio greater than 92%,the?d value of1.07‰/%will be reduced in precipitation for each increase of 1%evaporation.