Thermal stability of permafrost under U-shaped crushed rock embankment of the Qinghai-Tibet Railway

The U-shaped crushed rock embankment(UCRE),of which widely utilized in the permafrost regions along the Qinghai-Tibet Railway,has the capability to rapidly reduce the ground temperature of the underlying permafrost.However,there remains uncertainty regarding the adaptation of UCRE to climate change and its long-term cooling trend.This study focuses on nine UCRE monitoring sites along the Qinghai-Tibet Railway to analyze the dynamic variations of the ground temperature underlying permafrost from 2006 to 2020.The efficiency of UCRE in stabilizing permafrost temperature in different permafrost zones is evaluated by considering the permafrost table,ground temperature,and MAGT,as well as the temperature difference between the top and bottom of the crushed rock layer and the ground temperature variation index(GTVI).The results show that UCRE is suitable for application in extremely unstable warm permafrost regions where the MAGT is higher than-0.5℃.Moreover,UCRE effectively diminishes the disparity in permafrost thermal stability between the sunny and shaded shoulders of the embankment.The short-term and long-term effect of cooling permafrost is experiencing a change related with permafrost stability.Notably,in stable cold permafrost regions with MAGT lower than-1.5℃,the long-term cooling effect of UCRE on permafrost seems to gradually di-minishes,but UCRE continues to fulfill the role of stabilizing the underlying permafrost thermal state over the long-term.These results show that UCRE can quickly restore and stabilize the thermal state of permafrost in the early stages of construction,and adapt to the influence of future climate change.The findings provide important guidance for understanding the variations of permafrost thermal stability beneath the embankment in permafrost regions,as well as for improving the embankment stability and operational safety of the Qinghai-Tibet Railway.

Climate warming is likely to weaken the performance of two-phase closed thermosyphon on the Qinghai-Tibet Plateau

Over the years,numerous geotechnical approaches have been implemented to mitigate the adverse effects of climate warming on various infrastructures in the permafrost region of the Qinghai-Tibet Plateau(QTP),such as the Qinghai-Tibet Highway and Railway,and achieved the expected engineering outcomes.However,little attention has been given to whether the performance of these geotechnical approaches has changed during the ongoing process of climate warming.To investigate the performance variation of one of these geotechnical approaches,which is two-phase closed thermosyphon(TPCT),during sustained climate warming,we conducted a statistical analysis of soil temperature monitoring data in 2003-2020 from eight regular embankments and six TPCT embankments in our permafrost monitoring network.The results indicate that TPCT undeniably has a cooling effect on the permafrost beneath embankments,even rapidly eliminated previously formed taliks beneath embankment.However,further analysis reveals that the performance of TPCT has been weakening during sustained climate warming,which has confirmed by the re-forming of the taliks beneath embankment where they had been previously eliminated.Based on the current understanding,we attributed the weakening of thermosyphon performance to a significant reduction in the air temperature freezing index caused by ongoing climate warming.Through this study,we aimed to draw attention to the evolving performance of geotechnical approaches in permafrost regions amid climate warming,prompting necessary engineering innovations to address this situation and ensure the sustainable development of the permafrost region on the QTP.

Enhanced detection of freeze‒thaw induced landslides in Zhidoi county(Tibetan Plateau,China)with Google Earth Engine and image fusion

Freeze‒thaw induced landslides(FTILs)in grasslands on the Tibetan Plateau are a geological disaster leading to soil erosion.These landslides reduce biodiversity and intensify landscape fragmentation,which in turn are strengthen by the persistent climate change and increased anthropogenic activities.However,conventional techniques for mapping FTILs on a regional scale are impractical due to their labor-intensive,costly,and time-consuming nature.This study focuses on improving FTILs detection by implementing image fusion-based Google Earth Engine(GEE)and a random forest algorithm.Integration of multiple data sources,including texture features,index features,spectral features,slope,and vertical‒vertical polarization data,allow automatic detection of the spatial distribution characteristics of FTILs in Zhidoi county,which is located within the Qinghai‒Tibet Engineering Corridor(QTEC).We employed statistical techniques to elucidate the mechanisms influencing FTILs occurrence.The enhanced method identifies two schemes that achieve high accuracy using a smaller training sample(scheme A:94.1%;scheme D:94.5%)compared to other methods(scheme B:50.0%;scheme C:95.8%).This methodology is effective in generating accurate results using only~10%of the training sample size necessitated by other methods.The spatial distribution patterns of FTILs generated for 2021 are similar to those obtained using various other training sample sources,with a primary concentration observed along the central region traversed by the QTEC.The results highlight the slope as the most crucial feature in the fusion images,accounting for 93%of FTILs occurring on gentle slopes ranging from 0°to 14°.This study provides a theoretical framework and technological reference for the identification,monitoring,prevention and control of FTILs in grasslands.Such developments hold the potential to benefit the management of grassland ecosystem,reduce economic losses,and promote grassland sustainability.

