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Ground surface temperature (GST), measured at approximately 5 cm in depth, is a key controlling parameter for subsurface biophysical processes at the land–atmosphere boundary. This work presents a valuable dataset of GST observations at various spatial scales in the Headwater Area of the Yellow River (HAYR), a representative area of high-plateau permafrost on the northeastern Qinghai–Tibet Plateau (QTP). GST was measured every 3 h using 72 iButton temperature loggers (DS1922L) at 39 sites from 2019 to 2020. At each site, GST was recorded in two plots at distances from 2 to 16 m under similar and different land-cover conditions (steppe, meadow, swamp meadow, and bare ground). These sensors proved their reliability in harsh environments because there were only 165 biased measurements from a total of 210 816. A high significant correlation (>0.96 , p<0.001) was observed between plots, with a mean absolute error (MAE) of 0.2 to 1.2 °C. The daily intra-plot differences in GST were mainly <2 °C for sites with similar land cover in both plots and >2 °C when GST of bare ground was compared to that of sites with vegetation. From autumn to spring, the differences in GST could increase to 4–5 °C for up to 15 d. The values of the frost number (FN) were quite similar between the plots with differences in FN <0.05 for most of the sites. This dataset complements the sparse observations of GST on the QTP and helps to identify the permafrost distribution and degradation at high resolution as well as to validate and calibrate the permafrost distribution models. The datasets are openly available in the National Tibetan Plateau/Third Pole Environment Data Center (10.11888/Cryos.tpdc.272945, Şerban and Jin, 2022). [ABSTRACT FROM AUTHOR]

The large variety of the semi-automated methods in mapping the surface water bodies and the frequent omission of ponds (< 10,000 m2) from permafrost regions inventories motivates this work. Based on the correlation matrix, several widely used classification methods for mapping the surface water bodies were assessed. Water bodies inventory was generated at a local and landscape scale in the Headwater Area of the Yellow River (HAYR) on a Sentinel-2 satellite image from 23 November 2015. The assessed methods are: spectral water indices, supervised and unsupervised classifiers (k-means, Density Slicing, Maximum Likelihood Classification ˗ MLC), and machine learning algorithms (Random Forest and Support Vector Machines). The MLC method applied on visible and near-infrared (NIR) bands represents the best ratio regarding the accuracy (96%), Kappa coefficient (0.87), and water surface (14.87 km2). However, misclassifications are still present, which requires manual editing. Based on the MLC approach, 651 more water bodies were identified than previous inventories. Ponds are account for up to 93% of the 966 of individual water bodies and contribute to 42% of the total water surface, in the context that were omitted before. This analysis emphasize the importance of method settings for the classifier performance, as well as the ponds abundance and substantial contribution to the total water surface. This inventory will improve the general circulation models and provides baseline information for sustainable management of water resources in the HAYR, one of the core Asian Water Towers. [ABSTRACT FROM AUTHOR]

