Guoan Yin

Effects of local factors and climate on permafrost conditions and distribution in Beiluhe basin, Qinghai-Tibet Plateau, China

Beiluhe basin is underlain by warm and ice-rich permafrost, and covered by vegetation and soils characteristic of the Qinghai-Tibet Plateau. A field monitoring network was established to investigate permafrost conditions and to assess potential impacts of local factors and climate change. This paper describes the spatial variations in permafrost conditions from instrumented boreholes, controlling environmental factors, and recent thermal evolution of permafrost in the basin. The study area was divided into 10 ecotypes using satellite imagery based classification. The field investigations and cluster analysis of ground temperatures indicated that permafrost underlies most of the ground in swamp meadow, undisturbed alpine meadow, degrading alpine meadow, and desert alpine grassland, but is absent in other cover types. Permafrost-ecotope relations examined over a 2-year (2014-2016) period indicated that: (i) ground surface temperatures varied largely among ecotopes; (ii) annual mean ground temperatures ranged from -1.5 to 0°C in permafrost, indicating sensitive permafrost conditions; (iii) active-layer thicknesses ranged from 1.4m to 3.4m; (iv) ground ice content at the top of permafrost is high, but the active-layer soil is relatively dry. Long-term climate warming has driven thermal changes to permafrost, but ground surface characteristics and soil moisture content strongly influence the ground thermal state. These factors control local-scale spatial variations in permafrost conditions. The warm permafrost in the basin is commonly in thermal disequilibrium, and is sensitive to future climate change. Active-layer thicknesses have increased by at least 42cm and the mean annual ground temperatures have increased by up to 0.2°C in the past 10years over the basin. A permafrost distribution map was produced based on ecotypes, suggesting that permafrost underlies 64% of the study region.

Long-term thermal regimes of the Qinghai-Tibet Railway embankments in plateau permafrost regions

Ten years of ground temperature data (2003–2013) indicate that the long-term thermal regimes within embankments of the Qinghai-Tibet Railway (QTR) vary significantly with different embankment structures. Obvious asymmetries exist in the ground temperature fields within the traditional embankment (TE) and the crushed-rock basement embankment (CRBE). Measurements indicate that the TE and CRBE are not conducive to maintaining thermal stability. In contrast, the ground temperature fields of both the crushed-rock sloped embankment (CRSE) and the U-shaped crushed-rock embankment (UCRE) were symmetrical. However, the UCRE gave better thermal stability than the CRSE because slow warming of deep permafrost was observed under the CRSE. Therefore, the UCRE has the best long-term effect of decreasing ground temperature and improving the symmetry of the temperature field. More generally, it is concluded that construction using the cooling-roadbed principle meets the design requirements for long-term stability of the railway and for train transport speeds of 100 km h−1. However, temperature differences between the two shoulders, which exist in all embankments shoulders, may cause potential uneven settlement and might require maintenance.

Morphological Characteristics of Thermokarst Lakes along the Qinghai-Tibet Engineering Corridor

Abstract In order to determine the distribution and morphometric characteristics of thermokarst lakes on the Qinghai-Tibet Plateau, SPOT-5 satellite images were acquired from the Chumaerhe High Plateau to Beiluhe Basin within a 10-km-wide corridor along the Qinghai-Tibet Railway. A total of 2163 water bodies, having a total area of 1.09 × 107 m2, were identified in the study area using unsupervised classification and image interpretation. Several shape metrics (area, perimeter, circularity index, elongation index, orientation of major axis, and curvature of lake shoreline) were determined for lakes from the imagery, and bathymetric profiles of lake bottoms were derived using ground-penetrating radar. The results highlighted significant morphometric differences between lakes larger than 5000 m2 among three subregions: the Chumaerhe High Plain (CHP), the Hoh Xil Hill region (HXR), and the Beluhe Basin region (BBR). The lakes in CHP usually have a more regular outline and smooth lake bottoms, while the lakes in HXR often have the greatest depths and the most complex shorelines. The most elongated and largest lakes were typically in BBR. Other than a minor NE-SE peak in HXR and BBR, the major axis orientation for lakes in the three subregions is mostly ENE-WSW. The differences in lake morphology between the three subregions are associated with differences in ground-ice content, local relief, and topography. The dominant factors controlling the development of orientated thermokarst lakes in the region are the prevailing summer wind direction and solar insolation.

