Study region: The source region of the Yangtze River in the Qinghai-Tibet Plateau, China. Study focus: In the context of global warming, conducting a comprehensive study on the hydrothermal processes and their influencing factors in the permafrost active layer of the Tibetan Plateau is crucial for gaining a better understanding of the ecohydrological processes in alpine grasslands. In this study, we analyzed differences in soil temperature and humidity change patterns in the active layer of four alpine grassland types in the Totuohe Basin of the Yangtze River source area. We aimed to discuss the influence of vegetation, soil, and other factors on the hydrothermal mechanism of the active layer. The main research results are as follows: (1) Significant differences in the active layer's hydrothermal regime, with higher vegetation cover correlating to lower thaw indices and better moisture conditions. (2) Vegetation and water content strongly influence thermal conditions and active layer thickness. In high-cover alpine meadows, ground surface temperature is lower with a 200 cm active layer, while swamp meadows have a shallowest layer at 160 cm. (3) Deeper active layer moisture is influenced by freezing and thawing, while shallower layers are affected by warm-season precipitation and soil texture. (4) Negative heat fluxes in the topsoil of alpine swamp and high-cover meadows indicate substantial heat release, likely contributing to permafrost preservation due to high active layer water content. New hydrological insights for the region: (1) Vegetation cover significantly influences the thermal and moisture conditions of the active layer, with higher vegetation associated with lower thaw indices and better moisture conditions. (2) Soil moisture distribution within the active layer is controlled by both freeze-thaw cycles and warm-season precipitation, indicating complex interactions between seasonal processes and soil properties.

The Qinghai-Tibet Plateau (QTP) is the largest permafrost region in the world at low and middle latitudes and high elevation. Permafrost is being degraded on the QTP due to global warming, which has a significant effect on regional climate, hydrological, and ecological processes. This paper provides a summary of recent progress in methods used in permafrost research, the permafrost distribution, and basic data relevant to permafrost research on the QTP. The area of permafrost was 1.32 x 106 km2 over the QTP, which accounts for approximately 46% of the QTP. Moreover, simulation studies of the hydrothermal process and permafrost change were reviewed and evaluated the effect of permafrost degradation on hydrological and ecological processes. The results revealed that the effects of permafrost on runoff were closely related to soil temperature, and the effect of permafrost degradation on the carbon cycle requires further study. Finally, current challenges in simulation of permafrost change processes on the QTP were discussed, emphasizing that permafrost degradation under climate change is a slow and non-linear process. This review will aid future studies examining the mechanism underlying the interaction between permafrost and climate change, and environmental protection in permafrost regions on the QTP.

The Qinghai-Tibet Plateau (QTP) is a region with an extensive area of permafrost that is very sensitive to climate change. In this study, soil water and thermal dynamic processes in the active layer of the QTP permafrost area at the profile and slope scales were investigated. The results showed that the hydrothermal process in the active layer soil was strongly affected by freezing and thawing processes and external weather conditions. The tem-perature of the active layer fluctuated in tandem with air temperature. The soil water content was more stable during the freezing period and showed a double-hump trend during the thawing period. The correlation co-efficient between soil and air temperature decreased with depth, from 0.896 in the surface soil layer to 0.082 in the deep soil layer; and the correlation coefficient between unfrozen water content and soil temperature during the freezing period showed an overall increasing trend with depth. The soil layers at different depths at the top and bottom of slope profiles differed in their hydrothermal processes due to the physicochemical properties and texture of the soil and vegetation types. The water-heat exchange of the surface soil is more frequent than that of the deep soil, and the frequency is more at the bottom than at the top of the slope. The soil water content at a depth of 0.25 m was the highest in the profile in association with a higher organic matter content and the blocking effect of dense roots. The changes of soil hydrothermal process in the active layer accelerated the hydrological cycle and the spatial-temporal variability in water resources in the frozen soil area. These effects might lead to a series of ecological and environmental problems, such as permafrost degradation and deserti-fication in the QTP alpine meadow ecosystem.

