Groundwater Circulation Evolution In Alpine Region Under Climate Change

In recent decades,global warming has accelerated the degradation of glaciers,permafrost,and precipitation phase transitions,significantly altering the rate of water circulation,which in turn has significant impacts on ecosystems,especially in high altitude cold regions（referred to as“alpine regions”）.Alpine regions are sensitive to climate change,with temperature rising twice as fast as the global average.Groundwater in alpine regions has its unique flow system and cycle characteristics.Affected by climate warming,its transportation,storage environment and water conservation functions have changed.Therefore,it is of great significance to understand the evolution mechanism and changing laws of groundwater cycles in alpine regions.Facing the practical needs of groundwater security,this thesis conducts a series of studies from three spatial dimensions:the global water tower,the Qinghai-Tibet Plateau（QTP）,and typical alpine watersheds.Firstly,based on multi-source remote sensing data and land surface hydrological model,the variation laws of groundwater storage in the global alpine region were clarified,and the interannual variation trends and driving factors of groundwater storage in the QTP were revealed.Then,the Naqu river basin in the source of Salween river basin,a typical alpine region,was taken as the study area.By carrying out multi-type field prototype observation experiments,the hydrochemistry,isotopic composition and spatiotemporal migration characteristics of continuous permafrost areas,discontinuous permafrost areas and seasonally frozen soil areas were analyzed,the potential water sources and origin of solutes of groundwater was revealed,and the groundwater age was quantitatively analyzed.On the basis of the above research,the evolution mechanism of groundwater circulation in different permafrost regions was clarified.Finally,the groundwater module was optimized,a distributed hydrological model suitable for the alpine basin was constructed,and the spatiotemporal variation of groundwater and its contribution to surface water in the alpine basin under climate change was identified.The main conclusions are as follows:（1）The variation laws of groundwater storage in global alpine regions.From 2004 to 2020,the alpine water towers were about 12.5°C lower than the average land temperature.In the past16 years,the temperature in most regions showed a significant upward trend,especially in the water tower.The annual average precipitation in the water tower is less than the global average,but the annual average snowfall is higher.The proportion of snowfall in the water tower area is decreasing.Under global warming,the terrestrial water storage showed a downward trend with different datasets,and the Arctic water storage decreased the most.The global average groundwater storage decreased by 17.65 mm,and the global average groundwater storage of water towers decreased by 41.09 mm.The decline in groundwater storage in the water tower is 2.3 times of the global average.（2）Interannual variation trend and driving factors of groundwater storage in the QTP.The climate of the QTP has changed from“warm-drying”to“warm-wetting”.Taking Pamir-Karakoram-Gangdise-Tanglha-Hengduan Mountains as the boundary,an accelerated increase trends can be found in groundwater storage in the northern QTP,while an accelerated decrease trend in the southwestern and southern QTP.The reduction of melt water recharge is the main controlling factor for the decrease of groundwater storage in the East Pamirs,Karakoram Mountains and West Kunlun Mountains in the northern QTP.The decrease of groundwater storage in the Indus River Basin in the south of the QTP and the Brahmaputra River Basin in the southwest of the QTP is controlled by two controlling factors.The external control factor is the increase of evapotranspiration,and the internal control factor is the undulating terrain,which is not conducive to water storage.The recharge of groundwater by glaciers and permafrost meltwater and precipitation is greater than the evapotranspiration in the central part of the QTP,and the increment of precipitation obviously exceeds that of evapotranspiration in the eastern QTP,resulting in an increase in water storage.（3）Groundwater formation mechanism and groundwater circulation evolution mode in typical alpine regions.The main controlling factor of groundwater chemical evolution in the Naqu river basin is water/ice–rock/soil interactions.In the permafrost region,the groundwater depth is shallow,the TDS,δ¹⁸O andδ²H is relatively depleted.The main chemical types of groundwater are Ca-HCO₃,（Ca-Na）-HCO₃ and（Ca-Mg）-（HCO₃-SO₄）.The water age of groundwater in the continuous permafrost region is 5 to 30 years,and the renewal rate is 0.83%to 10.2%.The water age of groundwater in the discontinuous permafrost region is 5 to 24 years,and the renewal rate is 1.01%to 13.71%.Groundwater is recharged by nearby high-altitude precipitation,ground ice,thawing water of snow and permafrost,and the deep groundwater passing through tailk and water-transmitted fault.In the seasonally frozen soil region,the groundwater becomes deeper and is rich in TDS,δ¹⁸O andδ²H.The chemical type of groundwater is mainly（Ca-Mg）-HCO₃.The groundwater age is 20 to 32 years,and the renewal rate is 0.61%to 2.15%.The groundwater is mainly recharged by the lateral runoff of the upstream groundwater,and partially recharged by the overflow of the deep groundwater.The interaction between surface water and groundwater is strong.（4）Groundwater storage changes and its contribution to surface water in alpine basins under different climate change scenarios.This study optimized the groundwater sub-module of the WEP hydrological model based on the principle of water balance and physical process,added groundwater depth calculation,and simulated the water cycle process in the Nagqu river basin.From 1961 to 2020,the growth rates of the annual precipitation,annual temperature and annual ground temperature were 27.1 mm/10a,0.35°C/10a and 0.06°C/10a,respectively.The future annual precipitation,temperature and ground temperature under the SSP126,SSP245,SSP370 and SSP585 scenarios will keep an increasing trend.From 1961 to 2020,groundwater storage and runoff depth showed an increasing trend,with growth rates of 22.87 mm/10a and0.65 mm/10a,respectively.On average 44.23%of the runoff comes from groundwater recharge.Under the SSP126,SSP245,SSP370 and SSP585 climate scenarios,the groundwater storage and runoff depth will continue to increase in the future.The contribution of groundwater to runoff will decrease,and the average contribution rate will drop to 28.01%,19.26%,24.31%and 20.59%,respectively.It may be because the groundwater level rise rate is lower than that of river level.The surface water mainly recharged by rainfall runoff and a small amount of snowmelt runoff.The ability of the basin to resist extreme climate risks will continue to decline,and the occurrence of droughts and floods may increase.