Changes in hydrological processes in the headwater area of Yellow River,China during 1956-2019 under the influences of climate change,permafrost thaw and dam

Discharge characteristics are crucial for detecting changes in hydrological processes.Recently,the river hydrology)in the Headwater Area of the Yellow River(HAYR)has exhibited erratic regimes(e.g.,monotonously declining/low/high hydrograph,even with normal precipitation)under the effects of climate change,permafrost thaw and changes in dam operation.This study integrates hydroclimatic variables(air temperature,precipitation,and potential evapotranspiration)with anthropogenic dam operation and permafrost degradation impact data to systematically examine the mechanisms of these hydrological process changes during 1956–2019.The results show the following:1)compared with the pre-dammed gauged flow,dam construction(January 1998–January 2000)and removal of dam(September 2018–August 2019)induced monotonously low(−17.2 m^(3) s^(−1);−61%)and high(+54.6 m^(3) s^(−1);+138%)hydrographs,respectively;2)hydroclimatic variables mainly controlled the summer–autumn hydrological processes in the HAYR;3)the monotonous decline of the hydrograph of Yellow River in the HAYR in some hydrological years(e.g.,1977,1979,1990 and 1995)was closely related with unusually high atmospheric demands of evaporation and low-intense rainfall during summer–autumn seasons;and 4)the lengthening of subsurface hydrological pathways and residence time,permafrost degradation reduced the recession coefficient(−0.002 per year)of winter flow and altered the hydrological regimes of seasonal rivers,which resulted in flattened hydrographs that reduced and delayed the peak flow(of 0.05 mm per year and 1.65 d per year,respectively)as well as boosted the winter baseflow(0.01 mm per year).This study can provide updated and systematic understanding of changing hydrological processes in typical alpine catchments on northeastern Qinghai–Tibet Plateau,China under a warming climate.

Non-climate environmental factors matter to Holocene dynamics of soil organic carbon and nitrogen in an alpine permafrost wetland,Qinghai‒Tibet Plateau

Studies on the responses of soil organic carbon(SOC)and nitrogen dynamics to Holocene climate and environment in permafrost peatlands and/or wetlands might serve as analogues for future scenarios,and they can help predict the fate of the frozen SOC and nitrogen under a warming climate.To date,little is known about these issues on the Qinghai‒Tibet Plateau(QTP).Here,we investigated the accumulations of SOC and nitrogen in a permafrost wetland on the northeastern QTP,and analyzed their links with Holocene climatic and environmental changes.In order to do so,we studied grain size,soil organic matter,SOC,and nitrogen contents,bulk density,geochemical parameters,and the accelerator mass spectrometry(AMS)^(14)C dating of the 216-cm-deep wetland profile.SOC and nitrogen contents revealed a general uptrend over last 7300 years.SOC stocks for depths of 0-100 and 0-200 cm were 50.1 and 79.0 kgC m^(-2),respectively,and nitrogen stocks for the same depths were 4.3 and 6.6 kgN m^(-2),respectively.Overall,a cooling and drying trend for regional climate over last 7300 years was inferred from the declining chemical weathering and humidity index.Meanwhile,SOC and nitrogen accumulated rapidly in 1110e720 BP,while apparent accumulation rates of SOC and nitrogen were much lower during the other periods of the last 7300 years.Consequently,we proposed a probable conceptual framework for the concordant development of syngenetic permafrost and SOC and nitrogen accumulations in alpine permafrost wetlands.This indicates that,apart from controls of climate,non-climate environmental factors,such as dust deposition and site hydrology,matter to SOC and nitrogen accumulations in permafrost wetlands.We emphasized that environmental changes driven by climate change have important impacts on SOC and nitrogen accumulations in alpine permafrost wetlands.This study could provide data support for regional and global estimates of SOC and nitrogen pools and for global models on carbon‒climate interactions that take into account of alpine permafrost wetlands on the northeastern QTP at mid-latitudes.

Response of soil hydrothermal processes within the active layer to variable alpine vegetation conditions on the Qinghai‒Tibet Plateau

