The characteristics and changing tendency of permafrost in the source regions of the Datong River,Qilian Mountains

The source regions of the Datong River are located in the mid-east part of the Qilian Mountains with alpine permafrost. The characteristics and changing tendency of the permafrost were analyzed by using data from boreholes and surface observation. Regression analysis indicates that the mean annual ground temperature of permafrost in the source regions decreases with altitude with a gradient of about 3. 82 ℃ · km- 1. The mean annual ground temperature within the permafrost is affected by surface condition closely. The permafrost depth in the source regions is also significantly dependent on altitude. With the rising of elevation by 100 m,the thickness of permafrost increases about 10 m. In the plain areas,the depth of permafrost is ranging from 17 to 86 m.Active layer thickness is closely related to local factors,especially land cover condition. There is no clearly zonality relationship between active layer thickness and altitude. Under the impact of rising temperature and human activities,permafrost in the source regions is overall in the state of degradation,and the mean annual ground temperature is increasing at a rate of about 0. 0075 ℃ ·a- 1.