Alpine wet meadow (AWM), an important wetland type on the Qinghai-Tibet Plateau (QTP), is sensitive to climate change, which alters the soil hydrothermal regime and impacts ecological and hydrological functions in permafrost regions. The mechanisms underlying extreme AWM degradation in the QTP and hydrothermal factors controlling permafrost degradation remain unclear. In this study, soil hydrothermal processes, soil heat migration, and the permafrost state were measured in AWM and extremely degraded AWM (EDAWM). The results showed that the EDAWM exhibited delayed onset of both soil thawing and freezing, shortened thawing period, and extended freezing period at the lower boundary of the active layer. The lower ground temperatures resulted in a 0.2 m shallower active layer thickness in the EDAWM compared with the AWM. Moreover, the EDAWM altered soil thermal dynamics by redistributing energy, modifying soil moisture, preserving soil organic matter, and adjusting soil thermal properties. As for energy budget, a substantial amount of heat in the EDAWM was consumed by turbulent heat fluxes, particularly latent heat flux, which reduced the amount of heat transferred to the ground. Additionally, the higher soil organic matter content in EDAWM decreased the annual mean soil thermal conductivity from 1.42 W m- 1 K-1 in AWM to 1.26 W m- 1 K-1 in EDAWM, slowing down heat transfer within the active layer and consequently mitigating permafrost degradation. However, with continued climate warming, the soil organic matter content in EDAWM will inevitably decline due to microbial decomposition in the absence of new organic inputs. As the soil organic matter content diminishes, soil heat transfer processes will likely accelerate, and the permafrost warming rate may surpass that in undistributed AWM. These findings enhance our understanding of how alpine ecosystem succession influences regional hydrological cycles and greenhouse gas emissions.

River-controlled permafrost dynamics are crucial for sediment transport, infrastructure stability, and carbon cycle, yet are not well understood under climate change. Leveraging remotely sensed datasets, in-situ hydrological observations, and physics-based models, we reveal overall warming and widening rivers across the Tibetan Plateau in recent decades, driving accelerated sub-river permafrost thaw. River temperature of a representative (Tuotuohe River) on the central Tibetan Plateau, has increased notably (0.39 degrees C/decade) from 1985 to 2017, facilitating heat transfer into the underlying permafrost via both convection and conduction. Consequently, the permafrost beneath rivers warms faster (0.37 degrees C-0.66 degrees C/decade) and has a similar to 0.5 m thicker active layer than non-inundated permafrost (0.17 degrees C-0.49 degrees C/decade). With increasing river discharge, the inundated area expands laterally along the riverbed (16.4 m/decade), further accelerating permafrost thaw for previously non-inundated bars. Under future warmer and wetter climate, the anticipated intensification of sub-river permafrost degradation will pose risks to riverine infrastructure and amplify permafrost carbon release.

We investigate the Gisla perched talus slope (Trollaskagi peninsula, northern Iceland), from which a landslide (more specifically a debris avalanche) occurred in October 2020. Although this talus slope is located outside of the permafrost climatic boundaries, geomorphological evidence (i.e., molards in the landslide deposits) suggest that degradation of azonal permafrost could be among the destabilising factors of the landslide. The thermal dynamics of talus slopes is currently poorly understood, as air convection ( the 'chimney effect') can play a role in the persistence of permafrost at the base of talus slopes. We use the software FEFLOW to run physical-based simulations of heat transfer within a cross- of the Gisla talus slope, from -20,000 years to present. We explore the sensitivity of our model to document the initial porosity/ ice content of the talus slope (0.3, 0.5 and 0.8), and the thermal conductivity (TC) of the rock phase (0.75, 1.1 and 1.75 W.m(-1).K-1). Analysis of air temperature data show that the region has been undergoing a general temperature increase for the last similar to 40 years, supporting the possibility that permafrost degradation is among the destabilising factors of the landslide. Our temperature measurements show that a chimney effect indeed occurs at the Gisla talus slope. Although our modelling approach does not simulate air convection itself, permafrost persists at the base of the talus slope in all model scenarios. Increasing the initial porosity/ice content and decreasing the TC of the rock phase enhances persistence of permafrost in the Gisla talus slope. Our approach is unconventional as we initially know that ground ice was present in the Gisla talus slope at the time of the landslide; it attests that the permafrost dynamics in the talus slope is best represented by our most ice-conservative scenario - i.e., with a TC of 0.75 W.m(-1).K-1.