Under the condition of warming and wetting trend on Qinghai-Tibet Plateau due to climate change, summer rainfall infiltration alters the change of the hydrothermal state and may impact the cooling performance of crushed-rock interlayer embankment. Herein, two experimental models with the 1.4-m-thickness (M1) and 0.6m-thickness (M2) crushed-rock layer (CRL) were conducted in an environmental simulator experiencing the freezing and thawing cycles. The hydrothermal response to rainfall events was investigated and quantified by analyzing the variations of measured soil temperatures, volumetric water contents, and heat fluxes. Thermal observations show that rainfall infiltration caused heat advection and resulted in step change of soil temperature at depth. Rainfall infiltration reduced the surface temperature of the CRL, but warmed the soil layer at depth by up to 2.13 degrees C. The average temperature of the base soil layer under the action of concentrated rainfall basically showed an increasing trend. In particular, the CRL with a smaller thickness (M2) had a more significant thermal response to rainfall event. In addition, the moisture pulse, experiencing a step increase and following a gradual decrease caused by rainfall water infiltration, appeared several hours earlier than the temperature pulse. Moreover, infiltrated water produced an additional energy to warm the soil at depth, with maximum heat flux of 12.13 W/m2 and 79.90 W/m2 for the M1 and M2, respectively. The infiltrated water accumulated at the top of base soil significantly delayed the refreezing processes in cold period due to the latent heat effect. The net founding of this study provide an insight into improving the design crushed-rock embankment in permafrost regions.

In boreal and arctic regions, forest fires exert great influences on biogeochemical processes, hydrothermal dynamics of the active layer and near-surface permafrost, and subsequent nutrient cycles. In this article, the studies on impacts of forest fires on the permafrost environment are reviewed. These studies indicate that forest fires could result in an irreversible degradation of permafrost, successions of boreal forests, rapid losses of soil carbon stock, and increased hazardous periglacial landforms. After forest fires, soil temperatures rise; active layer thickens; the release of soil carbon and nitrogen enhances, and; vegetation changes from coniferous forests to broad-leaved forests, shrublands or grasslands. It may take decades or even centuries for the fire-disturbed ecosystems and permafrost environment to return to pre-fire conditions, if ever possible. In boreal forest, the thickness of organic layer has a key influence on changes in permafrost and vegetation. In addition, climate warming, change of vegetation, shortening of fire return intervals, and extent of fire range and increasing of fire severity may all modify the change trajectory of the fire-impacted permafrost environment. However, the observations and research on the relationships and interactive mechanisms among the forest fires, vegetation, carbon cycle and permafrost under a changing climate are still inadequate for a systematic impact evaluation. Using the chronosequence approach of evaluating the temporal changes by measuring changes in the permafrost environment at different stages at various sites (possibly representing varied stages of permafrost degradation and modes), multi-source data assimilation and model predictions and simulations should be integrated with the results from long- and short-term field investigations, geophysical investigations and airborne surveys, laboratory testing and remote sensing. Future studies may enable quantitatively assess and predict the feed-back relationship and influence mechanism among organic layer, permafrost and active layer processes, vegetation and soil carbon under a warming climate at desired spatial and temporal scales. The irreversible changes in the boreal and artic forest ecosystem and their ecological and hydrothermal thresholds, such as those induced by forest fires, should be better and systematically studied.

Qinghai-Tibet Plateau (QTP) is the largest high-altitude permafrost zone in the mid-latitudes. Due to the climate warming, permafrost degradation on the QTP has been widely recorded in the past decades. Since it greatly affects the East Asian monsoon, and even the global climate system, it is extremely important to understand permafrost current state, changes and its impacts. Based on literature reviews and some new data, this paper summarizes the characteristics of the current state permafrost on the QTP, including the active layer thickness (ALT), the spatial distribution of permafrost, permafrost temperature and thickness, as well as the ground ice and soil carbon storage in permafrost region. The new results showed that the permafrost and seasonally frozen ground area (excluding glaciers and lakes) is 1.06 million square kilometers and 1.45 million square kilometers on the QTP. The sub-stable, transitional, and unstable permafrost accounts for 30.4%, 22.1% and 22.6% of the total permafrost area. The permafrost thickness varies greatly among topography, with the maximum value in mountainous areas, which could be deeper than 200 m, while the minimum value in the flat areas and mountain valleys, which could be less than 60 m. The mean active layer thickness of the permafrost on the QTP is 2.3 m, with 80% of the permafrost regions ranges from 0.8 m to 3.5 m. During 1980 to 2015, soil temperature at 0-10, 10-40, 40-100, 100-200 cm increased at a rate of 0.439, 0.449, 0.396, and 0.259 degrees C/10 a, respectively. From 2004 to 2018, the increasing rate of the soil temperature at the bottom of active layer was 0.486 degrees C/10 a. These results show that the permafrost degradation has been accelerating. The permafrost degradation largely reduces the soil moisture. The ground ice volume of the permafrost is estimated up to 1.27x10(4) km(3) (liquid water equivalent). The soil organic carbon in the upper 2 m of permafrost region is about 17 Pg; there is a large uncertainty in this estimation however due to the great heterogeneities in the soil column. Although the permafrost ecosystem is a carbon sink at the present, it is possible that it will shift to a carbon source due to the loss of soil organic carbon along with permafrost degradation. Overall, this paper shows that the plateau permafrost has undergone remarkable degradation during past decades, which are clearly proven by the increasing ALTs and ground temperature. Most of the permafrost on the QTP belongs to the unstable permafrost, meaning that permafrost over TPQ is very sensitive to climate warming. The permafrost interacts closely with water, soil, greenhouse gases emission and biosphere. Therefore, the permafrost degradation greatly affects the regional hydrology, ecology and even the global climate system. This paper also proposes approaches and methods to study the interactive mechanisms between permafrost and climate change, and the results can serve as a scientific basis for environmental protection, engineering design and construction in cold regions.