Hydro-meteorological variations are fundamental environmental factors affecting site-level ecosystem carbon balance variations; however, how hydro-meteorological variations influence continuous CO2 exchange over alpine meadows during the growing season in permafrost regions on the Tibetan Plateau is poorly understood. Net ecosystem CO2 exchange (NEE) measured using the eddy covariance method was partitioned into its components of ecosystem respiration (ER) and gross ecosystem CO2 exchange (GPP). ER and GPP had a significant relationship with air temperature, vapor pressure deficit, and net radiation, during the growing season of an alpine meadow. There was a positive correlation between GPP and active layer soil temperatures at 5–40 cm except soil temperature at 80 cm, located in the vicinity of the bottom of the active layer. The microbial activity of underground rhizospheric microorganism, mainly distributed in in shallow soil layers at 0–20 cm in alpine permafrost regions on the Tibetan Plateau, can significantly promote plant photosynthesis. Thus, the correlation between GPP and soil temperatures at 5–20 cm was stronger than that between GPP and air temperature. The correlation between ER (GPP) and active layer soil moisture at 5–80 cm was complicated, especially for soils at 5–20 cm. With the increase of active layer soil moisture, ER (GPP) increased first and then decreased. This was related to the thawing process of the active layer, increase of rainfalls, the frozen layer acting as a weak permeable layer, and the replenishment of super-permafrost water. Our results suggested that long-term observations on how hydro-meteorological variations affect the continuous CO2 exchange of alpine meadows with different active layer thicknesses should be carried out in alpine permafrost regions. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the thermal characteristics and dynamics of permafrost as well as seasonally frozen ground over the upper reaches of the Heihe River Basin (URHR), an observation network with 14 boreholes was established during 2011–2014. The in situ measurements indicated mean annual air temperature that ranged from −5.2 to −2.3°C at the monitored elevation range of ∼3,600–4,150 m, and mean annual ground surface temperature that ranged from −1.3 to 1.7°C during 2013–2017. The mean annual ground temperature at 16‐ to 18‐m depth ranged from −1.71°C on the high (>4,000 m above sea level) north facing slope to about 0‐C around areas near the lower limit of permafrost. Active layer thickness at the monitored sites varied significantly with the range of 0.77–4.90 m during 2011–2017, and maximum frozen depth in seasonally frozen ground was about 5 m. Permafrost thickness was between ∼136 m and less than 10 m. Both permafrost and seasonally frozen ground were found to be subject to serious warming during the measured period in the URHR. This study provides new quantitative insights for permafrost and seasonally frozen ground in the URHR. [ABSTRACT FROM AUTHOR]

Observational data collection on permafrost and active layer freeze–thaw cycle is extremely limited in the upper reaches of the Heihe River (URHHR) in the Qilian Mountains of the north-eastern Qinghai-Tibet Plateau. It acts as a bottleneck, restricting the hydrological effects of the changes in the permafrost and active layer in the Heihe River Basin. Using soil temperature, moisture and air temperature data collected from the four active layer observation sites (AL1, AL3, AL4 and AL7) established in the alpine permafrost regions in the URHHR, from 2013 to 2014, the region's active layer freeze–thaw cycle and the soil hydrothermal dynamics were comparatively analysed. As the elevation increased from 3700 m a.s.l. to 4132 m a.s.l., the mean annual ground temperatures (MAGTs) of the active layer and the active layer thicknesses (ALTs) decreased, the onset date of soil freeze of the active layer occurred earlier and the soil freeze rate increased. However, the onset date of soil thaw and the thaw rate did not exhibit significant trends. Compared to the thaw process, the duration of the active layer freeze process was significantly shortened and its rate was significantly higher. The soil freeze from bottom to top did not occur earlier than that from top to bottom. Furthermore, as elevation increased, the proportion of the bottom-up freeze layer thickness increased. The soil moisture in the thaw layer continuously moved to the freeze front during the active layer's two-way freeze process, causing the thaw layer to be dewatered. The seasonal thaw process resulted in significant reduction of the soil water content in the thaw layer, accounting for the high ice content in the vicinity of the permafrost table. Controlled by elevation, the active layer's seasonal freeze–thaw cycle was also affected by local factors, such as vegetation, slope, water (marsh water and super-permafrost water), lithology and water (ice) content. This study provides quantitative data that identify, simulate and predict the hydrological effects of the changes in the permafrost and active layer of the Heihe River Basin. [ABSTRACT FROM AUTHOR]

The sensitivity of soil carbon and nitrogen to warming is a major uncertainty in projections of climate. However, previous studies about soil organic carbon (SOC) stocks and potential emission predominantly concentrated on the shallow soil layer in high latitude ecosystems. In this study, we analyzed the SOC, total nitrogen (TN) and soil inorganic carbon (SIC) stocks, C/N ratios, and stable carbon isotope (δ13C) in the active layer and permafrost layer on the Eboling Mountain in the upper reach of Heihe River basin, northwestern China. Our results showed that the average stocks of SOC, TN, and SIC in permafrost layer above soil parent materials (71.7 kg m-2, 8.0 kg m-2, 34.7 kg m-2) were much higher than those in the active layer (44.3 kg m-2, 5.3 kg m-2, 12.2 kg m-2). The δ13C pattern in the soil profiles indicated that historical drainage conditions and pedogenesis were important factors in determining soil organic matter (SOM) stocks in this permafrost region. The δ13C and C/N ratios of the transient layer and some layers of permafrost implied that the degradation of SOM was different. These results highlight that carbon and nitrogen in permafrost regions with Alpine Kobresia meadow could make significant contribution to China's terrestrial carbon cycle. [ABSTRACT FROM AUTHOR]