Alpine vegetation plays an important role in the thermal stability of the permafrost under a warming climate,as it affects ground hydrothermal dynamics.The response of soil hydrothermal dynamics in the active layer to permafrost degradation under different alpine grassland types is unclear on the Qinghai‒Tibet Plateau.In this study,long-term soil temperature and soil water content in the active layer were monitored in situ from October 2010 to December 2018 at five sites in the Kaixinling permafrost region on the interior Qinghai‒Tibet Plateau along the QinghaieTibet Railway.The sites included an alpine steppe(AS),three alpine meadows(AM)with different degrees of degraded vegetation,and an alpine swamp meadow(ASM).Based on field-monitored data,the variations in soil temperature,soil water content,and freezeethaw processes were examined in the active layer.The response characteristics of the soil hydrothermal processes to climate change were analysed under the different alpine grasslands.The results showed that the duration of the thawing and freezing stages of the active layer of the AMs was shorter than that of the ASM and the AS.The average mean annual soil temperature(MAST)in the active layer of the AM((-1.25±0.50)℃)was lower than those in the AS((-0.71±0.39)℃)and ASM((-0.45±0.57)℃),while the AM had the highest rate of soil temperature increase((0.2±0.06)℃ per year).The annual amplitude of ground temperature in the active layer increased with the transition direction of the alpine vegetation type from ASM to AM to AS.The small surface offset(SO)and thermal offset(TO)(absolute values)indicated that the ground thermal state of the AM was more unstable,as it was more sensitive to the increase in air temperature than the ASM or the AS.Soil properties controlled the distribution of soil water content within the active layer,but vegetation improved the shallow soil structure by producing more belowground phytomass,thus,enhancing soil water content in the 0-30 cm layer.The average soil water content at depths of 0-30 cm was directly proportional(p<0.05)to the phytomass.Soil water contents at depths of 0-30 cm in the ASM((37.7±5.3)%)and the AM((40.8±5.9)%)were significantly higher than those in the AS((22.7±3.2)%).These results provide valuable insight into the hydrothermal interactions between the degradation of permafrost and alpine vegetation under a warming climate.

Differences in respiration components and their dominant regulating factors across three alpine grasslands on the Qinghai−Tibet Plateau

Greenhouse gases(GHGs)emissions from high-cold terrestrial ecosystems underlain by permafrost on the Qinghai–Tibet Plateau(QTP)have received widespread attention.However,the dominant factors regulating ecosystem respiration(Re)and its components(soil respiration Rs and heterotrophic respiration Rh)and how the differences in carbon emissions from different ecotypes and seasons remain are still unclear.We conducted a 2-year field investigation(August 2018 to October 2020)and applied the structural equation model(SEM)to clarify the changes in the factors controlling the respiration components during different seasons.The results indicate that the R_(e)and its controlling factors in three alpine grassland ecosystems(alpine steppe,alpine meadow,and swamp meadow)vary with seasons.Furthermore,autotrophic respiration(Ra)contributes the most to the seasonal changes in R_(e).The R_(e)gradually increases in the early growing season and rapidly decreases in the late growing season.Rh remains relatively stable during the year.Under these seasonal variations in the respiration components,the dominant factors controlling R_(e)in the nongrowing season are the temperature of the atmosphere–soil interface(heat flux,atmospheric temperature,and soil temperature at 5 cm depth)and microbial activity(microbial carbon and pH)with the variable importance projections>1.5.During the growing season,the dominant factors regulating R_(e),Rs,and Rh are the soil temperature with a standardized direct effect(SDE)of 0.424,soil nutrient conditions(total nitrogen and pH)with SDEs of 0.570–0.614,and microbial activity(microbial carbon)with a SDE of 0.591,respectively.In addition,meteorological conditions have an important impact on the respiration components during the growing season.Specifically,the atmospheric vapor pressure is the dominant factor regulating the three respiration components(standardized total effects=0.44−0.53,p<0.001).The optimal soil water contents during the growing season(water content at which R_(e)reaches the maximum)are 10%in the alpine steppe,13%–15%in the alpine meadow,and 40%–43%in the swamp meadow,respectively.The effect of the soil water content on R_(e)is more important in arid ecosystems(alpine steppe and alpine meadow)than in wet ecosystem(swamp meadow).The alleviation of water limitations in arid ecosystems may potentially increase R_(e).

Thermal and deformational repairing effect of crushed rock revetment acting as reinforcement along Qinghai–Tibet railway in permafrost regions

Due to the particularity and complexity of permafrost subgrade,research on its long-term maintenance and reinforcement under climate warming and engineering activities is of great significance.To mitigate subgrade diseases caused by thermal disturbance during the engineering construction and operation in the initial stage,the crushed-rock revetment(CRR)was additionally paved with a thickness of 1.5 m and 1.0 m on some sunny and shady shoulders of the traditional embankments along the Qinghai-Tibet railway,respectively.The improving effects for thermal and deforming stability are evaluated based on observation data of ground temperatures and embankment deformations at two sites from 2002 to 2014.The results show that a larger uplifting magnitude in the artificial permafrost table(APT),greater ground temperature decreasing amplitudes and reduction ranges of settling rate appear under the shady embankment shoulder in warm permafrost region,and both sides in the cold permafrost region when reinforcing with CRR.However,in warm permafrost region,the laying of CRR on the sunny slope of subgrade may cause considerable thermal disturbance to the underlying permafrost foundation,combined with the resulting additional stress,induce the further expansion of differential settlement.Moreover,the thermal stability strengthening effect of the CRR is closely related to the variation of the APT thickness in the earlier stage,convection intensity inside the CRR,‘cold energy reserve’in the deeper permafrost,and amount of solar radiation received by the CRR.More effective reinforcements should be implemented to alleviate the potential threaten beneath sunny embankment slope in warm permafrost regions.