Variations in the ground temperatures of permafrost in the two watersheds of the interior and eastern Qilian Mountains

The Qilian Mountains, which are composed of a series of approximately northwest-southeast-oriented mountains, are one of main alpine permafrost areas in the Qinghai-Tibet Plateau in northwestern China. From east to west, variations in permafrost environments are remarkable. The source areas of the Shule River (SASR) and the Datong River (SADR) are located in the interior and eastern part of the Qilian Mountains. In this study, variations in the ground temperatures of permafrost in the two watersheds represented the characteristics of interior and eastern permafrost environments in the Qilian Mountains. A total of 20 and 30 boreholes, along with ground temperatures measured at 10-15 m depths, were collected in the two areas. The maximum ground temperature in the SASR was 0.7 degrees C, and the minimum was -3.4 degrees C. The variations in the ground temperatures can be explained by elevation and local slope facing. The lapse rates of the ground temperatures with elevation were 5.9 degrees C/km for the south-facing slope, 8.0 degrees C/km for the gentle flat terrain and approximately 5.7 degrees C/km for the north-facing slope. It was estimated that the 0 degrees C ground temperature was located at approximately 3800 min the flat terrains, at approximately 3880 mon the south-facing slope terrains, and at approximately 3710 m on the north-facing slope terrains. In the SADR, the maximum and minimum ground temperatures were 2.54 and -2.78 degrees C, respectively. The elevation, vegetation types and soil moisture content can explain most of the variations in the ground temperatures. The lower limits of the permafrost were 3620 m for the wet meadow and 3710 m for the moist meadow. The lapse rates of the ground temperatures with elevation were 3.4 degrees C/km for wet meadow and 3.0 degrees C/km for moist meadow. The differences in the ground temperatures of the two areas were primarily caused by regional climatic differences, particularly the varying precipitation and evaporation. However, more areas must be studied to elucidate the regional differentiation of permafrost distribution and the underlying reasons for this differentiation.