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Ground ice is a distinctive feature of permafrost, and its thawing under climate change can alter the regional hydrological and biogeochemical cycles. Spatial variations and determinants of ground ice isotopes are critical to understand subsurface water cycling during freeze-thaw process in the context of climate change, while they are not well known in permafrost region due to lack of field investigation. We examined spatial distributions and controlling factors of ground ice isotopes using data of 8 soil profiles surveyed in permafrost areas of the Qinghai-Tibet Plateau (QTP). The stable isotope values (δ2H and δ18O) of subsurface water on the QTP were higher than those in Arctic tundra ecosystem and East Siberian permafrost region. Isotopic values of water components differed each other, and varied significantly among the sampling sites. The spatial distribution of isotopes was complex. Isotopes generally decreased with depth within the soil profile, implying a general isotope depth gradient across different permafrost-affected areas. Water source, evaporative and freeze-out fractionation, and cryoturbation affect soil water isotopes. Correlation analyses showed that δ2H and δ18O in soil water positively related to air temperature and soil temperature, while negatively related to soil moisture, depth, active layer thickness, vegetation coverage, elevation, and precipitation. Elevation and soil depth mainly controlled spatial distributions of ground ice isotopes. The results could provide a new insight into soil moisture movement and cycling during freeze-thaw process in the permafrost region of the QTP, which is helpful to understand subsurface water cycle mechanism in the context of permafrost degradation. Unlabelled Image • The stable isotope values of ground ice on the Tibetan Plateau were measured. • Generally stable isotopes decreased as soil depth increased. • The spatial distributions of ground ice isotopes were mainly dominated by elevation and soil depth. [ABSTRACT FROM AUTHOR]

Abstract Stable isotopic tracing has proven to be a useful tool for assessing surface water source dynamics and hydrological connectivity in permafrost regions. This study has investigated the contribution of precipitation to water within the active layer at three long-term observation sites, including Fenghuoshan (FHS), Hoh Xil (KKXL) and Wudaoliang (WDL), by using isotopic tracer technique and two-component mixing model. The results showed that precipitation was the predominant source for water within the active layer, permafrost and ground ice near permafrost table at the three sites. Precipitation in August was the predominant source for water within the active layer at FHS, and precipitation in September was the main source at KKXL and WDL. The variation of isotopic values at different levels indicated that the water sources within the active layer could vary as the depth increases. The evaporation fractionation of water within the active layer at WDL was noticeable at depths of 0–50 cm, and the evaporation intensity decrease gradually from late June to late September. The relationship of isotopic tracing values between precipitation and water within the active layer at depths of 0–50 cm becomes more significant as the amount of the recently-occurring precipitation increases. Moreover, the relatively higher d-excess in precipitation indicates that local recycled moisture has greater contribution to precipitation. The differences of d-excess in most water within the active layer, permafrost and ground ice near permafrost table revealed that there were isotopic fractionation when precipitation supplying to above-mentioned three water bodies. The precipitation event amounted to 8.1 mm at KKXL can exert 49% ± 7.1% and 30.8% ± 3.6% contribution to water within the active layer at depths of 0–10 cm and 10–20 cm, respectively. While the long-period contribution cannot be identified because of the impact of evaporation. The results would provide new insights into the contribution of precipitation to water within the active layer on the QTP, which is also helpful to improve process-based hydrological models in the permafrost regions. Graphical abstract Unlabelled Image Highlights • Precipitation is the predominant source for water within the active layer. • Obvious evaporation fractionation sign for water within the active layer exists at depths of 0–50 cm. • The contribution ratio of single precipitation event to water within the active layer can be identified. [ABSTRACT FROM AUTHOR]

Ground ice is a distinctive feature of permafrost terrain. The vertical distribution and factors controlling the hydrochemistry of ground ice are important for studying soil moisture and salt migration during the freeze–thaw process in soil. These factors are also important components of hydrological cycles in cold regions. The hydrochemical characteristics of ground ice on the Qinghai-Tibetan Plateau (QTP) are not well known. We examined the characteristics of ground ice hydrochemistry using data from 9 soil profiles in permafrost regions of the central QTP. The isotopes and anion concentrations of subsurface water on the QTP were higher than those in Arctic polygonal ground regions. The spatial distribution of anions was complex. Well-developed hydrochemical depth gradients occurred within the soil profile. Isotopes decreased and anions increased with depth, suggesting general vertical patterns of soil hydrochemistry across different permafrost regions. Cl − and SO 4 2− concentrations in soil water increased with depth, while NO 3 − concentration did not change with depth. Freeze-out fractionation, self-purification, and desalination greatly impact soil hydrochemistry. The major factors controlling variation of soil water chemistry were soil moisture, air temperature, and active layer thickness. The results could provide a framework for understanding ground ice origins and the moisture and salt migration pathways in the context of permafrost changes. This information could be useful in developing process-based permafrost hydrologic models. [ABSTRACT FROM AUTHOR]