Degradation of permafrost with a thin overlying active layer can greatly affect vegetation via changes in the soil water and nutrient regimes within the active layer, while little is known about the presence or absence of such effects in areas with a deep active layer. Here, we selected the northeastern Qinghai-Tibet Plateau as the study area. We examined the vegetation communities and biomass along an active layer thickness (ALT) gradient from 0.6 to 3.5 m. Our results showed that plant cover, below-ground biomass, species richness, and relative sedge cover declined with the deepening active layer, while the evenness, and relative forb cover showed a contrary trend. The vegetation indices and the dissimilarity of vegetation composition exhibited significant changes when the ALT was greater than 2.0 m. The vegetation indices (plant cover, below-ground biomass, evenness index, relative forb cover and relative sedge cover) were closely associated with soil water content, soil pH, texture and nutrient content. Soil water content played a key role in the ALT–vegetation relationship, especially at depths of 30–40 cm. Our results suggest that when the ALT is greater than 2.0 m, the presence of underlying permafrost still benefits vegetation growth via maintaining adequate soil water contents at 30–40 cm depth. Furthermore, the degradation of permafrost may lead to declines of vegetation cover and below-ground biomass with a shift in vegetation species. [ABSTRACT FROM AUTHOR]

Highlights • The recharge ratios of three source waters to stream were quantitatively estimated. • The melting ground ice contributes substantial (13.2%–16.7%) input to stream water. • The near-surface ground ice exhibited different formation mechanism and source water. • Seasonal hydrological variations and hydraulic connections were conceptually modeled. Abstract Climatic warming has resulted in permafrost degradation and it is expected to alter the hydrological processes and hydraulic connectivity on the Qinghai-Tibet Plateau (QTP). Some important and pending issues for understanding the hydrological processes in permafrost regions are how much melting water from thawing permafrost can feed the stream water and how the hydraulic connectivities will be altered under continual permafrost degradation. In this study, the Source Area of the Yellow River locating on the Northeast QTP was studied by using stable isotopic method and field hydrological observation. Results exhibited significant seasonal hydrological variations of stream water, thermokarst lakes, and ground ice. Hydrograph separation suggested that precipitation is the main contributor to stream water in ice-free months, accounting for 53.5% and 52.2% of the streams on average for the two branches, respectively. The second source is springs, which contributed about 29.8% and 17.9%. However, the recharge from melting ice is also important; it exported an average of 16.7% and 13.2% to the surface stream. Conceptual model and stable isotopes emphasized the remarkable hydraulic connectivities between the precipitation, stream water, thermokarst lakes, spring, and the near-surface ground ice. Current findings provide a basic understanding of the log-term hydrological processes under permafrost degradation, and can offer an efficient way to assess future hydrological changes and water resource protection. [ABSTRACT FROM AUTHOR]

Abstract: To examine the permafrost carbon vulnerability of the Eboling Mountain in the upper reach of Heihe River basin, Qinghai-Tibet Plateau, the distributions of concentrations and δ13C values of TOC (total organic carbon), DOC (dissolved organic carbon) and HWC (hot water soluble carbon) with soil depth in permafrost were analyzed. Our results indicated the potential CH4 emission in the soils near the permafrost table and deep permafrost (414–448 cm) with climate warming. The labile carbon (DOC) and microbial biomass carbon (HWC) in deep permafrost (280–350 cm) have been utilized during soil forming processes, with an increase of δ13CDOC and δ13CHWC values about 3‰. The lower δ13C values showed that HWC was more easily available for microbial utilization. The δ13C values of TOC suggested that C3 plants prevailed on the Eboling Mountain during the last 7000 years. [Copyright &y& Elsevier]