Risk Analysis Of Source-Sink Of Hydrochemical Change In Lhasa River Basin

As the political,economic and cultural center of Tibet Autonomous Region,the water environment quality of Lhasa River Basin has a direct impact on the social and economic development of the region and the water safety of residents.Based on the analysis of the hydrochemical characteristics of the Lhasa River Basin,this study further identifies the regional environmental risk factors that affect the hydrochemical characteristics of the Lhasa River Basin.Combined with the analysis of hydrochemical sources,the characteristics of"source sink"risk of the basin hydrochemical changes under the comprehensive effect of various factors are analyzed by using the minimum cumulative resistance model.Using Piper three line chart and m-k trend test method,this paper analyzes the hydrochemical characteristics of the main stream and branches of Lhasa River from the perspective of watershed hydrochemical types and changing trends.The results show that the hydrochemical types of the main stream,Xuerong Tsangpo and Meldromarchu of Lhasa River are Ca²⁺-HCO₃^-,and the hydrochemical types of Lhachu and T(?)lungchu are Na⁺+K⁺-HCO₃^-.The trace elements in the middle reaches of the main stream have a significant increase in mutation characteristics due to the influence of tributaries such as Meldromarchu and Xuerong Tsangpo.In addition,the mutation characteristics of the latter trace elements are particularly obvious in the high and low water periods,while the mutation characteristics of the main ions are most obvious in the low water periods.The identification results of the regional environmental risk factors that affect the water chemistry characteristics of the Lhasa River Basin show that:related to the risk of the loss of ion elements in the soil of the Lhasa River Basin,the content of organic carbon and cation exchange in the soil of the catchment are the key factors that affect the hydrochemical types in Lhasa river basin.From the point of view of climate and environmental risk factors,rainfall erosivity,vegetation coverage and soil freeze-thaw erosion in low water period are the main reasons for the difference of risk regional distribution in three water periods.Based on the regional environmental risk factors,the minimum cumulative resistance model is applied to analyze the risk characteristics of"source-sink"of watershed hydrochemical change.The results show that Lhasa River Basin has two typical kinds of risk sources,among which the low-risk sources,which are mainly urban,construction,industrial and mining land,represent the impact of human activities and mineral development on the change of water chemistry.The high-risk sources of vegetation and unused land represent the influence of the weathering of calcium and magnesium bearing carbonates and various sedimentary rocks and migmatites on the changes of water chemistry.