Increase of surface temperatures has long been recognized as an unequivocal response to radiative forcing and one of the most important implications for global warming. However, it remains unclear whether the variation of ground surface temperature (GST) and soil temperatures is consistent with simultaneous changes of the near-surface air and land (or skin) surface temperatures (T-a and LST). In this study, a seven-year continuous observation of GST, T-a, and surface water and heat exchange was carried out at an elevational permafrost site at Chalaping, northeastern Qinghai-Tibet Plateau. Results showed a distinct retarding of warming on the ground surface and subsurface under the presence of dense vegetation and moist peat substrates. Mean annual T-a and LST increased at noteworthy rates of 0.22 and 0.32 degrees C/a, respectively, while mean annual GST increased only at a rate of 0.057 degrees C/a. No obvious trends were detected for the four radiation budgets except the soil heat flux (G), which significantly increased at a rate of 0.29 W.m(-2).a(-1), presumably inducing the melting of ground ice and resulted in much higher moisture content through the summers of 2015 and 2016 than preceding years and subsequent 2017 at the depths between 80 and 120 cm. However, no noticeable immediate variations of soil temperatures occurred owing to the large latent heat effect (thermal inertia) and the extending zero-curtain period. We suggest that a better protected eco-environment, particularly the surface vegetation, helps preserving the underlying permafrost, and thus to mitigates the potential degradation of elevational permafrost on the Qinghai-Tibet Plateau.

Hydrothermal processes are key components in permafrost dynamics; these processes are integral to global warming. In this study the coupled heat and mass transfer model for (CoupModel) the soil-plant-atmosphere-system is applied in high-altitude permafrost regions and to model hydrothermal transfer processes in freeze-thaw cycles. Measured meteorological forcing and soil and vegetation properties are used in the CoupModel for the period from January 1, 2009 to December 31, 2012 at the Tanggula observation site in the Qinghai-Tibet Plateau. A 24-h time step is used in the model simulation. The results show that the simulated soil temperature and water content, as well as the frozen depth compare well with the measured data. The coefficient of determination (R (2)) is 0.97 for the mean soil temperature and 0.73 for the mean soil water content, respectively. The simulated soil heat flux at a depth of 0-20 cm is also consistent with the monitored data. An analysis is performed on the simulated hydrothermal transfer processes from the deep soil layer to the upper one during the freezing and thawing period. At the beginning of the freezing period, the water in the deep soil layer moves upward to the freezing front and releases heat during the freezing process. When the soil layer is completely frozen, there are no vertical water exchanges between the soil layers, and the heat exchange process is controlled by the vertical soil temperature gradient. During the thawing period, the downward heat process becomes more active due to increased incoming shortwave radiation at the ground surface. The melt water is quickly dissolved in the soil, and the soil water movement only changes in the shallow soil layer. Subsequently, the model was used to provide an evaluation of the potential response of the active layer to different scenarios of initial water content and climate warming at the Tanggula site. The results reveal that the soil water content and the organic layer provide protection against active layer deepening in summer, so climate warming will cause the permafrost active layer to become deeper and permafrost degradation.