Spatial variability of permafrost soil-moisture on the slope of the Qinghai-Tibet Plateau

Permafrost degradation and its freezing-thawing process affect the physical, chemical and biological processes of ecological and hydrological systems in cold regions. In particular, the movement of water in frozen soil is the most important carrier or the main form of migration-transformation of matter and energy in each surface layer in cold regions. Thus, the movement of water in frozen soil is one of the primary forces driving water-resources deterioration and ecological-function degradation. One of the key problems is to reveal the spatial and temporal variability of soil moisture movement and its main controlling factors in permafrost regions. In the Qinghai-Tibet Plateau, the occurrence of permafrost is complex due to different terrain features. Therefore, the spatial variability of the permafrost soil-moisture is more significant. Thus, we explore the spatial variability and its main controlling factors of permafrost soil-moisture on the northern-slope of the Bayan Har Mountains in the Qinghai-Tibet Plateau. The method of classification and regression tree (CART) is adopted to identify the main controlling factors influencing the soil moisture movement. Additionally, the relationships between soil moisture and environmental factors are revealed by the use of canonical correspondence analysis (CCA). The results show the following:① This greatly enhances the horizontal flow in the freezing period due to the terrain slope and the freezing-thawing process. Vertical migration is the main form of soil moisture movement, and it causes the soil-moisture content in the up-slope to be higher than that in the down-slope. In contrast, the soil-moisture content in the up-slope is lower than that in the down-slope during the melting period. ② The main environmental factors that affect the slope-permafrost soil-moisture are elevation, soil texture, soil temperature and vegetation coverage. However, there are differences in the impact factors of the soil moisture in different freezing-thawing stages. The main factors are elevation, soil texture and soil temperature in the freezing stage, with relative contribution rates of 19.97%, 19.45% and 9.56%, respectively. In the melting stage, the main factors are elevation, vegetation coverage and soil texture, with relative contribution rates of 37.4%, 14.9% and 10.7%, respectively. ③ The main factors that affect the slope-permafrost soil-moisture at the shallow depth of 0-20 cm are slope, elevation and vegetation coverage, with correlation coefficients of 0.941 2, 0.903 9 and 0.563 1, respectively. At the middle and lower depths, the main factors influencing the soil moisture are complex.