Modeling permafrost distribution using remote sensing-derived vegetation data in the source region of the Datong River in the northwestern China

Abstract

The source region of the Datong River is an inland watershed and located at the northeastern edge of the Qinghai-Tibetan Plateau. There is relatively plentiful rainfall in the source region. Vegetation, mainly composed of alpine meadow and alpine swamp meadow, developed well and covered mostly the ground surface of the source region. The equivalent-elevation approach has been proved a valid method to evaluate the double effects of latitude and altitude on permafrost in the Qilianshan Mountains, northwestern China. This method was used in this research. The field investigation indicated that vegetation class was an important local factor determining permafrost development and distribution in the study area. According to the vegetation samplings, the Landsat TM images of the source region were interpreted using the maximum likelihood algorism. Ground surface of the study area were classified into three classes, alpine meadow, alpine swamp meadow and the bare ground. According to the classification, boreholes were also divided into three classes correspondingly. As the small amount of boreholes drilled in the bare ground, permafrost distribution in the vegetated areas was studied. By setting up different datum point suitable for different vegetated covering areas, equivalent elevations of each borehole were calculated. As far as the ground temperatures of permafrost are concerned, the ground temperatures at 15m depth were usually chosen as the proxy of mean annual ground temperature (MAGT) because of the index has a tiny seasonal variation although the large seasonal changes of ambient temperatures. Permafrost models, between permafrost MAGT and the calculated equivalent-elevations, in different vegetated areas were constructed. Using the GIS software combined with the DEM data, permafrost MAGT of the vegetated areas in the source region were computed and mapped. The calculation results indicated that permafrost MAGT varied from -3.1°C to 1.1°C and from -4.6°C to 1.7°C in the alpine swampy meadow and alpine meadow vegetation areas, respectively. The vegetated areas had a variation of permafrost MAGT from -4.6°C to 1.7°C. In the vegetated areas, the areal percentages of permafrost and the seasonally frozen ground were 94% and 6%.