| The Qinghai-Tibetan Plateau is characterized by mountainous frozen region and is covered by permafrost and seasonally frozen ground.Under the global climate warming,the degradation of frozen soils over the plateau region is significant and has triggered a number of ecological and hydrological problems.In order to preserve the delicate eco-environment in the plateau and ensure the water safety in the lower regions,it is imperative to know the spatial pattern and temporal changes of frozen soils over the entire plateau.Since the Qinghai-Tibetan Plateau has a complex topography and landscape,and the ground observatory sites of frozen soil and meteorology are quite sparse,the dissertation aimed to develop a soil freeze-thaw process model fully driven by satellite remote sensing data,and used the model to simulate the spatiotemporal changes of frozen soil over the plateau region and to analyze the relevant influential factors.Firstly,this study developed a soil freeze-thaw process model fully driven by satellite remote sensing data based on a geomorphology-based eco-hydrological model(GBEHM),and the new model is referred to as GBEHM-RS.In GBEHM-RS,satellite remotely sensed land surface temperature was used as the upper boundary condition for thermal transfer equations and the principle of maximum enthropy prodctiuon was used to solve the surface energy balance.This study developed a physically-based parameterization for soil albedo estimation to make it more suitable for the Qinghai-Tibetan Plateau.By introducing the influences of solar zenith angle and near-surface liquid soil moisture in formulation and using soil compositions in parameter estimations,the newly developed parameterization of soil albedo reduced errors in calculated surface albedo.Additionally,the validation results indicated that satellite-based surface albedo obtained reliable accuracy,which was also suitable for frozen soil simulations in the plateau region.Next,this study carefully evaluated GBEHM-RS in simulations of frozen soils on the Qinghai-Tibetan Plateau.Compared with the tradiational simulations driven by ground observed meteorological data,using satellite-based land surface temperature could obviously reduce errors associated with the calculation of surface heat flux and avoid additional errors introduced by the spatial extrapolation of ground meteorological observations.The simulated frozen soils thus had higher accuracy and better reproduced the spatial patterns of ground observed frozen soils.Additionally,those previously developed satellite-based frozen soil models did not consider soil water movement;in comparison,GBEHM-RS fully coupled the heat and water transfers processes to ensure the soil freeze-thaw simulated results having higher accuracy.Finally,this study employed GBEHM-RS to simulate the spatiotemporal changes of frozen soil over the entire Qinghai-Tibetan Plateau during 2002?2016 and to analyze on the influential factors.Results showed that the permafrost covered an area of 1.22 million km~2,and the seasonally frozen ground covered an area of 1.86 million km~2.Due to rapid rising of land surface temperature during the thawing season,the permafrost degraded evidently during the study period with permafrost area decreasing by 6%and active layer thickness increasing in most places.Land surface temperature is the most important factor in influencing on the changes of frozen soils,dominating in over 80%of permafrost and seasonally frozen ground regions.The snowfall and rainfall influenced on the frozen soils through snow processes and soil water movement,and dominated 11%and 6%of permafrost and seasonally frozen ground regions,repestively.The snowfall dominated area were primarily located on the Karakoram Mountain ranges,the Himalayan Mounatin ranges,the Nyainqentanglha Mountain ranges,and the source region of Yellow River;the rainfall dominated area were primarily located on the Qaidam Basin and the source regions of Yellow and Yangtze Rivers. |
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