Simulation And Analysis Of Changes In Frozen Ground And Hydrology In Typical Basins Of The Tibetan Plateau

Due to the global warming,the frozen ground of the Tibetan Plateau has been significantly degraded,and the changes of the frozen ground have further affected the hydrological processes of the plateau.As the headwater of many large rivers in Asia,the runoff changes in the Tibetan Plateau have important impacts on regional economic development.Thorough understandings of the responses of frozen ground to climate change and the impact of frozen ground changes on hydrological processes are of great significance to the rational estimation of the future runoff trends of the rivers located in the Tibetan Plateau,and the regional water resources sustainable development.Based on a distributed hydrological model,this paper simulated and analyzed the changes of frozen ground and hydrological processes over the past 30 years in two typical basins of the Tibetan Plateau,and the effects of frozen ground changes on hydrological processes were compared for different basins as well.Then the frozen ground changes and their hydrological impacts in the two basins under the climate scenario in the future 50 years were predicted.In addition,the thesis developed precipitation spatial distribution estimation methods based on multi-source data for the plateau with complex terrain and sparse ground gauges.The upper reaches of the Yaluzangbu River and the upper reaches of the Heihe River are two typical basins,which are located in the south and north of the Tibetan Plateau,respectively.The similarities and differences of the frozen ground and hydrological characteristics in the two basins were compared.The distributed hydrological model GBEHM was established in the upper reaches of the Yaluzangbu River.The simulation results of frozen grounds and runoff processes were verified based on gauge observations.Based on simulation results,the effects of the changes in precipitation,temperature,frozen ground and other factors on the hydrological process of the basin were analyzed.To estimate the spatial distribution of precipitation in the Tibetan Plateau with complex terrain and sparse ground gauges,this thesis proposed a multi-source precipitation data assimilation method.The method estimated precipitation-elevation relationship based on the reference precipitation data such as satellite remote sensing or climate model output,to make the estimation the spatial distribution of precipitation more reasonably.The precipitation-elevation relationship were corrected based on ground gauge observations since the reference precipitation data may be uncertain.If the ground gauges in the study region were not sufficient to correct the relationship,the precipitation data assimilation method based on Bayesian model was used to estimate the uncertainties of the assimilated precipitation.The assimilation methods were applied in the Tibetan Plateau.Based on cross-validation,independent gauge verification and other indices and methods,the methods were validated to be reasonable to estimate the spatial distribution of precipitation in the plateau basins.The precipitation and temperature in the upper reaches of the Heihe River Basin and the Yaluzangbu River have increased significantly over the past 30 years.The frozen ground in the two basins showed a significant degradation trend in the past 30 years.The effects of frozen ground degradation on hydrological processes were mainly in permafrost regions and permafrost degraded regions.The mechanism is that the thickness of the active layer of the permafrost increases and the permafrost degradation lead to an increase in the water storage capacity of the basin,which futhermore reduces the saturation-excess runoff.Due to the large proportion of permafrost in the upper reaches of the Heihe River,the runoff showed a significant decrease trend due to the frozen ground degradation;while the upper reaches of the Yaluzangbu River mainly distributed seasonal frozen ground,the degradation of frozen ground had less impact on river runoff.The changes in frozen grounds and hydrological processes in the two typical basins over the next 50 years showed that the frozen ground in the two basins will be further degraded,which lead to runoff decreases.Comparing the differences in the sensitivities of past and future runoff changes to temperature and frozen ground changes,it is found that runoff changes would be more sensitive to temperature rise and frozen ground degradation in the future.