Permafrost Simulation In The Qilian Mountains Over The Upper Reaches Of Heihe River Basin

Permafrost,defined as ground that remains below 0℃for at least two consecutive years,is a product of cold climates.Permafrost has substantial impacts on energy,water and carbon exchange between the atmosphere and the land surface,surface and sub-surface hydrologic processes,vegetation growth,the ecosystem,carbon dioxide cycle,agriculture,and engineering constructuion,as a whole.Previous studies mostly focus on the continuous permafrost region,but less on the the lower limit of alpine permafrost（LLAP）.In order to study the characteristics of permafrost in the Qilian Mountains in the upper reaches of the Heihe River,our group established a permafrost monitoring network to observe the permafrost.Based on this,in this study we collected high-resolution climate and soil data and used one-dimensional and two-dimensional numerical models to analyze the degradation characteristics of the permafrost in the study area,and solved how the permafrost degraded at the LLAP.The main research contents include:1)Change characteristics of surface freeze-thaw in the study area,2)Distributive characteristics of permafrost in the study area,3)Change characteristics and environmental control factors of LLAP and 4)Study on the model simulation of permafrost degradation at the LLAP.The main conclusions are drawn as follws:According to the assessment,the Thermal and Reanalysis Integrating Moderate-resolution Spatial-seamless LST–Tibetan Plateau（TRIMS LST）data set is in good consistency with the observed values,which can be used to characterize the surface freezing and thawing characteristics of the study area.From 2000 to 2021,the multi-year average surface ground temperature in the study area was 2.52℃,the average warming rate of mean annual ground surface temperature was 0.26℃decade^-1（p<0.05）,and the elapse rate of ground surface temperature was 10℃km^-1.The onset date of ground surface freezing was delayed by about 3 days,the end time of ground surface freezing was advanced by about 5 days,the duration of ground freezing was reduced by 8 days,and the days of ground freezing were shortened by 9 days.From 2000 to 2021,the interannual change of the ground surface thawing index in the study area showed a significant upward trend,with a change trend of about 2.09±0.96℃·d yr^-1.The ground surface freezing index showed a significant downward trend,with a change trend of-5.32±2.41℃·d yr^-1.One dimensional heat conduction model can well simulate the thermal state and the change characteristics of permafrost in the study area.From 2001 to 2021,the average area of permafrost in the study area was 15747 km²（excluding the area of glaciers）,and the area of permafrost decreased significantly（592 km²）,with a decreasing trend of28.2 km² yr^-1.The LLAP is the main area of permafrost area change.The average thickness of active layer is 1.28 m,the average depth of permafrost base is 22.4 m,and the average thickness of permafrost is 21.1 m.Unstable permafrost mainly occurs near the LLSP.With the continuous warming of the climate,the unstable permafrost will be at high risk of complete degradation in the future.The observation results show that the mean annual ground temperatures（MAGT）of the permafrost at the LLAP（2014～2021）was rising,indicating a degradation trend of permafrost.The average heating rate in the SFGT borehole is about 0.01℃yr^-1.Our results show that permafrost has significantly degraded in the study area:the mean downward thawing rate from the permafrost table was about 0.16 m per year while the mean upward thawing rate from the permafrost base was about 0.23 m per year.We estimated the mean lateral degradation rate of permafrost in this area was～4.14 m per year.The rate of lateral degradation is 26 and 18 times of that of the former two,respectively.It is found that lateral degradation is the main degradation mode of the LLAP.In the overall context of regional climate change,the specific environmental conditions at the site,especially water content and soil conditions,play a key role in the melting and disappearance of permafrost.The 2-D model by taking into account of vertical and horizontal heat transfer s,the 2-dimensional model can show the thermal state around the permafrost island at the LLAP and the change characteristics of permafrost.According to the 2D simulation results,the downward average degradation rate of the near-surface permafrost in the historical period（1960～2014）was 0.06 m yr^-1,and the upward degradation rate from permafrost base was 0.25 m yr^-1.Vertically,after the complete thaw if the middle part of permafrost,the average rate of lateral degradation of permafrost about 1.8 m yr^-1.Under the future warming scenario,the difference of ground surface warming will not lead to significant difference in the degradation rate of alpine permafrost.Simulation results show 1)island permafrost degradation was dominated by lateral degradation,which was about one order of magnitude greater than that of the vertical thaw;2)in the vertical direction,downward permafrost thaw raised air temperature was restricted by ground ice phase change near the permafrost table during the chosen simulation period,leading to little contribution to permafrost degradation;3)1D simulation without considering the effect of lateral heat flux remarkably underestimates permafrost degradation rates（roughly 1.7 times slower）at the study site.Both the field monitoring and numerical simulation studies highlight the importance of coupling a lateral heat transfer in the permafrost model and require a rethinking of the current strategy and comprehensive design in model simulations and prediction o permafrost distribution and it changes.Based on the refined soil parameters and high-resolution forced data,this study reveals the temporal and spatial variation characteristics of frozen soil in the Qilian Mountains in the upper reaches of the Heihe River during the past 20 years.The lateral degradation rate of permafrost is quantified from the perspective of observation and simulation,the degradation mechanism of permafrost at the lower boundary of permafrost is revealed,and the dominant environmental factors of permafrost change are identified.These results provide basic information for other relevant studies in the study area（such as underground ice content,ecological hydrological process,carbon cycle）.