Temperature is an important physical variable of soil. Heat transfer in soils predominantly occurs due to conduction and convection. In this paper, we present a new analytical solution, based upon the Fourier series boundary conditions for soil surface temperature, in which the separation of variables for the heat conduction–convection equation was established. Data that had been collected from the Qinghai-Xizang (Tibet) Plateau (QXP) were used to calculate the thermal diffusivity and the liquid water flux density using different methods. The results of the soil thermal diffusivity using 5 cm as the upper boundary for the soil depth were used to calculate the soil temperature using both the single sine wave conduction and conduction–convection model and the Fourier series conduction–convection model. These results were then compared with the values for the temperature of the field soil measured at a depth of 10 cm. The average standard error of the estimate (SEE), the normalized standard error (NSEE) and Bias were 0.16 °C, 2.70% and 0.11 °C for the Fourier series conduction–convection method. The results indicate that the Fourier series model provides a better estimate of observed field temperatures than the sine wave model. This method provides a useful tool for determining soil thermal parameters, simulating soil temperature and the parameterization of land surface processes for modeling permafrost changes under global warming conditions. [ABSTRACT FROM AUTHOR]

Soil thermal conductivity (λ), describing the ability of transferring heat in the soil, plays an important role in soil thermal behavior. The estimation of λ at dryness (λ dry) is essential for obtaining accurate λ. This study aims to develop a new model for λ dry across a wide range of the soil dry density (ρ d) for soils with different textures. The λ dry measurements of 75 soil samples from literature and 19 new soils from Qinghai-Tibet Plateau are used to establish the segmented relationships between λ dry and ρ d based on clustering algorithms. Our analyses reveal that when ρ d < 1.4 g cm−3, ρ d significantly influences λ dry , while for ρ d ≥ 1.4 g cm−3, other soil properties must be taken into account. So, the performances of 12 widely used models are evaluated in these two different ρ d ranges. Results show that when ρ d < 1.4 g cm−3, the previous developed models generally perform well. On the contrary, all these models produce large calculation errors when ρ d ≥ 1.4 g cm−3. This further confirms the necessity of segmentation. Finally, with a demarcation point of 1.4 g cm−3, a new model with different calculation methods is proposed herein for predicting λ dry. The new model exhibits the highest accuracy in predicting λ dry with the highest correlation coefficient (R), lowest root mean square error (RMSE), and smaller mean bias error values; compared to the previous models, the new model RMSE values are reduced by 16.6% on average for soils with ρ d < 1.4 g cm−3 and 21.0% for ρ d ≥ 1.4 g cm−3, respectively. Namely, the new model is highly suitable for studying λ dry for different ρ d due to its simplicity and applicability. • It is necessary to predict dry soil thermal conductivity in segment. • With a demarcation point of 1.4 g cm−3, a new model was established. • The new model performed well under different ranges of dry soil density. [ABSTRACT FROM AUTHOR]

Abstract: Data observed at monitoring stations in the Tanggula Pass (TGLMS) and in the Xidatan region (XDTMS) were used to calculate the surface energy fluxes, that is, net radiation, ground surface heat flux, sensible heat flux, and latent heat flux. The energy closure statuses obtained by the daily accumulated energy fluxes at the TGLMS and XDTMS sites were good and the turbulent fluxes were slightly higher than the available energy, but this was influenced by differing time-scales. From an energy perspective, the phenomenon of turbulent fluxes being greater than the available energy is advantageous to the development of permafrost, and snow cover is advantageous to storing cold energy in soil. The sensible heat flux and the latent heat flux exhibited a seasonal alternation characteristic, that is, the sensible heat flux was higher than the latent heat flux in winter and spring, but was lower than the latent heat flux in summer and autumn. This is mainly related to the precipitation influenced by the monsoon and the freezing-thawing processes of the active layer. Analysis of diurnal flux variations included the daily freezing-thawing processes. Variations of the Bowen ratios at the two sites were greater than 1.0 from June to September, and less than 1.0 in the other months. Negative Bowen ratios appeared when the snow cover reached a certain thickness (in this case, 10 cm). [Copyright &y& Elsevier]

