Research On Water Environment And Its Evolution For Lakes In Arid Region

As an important part of hydrological cycle,lake has high water storage capacity which involves multiple spheres and systerms interaction in the Earth system.It is also an essential part in Earth Critical Zone.Given the global warning and the acelerated urbanization and modernazation processes,the contradication between water supply and demand is becoming increasingly acute,and the status of water environment is gradually deteriorating.The arid areas,with water resoures shortage and fragile ecological environment,is intrinsically more vlunerable to climate change and anthropogenic activities.In this context,it is of great significance to ensure ecosystem health and sustianble utilization for lake in social-economic development.There is no hydualic connection between lakes and sea through surface and subsurface runoff,and even water bodies in other watersheds in arid regions.Thus,the lakes show high sensitivity to climate change ans anthropogenic antivities due to the unique characteristics.The water evolution for lakes in arid regions is distinct different from lakes in humid regions.Due to the special characteristics in hydrology,hydrophysics,hydrochemistry and hydroecology,it is incorrect to identify the water environment and its evolution for lakes in arid region using the relative theories and approaches for lakes in humid regions.Shahu Lake is the biggest brackish water lake in Ningxia,China.The local tourism economy greatly depends on the lake.Due to the intensive evaporation,the water balance and environment health for Shahu Lake is greatly dependent on replenishment water.However,the status of water environment,water circulation,the effects of water replensishment and the related impacts of evaporation and cirulation on lake evolution are not yet clear.The lack of in-depth study greatly impeded lake restoration and management.This paper studied the status of water environemnt and chemical evolution for lakes in arid and semi-arid region,and took Shahu Lake as an exzample.The findings are of great theoretical significance and practical value to maintain water sustainability for lakes in arid regions,and to supprot ecology construction in arid regions.Based on extensive literature review,supported by National Natural Science Foundation of China,the Foundation for the Excellent Doctoral Dissertation of Chang'an University and other projects,the author carried out field investigation,performed field,laboratory tests and theoretical analyses to elucidate geochemistry,eutrophic status,lake circulation and chemical evolution for Shahu Lake.The main conclusions and highlights of innovation are shown as follows.(1)The water environment and its evolution for Shahu lake were identified.Artificial replenishment water and evaporation were the main recharge of discharge for Shahu lake,respectively.The process of lake's evolution can be divided into three stages: initial stage,rapid salinization and slow salinization.The climate change,groundwater environment and human activities had great impacts on water environment and its evolution for Shahu lake.(2)A systematic research regarding multiple advanced techniques and methods,such as hydrogeochemistry,isotopes and uncertainty theories and numerical simulation,were conducted for the first time for lakes in arid and semi-arid area,highlighting the interdisciplinarity of the research on water environment under human interferences.The major factors controlling the chemical compositions of water included water-rock interaction processes,ion exchange,evaporation and human activities.(3)The Monte Carlo-Variable Fuzzy Sets model was first proposed based on fuzzy and stochastic mathematical theories to assess eutrophication for lakes.Compared to the comprehensive nutrition state index method,the model can better quantify uncertainty in assessment results.It is beneficial to reduce uncertainty and gives new insights into risk management.In addition,Bayes theory was introduced in this study in order to resolve the information loss in eutrophication assessment.The approach can realize deep dig of the values of information,and have the guide and the reference value in actual work.(4)A comprehensive and systematic research was carried out to identify characteristics of water circulation and balance of lake in arid and semi-arid areas.On the basis of water balance principle and isotopic mass conservation,the characteristics of lake water circulation were identified using Craig-Gordon non-equilibrium evaporation simulation.The approach is simple and feasible,which is worthy of popularizing in the future.The E/I ratios and evaporation ratios were 0.62 and 0.444,respectively.The hydrochemical characteristics of lake water were affected by yellow river water,groundwater and drainage water.(5)As a regulator for lakes' health,groundwater plays an important role in water circulation for lakes in arid regions.On the one hand,due to the lack of surface water,the subsurface runoff become a timely and effective water source to maintain water balance for Shah lake due to water shortage in the arid region.On the other hand,the salinity in the lake was taken away with the subsurface runoff,decreasing the TDS level for lake water.Because of the specific water circulation characteristics,there is a time lag between salinity discharge and the decrease of TDS concentration.In addition,the water quantity of subsurface runoff cannot be completely described when the monthly average lake water levels were used.The actual quantities between lake and groundwater were higher than simulated results.Recognizing the interaction between groundwater and lake is prerequisites for effective management for Lakes in arid regions,and it has a very high academic value and practical value.