Analysis Of Water Sources And Confluence Pathways Based On Environmental Tracers In Rainy Season In Alpine Regions Of Naqu River Basin

In recent years,due to the influence of climate change,the glaciers on the Qinghai-Tibet Plateau(as the "Asian water tower")have retreated,the snow line has risen,the permafrost has been seriously degraded,and the formation mechanism of runoff has become more difficult to study,which poses a more severe challenge to the comprehensive utilization of local water resources and environmental protection.In the Qinghai-Tibet Plateau,seasonal frozen soil and permafrost coexist,which leads to the diversity of runoff sources during rainy season,the spatial combination characteristics and the water pathways are very complex.At present,the research on the mechanism and law of water cycle in this area is still not in-depth,and there is no systematic theory and application results.The composition of runoff sources and the confluence path of water sources are two indispensable aspects of basin water cycle.In this paper,the Naqu River Basin was taken as the research area.Based on the tracing method of water environment tracer,and aiming at the actual characteristics of plateau area,the "error structure" of Bayesian mixing model was improved,and the composition of runoff sources in Naqu River Basin was analyzed qualitatively and quantitatively.The runoff sources pathways during rainy season were analyzed by using reactive solute tracer,and the runoff sources in Alpine basin were constructed by combining temporal and spatial sources of runoff and supporting qualitative and quantitative water sources.The analysis system reveals the formation mechanism of runoff in rainy season in Naqu basin,which provides a solid basis for water resources utilization and environmental protection in the Naqu River Basin.The main achievements are as follows:(1)In view of the difficulty in identifying the water source composition caused by the lack of basic data in the basin of the Qinghai-Tibet Plateau,the method of mutual verification and verification of stable isotope and hydrochemical tracer was used to qualitatively identify the runoff source composition in different periods of rainy season.The Local Meteoric Water Line(LMWL)is ?2H=7.6?18O+3.4,which shows that the water vapor of summer precipitation comes from the evaporation of water under humid conditions,which is brought by the southwest monsoon.During the rainy season,the isotopic and hydrochemical characteristics of the river water are characterized by the mixed supply of ice and snow melt water,rainfall and groundwater.At the beginning of the rainy season(May to June),the runoff is replenished by snowmelt;in the middle of the rainy season(June to August),with the significant increase of the monsoon precipitation,the rainwater gradually replaces the snowmelt and occupies a dominant position in the contribution of the runoff during the rainy season;by the end of the rainy season(August to October),the runoff supply by rainfall and snowmelt decreases,and the groundwater supply increases.(2)In view of the uncertainty of runoff segmentation results of traditional Bayesian mixing model,based on the actual characteristics of plateau frozen soil runoff,the Bayesian mixing model is improved.Considering the advantages of Bayesian mixing model in runoff segmentation,this scheme was used as the main method of runoff quantitative segmentation.The "process error" in the "error structure" of the model was multiplied by the "multiplication error term" to realize the "Scaling" function,which maked the segmentation result better controlled in the "95%confidence interval width" and "average absolute error";and different"covariants" were set in the improved model to quantitatively compare the impact of different covariates on the runoff source segmentation ratio.(3)The runoff sources in different periods of rainy season and different height vegetation types in Naqu basin were quantitatively segmented.The improved Bayesian mixing model was used to quantitatively analyze the runoff sources of Naqu River Basin in different periods and different height vegetation types during rainy season:at the beginning of the rainy season,the main water source of runoff is base flow,accounting for 86.9%;in the middle of rainy season,snowmelt and rainfall are the main water sources of runoff,accounting for 37%and 43.4%respectively;at the end of rainy season,the main water sources come from base flow and snowmelt,accounting for 85.9%and 14.1%respectively.At the same time,in the middle of the rainy season,the contribution ratio of runoff sources of different vegetation types is roughly the same.The reason is that the altitude gradient is not large and the vertical zonation of hydrological underlying surface is not obvious.Compared with similar research results,the results of runoff quantitative segmentation in this paper are more reasonable.(4)The hydrochemical tracer method was used to analyze the runoff sources pathways in different periods of the rainy season in Naqu basin under the influence of frozen soil thawing.The results show that in the early rainy season,the infiltration of snowmelt is limited by the unfreezing active layer and the permafrost below and the soil moisture content is maintained at a low level of 9.5%,and the snowmelt directly flows into the river channel in the form of slope flow;in the middle of the rainy season,the soil moisture content gradually increases to 12.5%and the soil surface near the surface and above the frozen active layer has reached saturation,and most of the precipitation and snowmelt directly flow into the river channel from the soil surface in the form of slope flow;in the middle rainy season,the thawing degree of frozen soil is strengthened or even completely melted,and the soil infiltration capacity is enhanced and continues to infiltrate,and most of the precipitation and snowmelt enter into the ground,which promotes the groundwater level to rise and displace more groundwater to supply the runoff;in the late rainy season,the soil moisture content reaches the maximum value of 15.7%and the water source confluence pathway is relatively single,and the groundwater supply is the main.In a word,the freezing and thawing changes of frozen soil significantly affect the water source confluence pathway in Naqu River Basin.This paper analyzes the variation law of soil freezing and thawing in different periods of rainy season in Naqu River Basin,explores the corresponding water pathways,and reveals the water cycle law of Naqu River Basin from another important aspect.