[Display omitted] • Soil water contents on the Tibetan Plateau during 1986–2016 were analyzed. • Soil water in permafrost areas had smaller changes than non-permafrost areas. • Climate factors could not explain the different soil water content patterns. • Permafrost favors the stability of soil water regimes. Permafrost is an important factor affecting soil hydrology in cold regions, while the effects of permafrost on temporal changes in soil water content largely remain unknown. Here, based on the calibrated Climate Change Initiative (CCI) soil moisture products using field observation soil water data at 5 cm depth from 8 representative sites, we examined changing trends of climate conditions and soil water contents during 1986–2016 between the permafrost and permafrost-free sites on the Qinghai-Tibetan Plateau (QTP). We found that all the sites have been experienced continuous warming during this period. Soil water contents showed significant increasing or decreasing trends at three of the four permafrost-free sites, but there were no significant increasing or decreasing changes at all the four permafrost sites. In addition, the Mann-Kendall (M−K) test showed that there were 2 change-points in soil water content for the sites with the active layer thickness was about 2 m, while the sites with active layer thickness larger than 3 m and permafrost-free sites showed 3–5 change-points, indicating that the soil water contents in areas with shallower active layer showed smaller changes. The different changing trends and change-points between permafrost and permafrost-free sites were associated with the existence of permafrost and active layer thickness. Although soil water contents can be affected by many factors, our results suggested that permafrost existence can affect interannual changes in soil water contents, and permafrost degradation including increasing active layer thickness and disappearance of permafrost may decrease ecosystem resilience in the face of climate change. [ABSTRACT FROM AUTHOR]

Long-term and high-quality surface soil moisture (SSM) and root-zone soil moisture (RZSM) data is crucial for understanding the land-atmosphere interactions of the Qinghai-Tibet Plateau (QTP). More than 40% of QTP is covered by permafrost, yet few studies have evaluated the accuracy of SSM and RZSM products derived from microwave satellite, land surface models (LSMs) and reanalysis over that region. This study tries to address this gap by evaluating a range of satellite and reanalysis estimates of SSM and RZSM in the thawed soil overlaying permafrost in the QTP, using in-situ measurements from sixteen stations. Here, seven SSM products were evaluated: Soil Moisture Active Passive L3 (SMAP-L3) and L4 (SMAP-L4), Soil Moisture and Ocean Salinity in version IC (SMOS-IC), Land Parameter Retrieval Model (LPRM) Advanced Microwave Scanning Radiometer 2 (AMSR2), European Space Agency Climate Change Initiative (ESA CCI), Advanced Scatterometer (ASCAT), and the fifth generation of the land component of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5-Land). We also evaluated three RZSM products from SMAP-L4, ERA5-Land, and the Noah land surface model driven by Global Land Data Assimilation System (GLDAS-Noah). The assessment was conducted using five statistical metrics, i.e. Pearson correlation coefficient (R), bias, slope, Root Mean Square Error (RMSE), and unbiased RMSE (ubRMSE) between SSM or RZSM products and in-situ measurements. Our results showed that the ESA CCI, SMAP-L4 and SMOS-IC SSM products outperformed the other SSM products, indicated by higher correlation coefficients (R) (with a median R value of 0.63, 0.44 and 0.57, respectively) and lower ubRMSE (with a median ubRMSE value of 0.05, 0.04 and 0.07 m3/m3, respectively). Yet, SSM overestimation was found for all SSM products. This could be partly attributed to ancillary data used in the retrieval (e.g. overestimation of land surface temperature for SMAP-L3) and to the fact that the products (e.g. LPRM) more easily overestimate the in-situ SSM when the soil is very dry. As expected, SMAP-L3 SSM performed better in areas with sparse vegetation than with dense vegetation covers. For RZSM products, SMAP-L4 and GLDAS-Noah (R = 0.66 and 0.44, ubRMSE = 0.03 and 0.02 m3/m3, respectively) performed better than ERA5-Land (R = 0.46; ubRMSE = 0.03 m3/m3). It is also found that all RZSM products were unable to capture the variations of in-situ RZSM during the freezing/thawing period over the permafrost regions of QTP, due to large deviation for the ice-water phase change simulation and the lack of consideration for unfrozen-water migration during freezing processes in the LSMs. • SSM and RZSM products are evaluated over the permafrost region of QTP • Uncertainties of SSM and RZSM products are analyzed • SSM products generally overestimate the in-situ measurements • Large errors were found in RZSM for both satellite-based and reanalysis products [ABSTRACT FROM AUTHOR]