This paper takes the representative buried structure in permafrost regions, a transmission line tower foundation, as the research object. An inverse prediction is conducted in a scaled-down experimental system mimicking actual heat conduction of the frozen ground in a tower foundation. In permafrost regions, global warming and the heat transfer through the buried structures bring significantly adverse thermal effects on the stability of the infrastructures. In modeling the thermal effects, it has been a challenge to determine the ground surface boundary condition and heat source strength from the buried structures due to the complex climate and environmental conditions. In this work, based on the improved model predictive inverse method with an adaptive strategy, an inverse scheme is successfully implemented to simultaneously identify the time-varying surface temperature and the time-space-dependent heat source representing the buried structures. In this scheme, an adaptive time-varying predictive model is established by the rolling update of the sensitivity response coefficients according to the predicted temperature field to overcome the influence of nonlinear characteristics in the heat transfer process. The inverse method is verified by simulation and experimental data. According to the experimental inversion results, the reconstructed temperature distribution efficiently predicts the thermal state evolution of the permafrost foundation under seasonal freezing and thawing. It is found that, under the experimental conditions, the intensified thawing and freezing are significantly severe, e.g., the increased area ratio of active layer thickness is as high as 28% after building a tower, and the depth of permafrost table ranges from about 14 mm to about 38 mm, which could be detrimental to the stability and safety of the tower foundation. This study will provide valuable guidance for risk assessments or optimizing the design and maintenance of the real tower foundation and similar buried structures.

The thermal parameters of adherent layer are of great significance to the distribution characteristics of temperature field and foundation stability control of runway in permafrost region. This paper investigated the effects of annual range of temperature (A), annual average temperature (T-A), and other factors on the adherent layer thickness (H), temperature amplitude (A(0)), and annual average ground temperature (T-0), and further analyzed the thermal parameters of the adherent layer by using the FEM (Finite Element Model) roadbed temperature field and experimental data. The results indicate: A and average monthly total solar radiation (Q) have the most serious on H. A numerical method for determining the parameters of the adherent layer based on various conditions such as A and T-A was proposed by multiple regression. The temperature fields of the three types of pavements obtained by FEM and the experimental data were compared with the numerical calculation results for verification, and the conclusions were in close agreement, illustrating that the proposed method for calculating the parameters of the adherent layer is reasonable and effective. The research results extend the application region of adherent layer theory and provide a reference for runway construction in the permafrost region of Northeast China.

Groundwater-surface water (GW-SW) interaction, as a key component in the cold region hydrologic cycle, is extremely sensitive to seasonal and climate change. Specifically, the dynamic change of snow cover and frozen soil bring additional challenges in observing and simulating hydrologic processes under GW-SW interactions in cold regions. Integrated hydrologic models are promising tools to simulate such complex processes and study the system behaviours as well as its responses to perturbations. The cold region integrated hydrologic models should be physically representative and fully considering the thermal-hydrologic processes under snow cover variations, freeze-thaw cycles in frozen soils and GW-SW interactions. Benchmarking and integration with scarce field observations are also critical in developing cold region integrated hydrologic models. This review summarizes the current status of hydrologic models suitable for cold environment, including distributed hydrologic models, cryo-hydrogeologic models, and fully-coupled cold region GW-SW models, with a specific focus on their concepts, numerical methods, benchmarking, and applications across scales. The current research can provide implications for cold region hydrologic model development and advance our understanding of altered environments in cold regions disturbed by climate change, such as permafrost degradation, early snow melt and water shortage.

Rainfall can potentially change upper thermal-moisture boundary conditions and influence the hydrological and thermal state of the active layer in permafrost regions. Studying the relationship between rainfall and ground temperature represents an emerging issue in permafrost engineering and environment but the interactive mechanisms of rainfall and the active layer are not well understood. This study aims to analyze the effects and mechanisms of summertime rainfall on the thermal-moisture dynamics of the active layer by field observations and simulation. The observation data demonstrated that frequent light rainfall events had a minor impact on the active layer, whereas consecutive rainfall events and heavy rainfall events had significant effects on soil temperature and water content. Moreover, the soil temperatures were more susceptible to summertime rainfall events. These rapidly cooled the shallow ground and delayed the temperature rise. Summertime rainfall significantly increased the surface latent heat flux, but decreased the net radiation, sensible heat flux, and soil surface heat flux. Rainfall also enhanced the amount of downward liquid water and water vapor, but the impact of rainfall on the increase in the convective heat transfer of the liquid water was lower than the decreases in the heat conduction flux, latent heat flux by vapor diffusion, and heat flux by convection of vapor. Thus, the reduction in the total soil heat flux caused by rainfall directly leads to a cooling effect on the soil temperature and delays the increase in soil temperature. The cooling effect of rainfall events may mitigate the warming rate and maintain the active layer at a relatively low temperature. The results provide new insights into understanding the inner mechanisms of the effect of rainfall on the active layer and on the possible long-term change trends of permafrost under increasing precipitation in the central Qinghai-Tibet Plateau. (c) 2021 Elsevier B.V. All rights reserved.

