Studies On Groundwater Environment Evolution And Regulation Mechanism In Manas River Catchment Under Changing Environment

In arid inland areas,surface water resources are scarce,while groundwater resources are relatively abundant and widely distributed,which maintains the security of food production and the health of the ecosystem in arid zones.In the context of climate warming,and with the continuous development of human society,the arid zone is suffering from extreme water scarcity and ecological crisis as human water and natural ecological water gradually become highly competitive.How to solve the problem of water scarcity and ecological degradation in arid inland areas on the basis of maintaining the systemic nature of mountains,water,forests,fields,lakes and grasses has become a frontier issue for research in disciplines at home and abroad.Due to the combined influence of natural factors and human activities,the dynamics and effects of the evolution of the groundwater environment in dryland areas are extremely complex and relatively poorly studied.To address this pressing challenge,this paper uses a combination of long-term groundwater dynamic observations,field surveys,sample collection and determination,in situ experiments and numerical simulations on the basis of systematic collection and analysis of historical data.The study takes the Manas River catchment,a typical watershed in the arid inland region of northwest China,as the research vehicle,and carries out research on the environmental evolution and regulation mechanisms of groundwater in the Manas River Catchment under changing environments through the integration of different scales along the time series,spatial structure and spatio-temporal coupling trajectories.The main results achieved are as follows:1.The mechanisms and effects of the spatial and temporal evolution of the groundwater dynamic and hydrochemical fields in the Manas River basin since 1957 are revealed,and the driving forces of the evolution of the groundwater environment are analysed.The increase in glacial snowmelt,precipitation and runoff,influenced by the warming trend,has alleviated water scarcity,but also increased the instability of hydrological processes in the basin.Groundwater reserves in the study area have declined by 45.11% due to water diversions,arable land expansion and groundwater over-extraction.This has further induced eco-hydrological problems such as a continuous decline in groundwater levels,a significant slowdown in groundwater flow rates,a downward shift in the extent of the overflow zone,a weakening or even a mutation of the surface-groundwater recharge and discharge relationship,a reversal of the deep groundwater hydraulic gradient in local areas,changes in the type of groundwater chemistry and a deterioration in groundwater quality.The research results have laid a solid theoretical foundation for groundwater protection and ecological restoration in the basin.2.A complete hydrogeological structure of the Quaternary strata in the Manas River basin was constructed,and key hydrogeological parameters of water transport were obtained;a numerical model of groundwater flow in the basin was constructed for a typical profile at the basin scale over the past 60 years.The simulation results reveal the dynamics of the groundwater flow system evolving from a simple flow hierarchy in its natural state to a complex multi-level nested flow system in the longitudinal profile of the basin under the changes of water diversion,groundwater extraction and evaporation.The study shows that the strength of surface-groundwater interconversion is weakening,the groundwater renewal rate of the whole basin is decreasing,the ecological water shortage in the downstream is serious,and there is an urgent need for reasonable regulation of groundwater resources.3.Four models of river-groundwater transformation from upstream to downstream are proposed;a numerical model of river-groundwater coupling in a typical profile at the regional scale is constructed.The simulation results investigate the response mechanisms of the river-groundwater transformation process to the depth of riverbed incision,surface conditions,thickness of the envelope and anisotropy of the envelope in the lateral profiles at different river sections under the condition that the river is artificially regulated.It also reveals the evolution of the thickness of the sub-suspension satiation zone,riverbed discharge and groundwater recharge as the width and depth of the river decrease.4.The kinetic processes of water migration and transformation in the soil profile under the change from bare land to cultivated land and the change of surface infiltration conditions from rainfall to diffuse irrigation and drip irrigation are revealed;a coupled surface-air zone numerical model of the soil profile under different irrigation conditions at the site scale is constructed.The simulation results analyse the kinetics of water infiltration at the soil-air interface and water migration in the unsaturated zone under different irrigation patterns,revealing that soil non-homogeneity affects soil water distribution in the profile,and that irrigation patterns control the water-holding capacity of the soil and affect the uptake of plant roots.5.From the perspective of groundwater environment,based on the concept of community of life in mountains,water,forests,fields,lakes and grasses,a "five-zone,one-network" approach to the regulation and control of groundwater renewable use in the Manas River Basin was constructed.The control measures were proposed to "enhance water connotation in mountainous areas,strengthen the infiltration of surface water in front of mountains to recharge groundwater,emphasize the restriction of groundwater extraction and water conservation in the middle and lower reaches,and maintain the ecological water level of the coccyx lake through artificial control".The study also constructs a "one vertical,four horizontal and one point" groundwater environment monitoring network in the basin.The results of this study can provide a reference for the sustainable development of water resources and ecological restoration in inland drylands and similar basins around the world.