• Adding unfrozen water improve soil hydrothermal conditions simulation. • Localized specific leaf area increase carbon flux simulation accuracy. • Land cover type determines carbon source/sink in the permafrost regions. • Carbon sink effect of alpine meadow may overturn in future warming. Permafrost carbon could produce a positive climate feedback. Until now, the ecosystem carbon budgets in the permafrost regions remain uncertain. Moreover, the frequently used models have some limitations especially regarding to the freeze–thaw process. Herein, we improved the IBIS model by incorporating an unfrozen water scheme and by specifying the parameters to estimate the present and future carbon budget of different land cover types (desert steppe, steppe, meadow, and wet meadow) in the permafrost regions. Incorporating an unfrozen water scheme reduced the mean errors in the soil temperature and soil water content by 25.2%, and the specifying leaf area parameters reduced the errors in the net primary productivity (NPP) by 79.9%. Further, the simulation results showed that steppes are carbon sources (39.16 gC/m2/a) and the meadows are carbon sinks (−63.42 gC/m2/a). Under the climate warming scenarios of RCP 2.6, RCP 6.0, and RCP 8.5, the desert steppe and alpine steppe would assimilated more carbon, while the meadow and wet meadow were projected to shift from carbon sinks to carbon sources in 2071–2100, implying that the land cover type plays an important role in simulating the source/sink effects of permafrost ecosystem carbon in the IBIS model. The results highlight the importance of unfrozen water to the soil hydrothermal regime and specific leaf area for the growth of alpine vegetation, and present new insights on the difference of the responses of various permafrost ecosystems to climate warming. [ABSTRACT FROM AUTHOR]

• The GLDAS-Noah had the best performance for soil temperature. • The ERA-Interim had the best performance for soil moisture. • Soil texture and forcing data may be the main causes of simulation errors. • Parameterization schemes and mismatch of spatial scales may also bring uncertainties. Long-term and large-scale reanalysis products of soil temperature and soil moisture are very important for understanding the hydrothermal regime in permafrost regions. However, it is necessary to evaluate the reliability of these products before using them. In this study, five in situ observed sites with different land cover types were collected to evaluate the performance of soil temperature and soil moisture in four reanalysis products from 2013 to 2014 in permafrost regions on the Qinghai-Tibetan Plateau (QTP). The four reanalysis products included three widely used products derived from the Climate Forecast System Reanalysis version 2 (CFSv2), the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim), and the Noah model driven by the Global Land Data Assimilation System (GLDAS-Noah), as well as a latest reanalysis product from the fifth-generation reanalysis product by the ECMWF (ERA5). The results showed that all of these products could capture temporal dynamics of soil temperature (R > 0.8) and soil moisture (R > 0.4) well. However, soil temperature was underestimated, and soil moisture was overestimated during the thawing period. The investigated results showed, those errors may mainly be caused by soil properties and forcing data. In addition, the mismatch of spatial scales between the reanalysis products and in situ observed sites, and parameterization schemes in the land surface models, such as soil thermal and hydraulic conductivity schemes, may also contributed partly causes of simulation errors. Overall, the statistical results showed that GLDAS-Noah product ranked at the top of the four products in simulating soil temperature, especially in the alpine desert, alpine swamp and alpine grassy meadow. And ERA-Interim product was superior to the other products in simulating soil moisture in permafrost regions on the QTP, especially in the alpine desert and alpine meadow. Additionally, we found that ERA5 product was better than ERA-Interim product in simulating soil temperature, especially in topsoil, but it did not show superior performance in simulating soil moisture in permafrost regions of the QTP. This may be related to the different land surface models and soil texture between the two products. [ABSTRACT FROM AUTHOR]

The spatial and temporal variations of the seasonal freeze–thaw cycles are important in understanding the ecological and hydrological processes and biogeochemical cycle associated with permafrost degradation caused by climate change, although observational data on the soil hydrothermal dynamics within the active layer of the permafrost region at the central and northern Qinghai–Tibet Plateau (QTP) are extremely scarce. In this study, soil temperature and moisture date from 11 observational sites along the Qinghai–Tibet Highway from 2010 to 2014 were used to analyze the freeze-thaw cycles of the active layer. The results revealed that mean annual ground surface temperature (MAGST) and mean annual temperature at the top of permafrost (TTOP) were the most closely related to the onset dates of soil freezing and thawing. The onset dates of soil freezing from bottom to top did not occur earlier than those from top to bottom. The differences between the onset dates of the two freezing directions and the proportion of bottom-up freezing depth increased with decreasing TTOP. The unfrozen water content of the cooling process was always higher than that of the warming process during the freezing stage. The hysteresis effect of the unfrozen water content could also be observed in the field experiment, and the maximum hysteresis levels occurred at their corresponding soil freezing points. Soil organic matter and soil moisture associated with vegetation cover are essential for water-heat exchanges between atmosphere and permafrost beneath active layer. We suggest that a better protected plant ecosystem, helps preserving the underlying permafrost on the Qinghai–Tibet Plateau. Unlabelled Image • A huge spatial variation of seasonal freeze-thaw cycles was observed. • The hysteresis effect of the unfrozen water can also be observed in the field. • TTOP and MAGST are the most important indicators of the freeze-thaw cycles. • Local factors are also vital for the freeze-thaw cycles in regional scale. [ABSTRACT FROM AUTHOR]