Soil hydrological process and migration mode influenced by the freeze-thaw process in the activity layer of permafrost regions in Qinghai-Tibet Plateau

Abstract

It hasn't yet fully understood the influence of freeze-thaw action on hydrological processes in the active layer of permafrost regions. So this article investigated one permafrost cross-section at the northern slope of Bayan Har Mountains in the Source Area of the Yellow River in Qinghai-Tibet Plateau. According to the data of field monitoring, seasonal variability of soil hydrological processes in the active layer of permafrost regions was studied and simulated by the freeze–thaw module of HYDRUS-1D software. The results show that: 1) The soil moisture and the suprapermafrost water-level are divided into four stages based on the freeze-thaw process. Rainfall infiltration is the main driving force of soil hydrological processes, and the freeze-thaw front are the main limiting factors. 2) The active layer thickness (ALT) is greater on the upslope than that on the downslope due to the influence of terrain slope. It leads to that the change variation of the soil moisture and the suprapermafrost water-level on the upslope is also more gently than that on the downslope. 3) Temperature is the key factor that affects soil hydrologic process of the active layer in permafrost regions. Soil temperature and soil moisture showed a significant exponential linear correlation at different depths. The relationship between soil temperature and suprapermafrost water level shows Boltzmann function relations. 4) Soil moisture has a downward migration trend after a freeze-thaw cycle. It will result in the formation of underground ice near the permafrost table. Driven by the temperature gradient, the unfrozen water in the active layer moves from bottom to top during the freezing stage.