Soil thermal state exerts an important role in soil physicochemical properties, nutrient content, soil carbon losses, and hydrological processes in cold regions. In the Qinghai-Tibet Plateau, desertification and aeolian sand accumulation greatly change the surface cover types and simultaneously alter the surface energy budget. However, the quantification of their impacts on the soil thermal state hasn't been studied methodically. Here, a laboratory experiment was conducted to investigate the impact of surface cover types, including bare surface, grass-coved surface, dry and wet (3%) aeolian sand-covered surface, on underlying soil thermal state. Our results demonstrate that there is a reciprocal relationship between environment change and permafrost degradation. The amount of heat entering the active layer was determined by the surface cover types and soil water content. Using the bare surface case as a reference, vegetation layer acted as a buffer to reduce the amount of heat propagation downwards the ground by 20% and to lower the near surface temperature by 0.7 degrees C. In contrast, dry aeolian sand acted as an insulation layer and warmed the ground by about 2 degrees C. Also, wet aeolian sand with high thermal conductivity facilitated the heat exchange with the atmosphere and warmed the ground about 1.5 degrees C. Our results have implications for thermal and hydrological processes in the atmosphere-ground-permafrost system and thermal stability of infrastructure under the effect of the desertification and aeolian sand accumulation. The hydrothermal interaction of desertification and permafrost needs to be quantified in the further study through long-term field observations and a fully-coupled water flow and heat transport model under a changing climate.

There is an obvious trend of climate warming and wetting on the Qinghai-Tibet Plateau during the past fifty years. Climate changes in air temperature or precipitation will inevitably influence the stability of permafrost. Previous studies mainly focus on the thermal influence of climate warming, but little is known about the induced rainfall infiltration and the hydrothermal response mechanism. Based on the meteorological data observed at Beiluhe observation station during 2013, the established water-vapor-heat transport model is used to predict the response under 1 degrees C and 2 degrees C increment of temperature, which considering the influences of rainfall. Climate change influences the thermal-moisture of permafrost mainly by changing the surface energy budget and soil hydrothermal transport components. The results show that climate warming greatly increased the surface net radiation, latent heat of evaporation and soil heat flux, decreased the sensible heat and rainfall infiltration. The rising air temperature reduces the soil moisture and soil hydraulic conductivity. Temperature gradient increases dramatically with temperature arising, further increases the moisture and energy components and reduces the components related to the water potential gradient. Climate warming increases the surface evaporation and thickness of active layer and accelerates the degradation of permafrost, which is contrary to the thermal effects of rainfall increasing.

Snow cover is the most common upper boundary condition influencing the soil freeze-thaw process in the black soil farming area of northern China. Snow is a porous dielectric cover, and its unique physical properties affect the soil moisture diffusion, heat conduction, freezing rate and other variables. To understand the spatial distribution of the soil water-heat and the variable characteristics of the critical depth of the soil water and heat, we used field data to analyze the freezing rate of soil and the extent of variation in soil water-heat in a unit soil layer under bare land (BL), natural snow (NS), compacted snow (CS) and thick snow (TS) treatments. The critical depth of the soil water and heat activity under different snow covers were determined based on the results of the analysis, and the variation fitting curve of the difference sequences on the soil temperature and water content between different soil layers and the surface 5-cm soil layer were used to verify the critical depth. The results were as follows: snow cover slowed the rate of soil freezing, and the soil freezing rate under the NS, CS and TS treatments decreased by 0.099 cm/day, 0.147 cm/day and 0.307 cm/day, respectively, compared with that under BL. In addition, the soil thawing time was delayed, and the effect was more significant with increased snow cover. During freeze-thaw cycles, the extent of variation in the water and heat time series in the shallow soil was relatively large, while there was less variation in the deep layer. There was a critical stratum in the vertical surface during hydrothermal migration, wherein the critical depth of soil water and heat change gradually increased with increasing snow cover. The variance in differences between the surface layer and both the soil water and heat in the different layers exhibited steady-rising-steady behavior, and the inflection point of the curve is the critical depth of soil freezing and thawing. This critical layer is a demarcation point between frozen soil and non-frozen soil, delineating the boundary between soil water and heat migration and non-migration. Furthermore, with increasing snow cover thickness and increasing density, the critical depth gradually increased.