Dynamic Evolution Of Groundwater Level In The Naqu River Basin Under Climate Change

The Qinghai-Tibet Plateau is an critical ecological security barrier in China and plays an essential role in maintaining water resources stability and climate stability.As the climate warms,glacial melting and soil freezing and thawing have influenced the spatial temporal evolution of water and energy,which in turn have changed the spatial temporal distribution characteristics of hydrological cycle elements.Groundwater as a crucial part of water resources in the Qinghai-Tibet Plateau,influenced precipitation,temperature,evaporation and other hydrological cycle elements under climate change also impact on the groundwater system,which further affects the subsurface hydrological processes.Therefore,the evolution of groundwater level dynamic change under climate change and its response measures are always be focused.In this article,the Naqu River Basin,source region of Nujiang River,is selected as study area.The field monitoring experiment of groundwater level is carried out to analyze the characteristics of current groundwater depth in the Naqu River Basin.Based on the data of topographic and hydrogeology in the Naqu River Basin,boundary conditions,aquifer structure,and condition of supplement diameter and drainage are reasonably generalized.On this foundation,The hydrogeological conceptual model and groundwater flow numerical model by using Visual MODFLOW are constructed,also the model is identified and validated.Finally,the dynamic change pattern of groundwater level in the Naqu River Basin in different scenarios is predicted,then targeted response and adaptation strategies are proposed.The main results are as follows:(1)Based on the field monitoring experiments,the spatial and temporal variation characteristics of current groundwater depth in the Naqu River basin are analyzed.In terms of temporal variation,the shallow groundwater depth in the study area shows a significant upward trend of 0.011 m/d during the freezing process.During the complete freezing period,the groundwater depth also tends to increase,but the rise rate is smaller than that during the freezing process,with a rate of 0.005 m/d.In the period of complete freezing,the groundwater depth fluctuation decreases at a rate of 0.002 m/d.During the complete melting period,the groundwater depth shows a rapid decreasing trend with a decreasing at the rate of 0.016 m/d,then shows an increasing trend with a rising rate of0.006 m/d after the rainy season.(2)The three-dimensional numerical simulation model of groundwater flow in the Naqu River basin is constructed,and the model is identified and verified by using the field measured groundwater level.More than 80%of the measured and simulated groundwater level data points are less than 0.6m.The total groundwater recharge is1.132 billion m~3/a during the simulation period.Precipitation is the most important source of groundwater recharge in the study area,the rainfall infiltration recharge is 957million m~3/a,accounting for 84.54%of the total groundwater recharge,and snowmelt recharge is 175 million m~3/a,the proportion is 15.46%.The total groundwater discharge is 996 million m~3/a,mainly by river discharge,which is 913 million m~3/a,accounting for 91.6%of the total groundwater discharge,and evaporation discharge is 81 million m~3/a,accounting for 8.13%.The groundwater is in positive equilibrium in the simulation period.(3)The evolution trend of groundwater level in the Naqu River Basin from 2021 to2050 is predicted.Taking 2020 as the current year,three climate scenarios,namely RCP2.6,RCP4.5,and RCP8.5 in the Nor ESM climate model,are selected to predict the evolution trends of precipitation,air temperature and evaporation in the study area in different scenarios from 2021 to 2050,and then the dynamic processes of groundwater level in different climate scenarios is analyzed.The results showed that:the precipitation decreases,while the temperature and evaporation increase in different scenarios;the variation range of groundwater level is-9.01cm/a,-8.64cm/a and-7.98cm/a from 2021 to 2035.The declining rate of groundwater level will slow down to-8.73cm/a,-7.28cm/a and-7.54cm/a from 2036 to 2050.The results supplement the basic research of water cycle in the Naqu River Basin,also have important practical significance to realize efficient utilization of water resources in the Naqu River Basin.