| The effect of vegetation on the water-heat exchange in the freezing-thawing processes of active layer is one of the key issues in the study of land surface processes and the prediction of the response of alpine ecosystems to climate change in permafrost regions.As the indicator and amplifier of global climate change and a vulnerable ecosystem,the Qinghai Tibet Plateau(QTP)has experienced serious permafrost and vegetation degradation under the influence of climate warming and human activities in the past few decades,which has changed the pattern of coupled thermal-moisture within active layer,leading to changes in patterns of energy and water cycling between the land surface and the atmosphere system,the regional hydrological cycle process,such as groundwater flow,runoff generation process,etc..In turn,these changes will affect the thermal-moisture dynamics of the permafrost and the active layer.How to qualitatively and quantitatively study the influence of vegetation cover change on the thermal-moisture dynamics of active layer and the energy and water cycle between the land surface and the atmosphere system has become the key and difficult problem in the study of land surface process in cold region.In addition,the interaction between groundwater flow and permafrost are often ignored in the previous studies in the simulation and prediction of permafrost distribution and change,and only the vertical heat conduction and water infiltration process are considered,thus affecting the accuracy and reliability of the simulation results of the thermal regimes in permafrost and active layer.To solve the above problems:1.we used the Simultaneous Heat and Water Model(SHAW)to simulate the dynamic change of soil temperature and moisture at an alpine meadow site in the Fenghuoshan area(FHS)of the QTP.For the simulation of soil temperature,the Nash efficiency coefficient(NSE)of each depth is between 0.89 and 0.99;for soil moisture,the NSE of each depth is between 0.62 and 0.78,and the simulation is in good agreement with the measurement.Based on the validated model,we simulated the dynamic changes of surface water-heat exchange and the thermal-moisture dynamics of the active layer under the condition of vegetation canopy change by changing leaf area index(LAI)and keeping other variables constant,then we quantitatively analyzed the effects of plant canopy changes on the above processes.Results showed that:(1)Under the influence of monsoon activity and freezing-thawing cycles of the active layer,the surface energy budget in permafrost region of QTP shows obvious seasonal variation characteristics.In this study area,the net radiation(Rn),latent heat(LE)and sensible heat fluxes(H)all increase with the increase of LAI,which is mainly caused by the difference of vegetation and soil albedo and the change of soil hydrothermal properties caused by vegetation change,while,there is a negative correlation between the surface heat fluxes(G0)and LAI,which shows that the existence of vegetation will be conducive to maintaining the thermal stability of the underlying permafrost,thus protecting the permafrost.(2)The annual total evapotranspiration(Etotal)and vegetation transpiration(Etrans)ranged from 10%to-14%and 22%to-100%respectively in response to extremes of doubled and zero LAI,and there is a logarithmic relationship between Etrans and LAI.However,the annual soil evaporation(Esoil)and shallow soil moisture(0.2 m,?0.2)decreased with the increase of LAI.There is a negative feedback cycle between plant canopy and the shallow soil moisture through changing evapotranspiration.In addition,the threshold for LAI to affect Esoil is 50%of the present level in our study area.(3)Simulation results of soil temperature and moisture suggested that better vegetation conditions are conducive to maintain the thermal stability of underlying permafrost,and the advanced initial thawing time and increasing thawing rate of soil ice with the increase of LAI may have great influence on the timing and magnitude of supra-permafrost groundwater,then affect the runoff generation process of the study area.To solve the above problems:2.we employed a numerical model of groundwater flow and energy transport,FEFLOW(Finite element subsurface FLOW system),to analyze to analyze the thermal effects of supra-permafrost groundwater on the active layer by developing two scenarios with and without considering groundwater flow respectively,and its response to the climate warming by setting different warming scenarios,and results showed that:(1)The groundwater heat advection,although has a minor effect on soil temperature,could significantly affect the freezing-thawing dynamics of the active layer especially in the thawing process.The simulated initial thawing time(ITT)of Water-flow scenario is earlier than that of No-water scenario at all depth,and this difference in simulated ITT increases from 3 days at 0.2 m to 33 days at 1.6m.In addition,the simulated active layer thickness(ALT)of Water-flow scenario is 0.23 m greater than that of No-flow scenario,and this difference in simulated ALT is gradually increase with increasing temperature(0.37 m under 3? warming scenario).(2)The groundwater flow discharge shows an increasing trend under warming scenarios especially for the winter minimum flow whose increasing rate is 1.75 times that of the maximum flow in autumn,led to a decreasing intra-annual temporal variability in groundwater flow,which could regard as an early indicator of permafrost degradation,and the variations of the freezing-thawing dynamics and groundwater flow response to the warming climate shows a positive feedback cycle between groundwater flow and permafrost degradation.This study quantified the impact of plant canopy change on the surface and subsurface water-heat transport processes in permafrost area,and analyzed the thermal effect of supra-permafrost groundwater on the active layer and its own response to climate warming,which will conducive to comprehensively and accurately predict the impact of vegetation degradation on water-heat exchange in permafrost region,as well as the complex interaction between permafrost degradation and groundwater flow under climate warming. |
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