Permafrost Distribution along the Qinghai-Tibet Engineering Corridor, China Using High-Resolution Statistical Mapping and Modeling Integrated with Remote Sensing and GIS

Permafrost distribution in the Qinghai-Tibet Engineering Corridor (QTEC) is of growing interest due to the increase in infrastructure development in this remote area. Empirical models of mountain permafrost distribution have been established based on field sampled data, as a tool for regional-scale assessments of its distribution. This kind of model approach has never been applied for a large portion of this engineering corridor. In the present study, this methodology is applied to map permafrost distribution throughout the QTEC. After spatial modelling of the mean annual air temperature distribution from MODIS-LST and DEM, using high-resolution satellite image to interpret land surface type, a permafrost probability index was obtained. The evaluation results indicate that the model has an acceptable performance. Conditions highly favorable to permafrost presence (≥70%) are predicted for 60.3% of the study area, declaring a discontinuous permafrost distribution in the QTEC. This map is useful for the infrastructure development along the QTEC. In the future, local ground-truth observations will be required to confirm permafrost presence in favorable areas and to monitor permafrost evolution under the influence of climate change.

Evaluation of thermokarst lake water balance in the Qinghai–Tibet Plateau via isotope tracers

Thermokarst lakes are a ubiquitous landscape feature, which widely distributed in the pan-arctic and some low latitude regions, and are associated with regional hydrological processes. The studies were taken to obtain a better understanding of the water balance of thermokarst lakes in the Qinghai-Tibet Plateau (QTP) in order to gain insight of the regional hydrological cycle. The characteristics of the stable isotopes δ 18O and δ D were investigated in precipitation, permafrost meltwater, and thermokarst lake water in the continuous permafrost region of the QTP and analyzed the lake water balance using the isotope mass model. The results showed that the δ D-δ 18O relationship in the thermokarst lakes (δ D = 5.45 δ 18O - 18.95) differed from that of the local precipitation (δ D = 8.30 δ 18O + 18.49) and permafrost meltwater (δ D = 5.78 δ 18O - 23.41), and the mean isotope compositions in the thermokarst lakes were -7.2‰ in δ 18O and -58.0‰ in δ D. The more positive isotope signals in thermokarst lakes than in the precipitation and permafrost meltwater revealed that the lakes had experienced stronger isotope enrichment. Additionally, the evaporation-to-inflow ratio (E/I) values were < 1 in most of the thermokarst lakes (84%), which might be explained by the recent expansion of the lake surfaces. However, 16% of the thermokarst lakes had shrunk, owing to thermokarst erosion, lateral expansion as the temperature increases, and lower recharge volume. Moreover, precipitation on the lake surface was only 14-18% of the inflow volume in the thermokarst lakes, and the surface-subsurface inflow and permafrost meltwater are very important for recharging the lakes and maintaining the water balance. The results of this study provide a comprehensive understanding of the influence of climate warming on hydrological processes in the permafrost regions in the QTP.

Variations in the northern permafrost boundary over the last four decades in the Xidatan region, Qinghai–Tibet Plateau

The distribution and variations of permafrost in the Xidatan region, the northern permafrost boundary of the Qinghai-Tibet Plateau, were examined and analyzed using ground penetrating radar (GPR), borehole drilling, and thermal monitoring data. Results from GPR profiles together with borehole verification indicate that the lowest elevation limit of permafrost occurrence is 4369 m above sea level in 2012. Compared to previous studies, the maximal rise of permafrost limit is 28 m from 1975 to 2012. The total area of permafrost in the study region has been decreased by 13.8%. One of the two previously existed permafrost islands has disappeared and second one has reduced by 76% in area during the past ~40 years. In addition, the ground temperature in the Xidatan region has increased from 2012 to 2016, with a mean warming rate of ~0.004°C a−1 and ~0.003°C a−1 at the depths of 6 and 15 m, respectively. The rising of permafrost limit in the Xidatan region is mainly due to global warming. However, some non-climatic factors such as hydrologic processes and anthropic disturbances have also induced permafrost degradation. If the air temperature continues to increase, the northern permafrost boundary in the Qinghai-Tibet Plateau may continue rising in the future.