Thermal State of Degrading Permafrost in the Source Region of Yellow River,Qinghai Province,China:Numerical Approach
1. Institute of Low Temperature Science,Hokkaido University2. National Institute of Polar Research,Japan,Research Fellow of the Japan Society for the Promotion of Science3. Graduate School of Life and Environmental Sciences,University of Tsukuba4. Geological Survey of Japan,National Institute of Advanced Industrial Science and Technology
摘要：A large part of the source area of the Yellow River（Hunag He）,in the northeastern margin of the Tibetan Plateau,is underlain by perennially or seasonally frozen ground,which faces a rapid warming in the past decades.Since 2002,we have investigated the permafrost distribution in the area to evaluate permafrost degradation and its impaots on groundwater hydrology.In this study,based on the data obtained during the first two years（Ikeda et al,2006）,involving the temperature profile from the 8m-depth borehole,the surface temperatures from distributed small loggers and the geophysical soundings,thermal history of the permafrost in the area was investigated through parameter studies using the numerical model.The problem was defined as the one-dimensional thermal conduction with phase change under the forcing of the surface boundary condition,which was given as ground surface temperature variations.Stable geothermal heat flux and homogeneous physical soil properties were assumed.Starting from various initial conditions,the rate of permafrost degradation was calculated under the different surface temperature history.Giving the same surface temperature conditions,the rate of degradation could vary even for the identical-thickness permafrost,depending on the initial temperature profile.Hence,several initial conditions as well as surface temperature scenarios are examined in the calculations to estimate the rate of degradation in the source area.In the previous studies,permafrost thicknesses in the source area in 1980 s are reported as ca.10 m or even less depending on the sites,which must have been already warm permafrost.Therefore,the reasonable initial temperature profile would be zero degree（or freezing point）throughout the whole frozen part,showing "zero curtain".This is the most sensitive condition for permafrost to respond the surface warming,and numerical experiments showed that such permafrost could degrade in the time scale of ten to several tens of years.Considering the time scale of the global wanning,there is a high possibility that the relict permafrost（perennially frozen part beneath the supra-permafrost talik） has widely degraded during 1990 s,which is considered to be related with the desertification of the grassland or lowering of the ground water level.Further analysis of borehole temperature data may provide more constraints for ground surface temperature history.
Asian Conference on Permafrost
地质学; 地质学; 建筑科学与工程