Study On The Weakening Mechanism Of Terrain Driving Force Caused By The Increase Of Groundwater Density In The Central Arid Basin

Qaidam Basin is a typical arid basin in inland China,where precipitation is scarce and groundwater is usually discharged by evaporation.Strong evaporation and concentration lead to the enrichment of hydrochemical components in the drainage area,and the groundwater density in the central basin will change greatly.The research on the action mechanism of this change on groundwater flow system is helpful to improve the accurate understanding of groundwater circulation law under the condition of variable density,and provide scientific basis for more accurate evaluation of groundwater resources and water resources security in arid basins.According to the data of geology,hydrogeology,meteorology and hydrology in the study area,as well as the borehole in Bulacun section in the east,a two-dimensional numerical model of variable density water flow and solute transport is established.The quantitative characterization model of the weakening degree of terrain driving force caused by density is established by the analysis method of average equivalent fresh water head difference.The weakening degree of terrain driving force caused by the increase of fluid density is calculated quantitatively by layers,and the sensitivity analysis of uncertain influencing factors is carried out.The main research results are as follows:According to the simulation results,with the increase of time,the evaporation and concentration in the central basin will lead to the increase of groundwater density,the increase of equivalent fresh water head difference,and then the decrease of terrain driving force,which is the fundamental reason affecting the regional water flow system.In addition,the weakening degree of terrain driving force of water flow system in lower region is obviously higher than that in upper region.After running for 100,000 years,the average equivalent fresh water head difference in discharge area of water flow system in lower region is 36.17 m,and the weakening degree is as high as 47.86%,which is 6.5 times higher than that in upper region.However,in the central arid basin with greater density difference,the weakening effect will be more obvious,and the regional water flow system will probably disappear with time.Through sensitivity analysis of uncertain influencing factors of weakening,it is found that weakening is more sensitive to diffusion coefficient than to permeability coefficient,and to horizontal permeability coefficient than to vertical permeability coefficient.However,on the whole,whether the diffusion coefficient or permeability coefficient changes or not,with the increase of time,the increase of groundwater density will weaken the topographic driving force of regional water flow system more and more obviously.