Thermal State and Characteristics of Permafrost in Qilian Mountains, Northwest China

Abstract:

A permafrost monitoring network with 20 boreholes was established over the upper reaches of Heihe River Basin in Qilian Mountains of northwestern China from 2011 to 2014. Boreholes were drilled along both east-west direction which is parallel to the Qilian Mountain ridges and the north-south transects. Elevation at these borehole sites varies from 3610 m to 4153m a.s.l. These boreholes cover different slope aspect, vegetation and land cover, and soil types. Five automatic meteorological stations were also established with additional measurements of soil moisture and temperature up to 2 m. Ground penetrating radar (GPR) method was used to detect active layer thickness and permafrost distribution over the study area. The results showed that the ground temperature at 16m depth ranged from -1.87°C to 2.25°C in Qilian Mountains and significantly influenced by topography and surface condition. There was a negative correction between elevation and mean annual ground temperature (MAGT), i.e., MAGT decreases as elevation increases. Based on data from GPR, the lower limit of permafrost was found at lowest elevation 3389m and 3657m on the north- and south-facing slopes, respectively. Active layer thickness varies from about 1.6 m at elevation about 4200 m a.s.l. to more than 4.0 m at 3650 m a.s.l on north-facing slope, and about 1m at about 3700m a.s.l on south-facing slope. Both the inverted ground temperature profiles and near-zero temperature gradients suggested that recent warming had already penetrated into the ground and influenced the thermal state of permafrost significantly. An intensive field survey and monitoring have been conducted over the area near the lower limit of permafrost to further investigate the response of permafrost to climate change.