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Investigating The Soil Freeze/Thaw Dynamics In The Tibet Plateau Based On In-situ And Simulated Data

Posted on:2022-08-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:H R JiangFull Text:PDF
GTID:1520306734977589Subject:Hydrology and water resources
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The consistent warming trend has induced significant changes in the cryosphere of the Tibetan Plateau(TP).As one of the major components of the cryosphere,frozen ground plays a crucial role in the local eco-hydro system due to the close association between the soil freeze/thaw(F/T)dynamics and the energy/water balances over land surface.However,the strong heterogeneity of land surface and sparse in-situ data in the TP limit the understanding of soil F/T dynamics.Therefore,this study aims to investigate the“soil F/T dynamics in the TP”.In the study area of central TP,the soil F/T processes are analyzed based on multi-resource observations including the satellite remote data.Then the environmental influence on soil F/T processes is investigated using a modified process-based model.By integrating the model and satellite retrieval,a high-resolution map of permafrost in the source region of Yangtze River is finally estimated.The main results are as follows:(1)The soil F/T processes between the seasonally frozen ground(SFG)and permafrost areas are contrasting and have different impacts on processes such as soil moisture transfer.In permafrost areas,the soil freezes downwards from the ground surface and upwards from the bottom of the active layer with distinct freezing zero-curtain,and the minimum soil moisture occurring in the intermediate observation layer,but the thawing process is unidirectional.However,vertical F/T directions are on the contrary situation in SFG area,where soil moisture generally increases with soil depth and the zero-curtain is much more distinct during the thawing process.The results indicate that the different soil moisture patterns between SFG and permafrost areas are likely influenced by the contrasting soil F/T processes,which may further impact the spring onset by coupling with the surface energy/water characteristics.(2)Apart from the well-known thermal effects,environmental conditions also have significant impacts on soil F/T processes.Model simulated maximum frozen depths(MFD)are closely related to elevation(R~2=0.23,p<0.01),soil moisture content(R~2=0.25,p<0.01),and soil organic carbon(SOC)content(R~2=0.18,p<0.01).Elevation can affect soil F/T dynamics by altering the land surface temperature and impacting the surface energy balance,while soil moisture will alter the soil properties.However,the impact of SOC on MFD may be due to the effect of SOC on soil water-holding capability.The main factors affecting MFD vary with scale.Among the environmental factors examined,elevation is the first-order factor controlling the MFD at the large-scale(~100 km);aspect shows sizeable impacts at the medium-scale(~30 km);while soil moisture plays an important role at the local-scale(~10 km).The influence of topography implicates the thermal effect on soil F/T processes.Soil thaw onset shows a close correlation with the examined environmental factors,including soil moisture,while freeze onset seems to be influenced more by other factors.(3)The process-based model results exhibit the permafrost degradation in the source region of the Yangtze River over the past decade.The model simulation results show that permafrost area significantly decreases while the SFG area increases during the period 2003-2018,accompanied by the increased active layer thickness(ALT)and decreased MFD.Integrating remote sensing with process-based models has a great potential in permafrost mapping.In SAR-based method can effectively improve the spatial resolution of ALT estimating.Moreover,the ALT retrieval results are more reasonable when the simulated soil moisture is introduced in the In SAR-based estimation.Therefore,it is highly important to integrate remote sensing with process-based models so as to obtain the long-period,high spatial-resolution and accurate estimation of permafrost distribution.This study 1)revealed the distinct differences of soil F/T processes between permafrost and SFG areas;2)clarified the main environmental factor impacting soil freeze at different spatial scales;and 3)integrated the remote sensing with a process-based model to improve the spatial resolution of regional permafrost estimation.This study also emphasizes the importance of soil moisture in soil F/T dynamics,and further promotes the understanding of soil F/T processes in the Tibetan Plateau,which provides theoretical support for projecting the response of permafrost to climate change and its impact on hydro-ecological processes.
Keywords/Search Tags:Tibetan Plateau, Soil moisture, Process-based permafrost model, Soil freeze/thaw processes, InSAR, Remote sensing
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