Seasonal variation of soil hydrological processes of active layer in source region of the Yellow River

In this paper, a permafrost cross-section at the northern slope of the Bayan Har Mountains in the source region of the Yellow River is studied. Based on the field observations, herein, the data has been collected from various sources including atmospheric precipitation, soil moisture, and supra-permafrost waterflow. According to the variations of the gathered data, the seasonal variability of soil hydrological processes in active layers is statistically investigated. In addition, the influence of freeze-thaw action on the hydrological process in an activity layer is numerically simulated by HYDRUS-1D software package. The obtained results are summarized in the following:① There is a close relationship between the supra-permafrost water flow and soil-water-heat at the slope scale. Using the variation of soil temperature in an active layer, the soil moisture and the supra-permafrost water flow are divided into four different phases based on the freeze-thaw action, including the frozen stability, the rapid thawing, the thawing stability, and the rapid frozen. ② Rainfall infiltration is regarded as the main driving force of soil hydrological processes in an active layer on the slope scale, as well as the freeze-thaw action in an active layer in which both are taken into account as major factors, imposing limitations. Due to the effect of freeze-thaw process in active layers, the precipitation in the freeze period decreases. In addition, due to the soil freezing, the soil water storage capacity reduces. Moreover, the soil water infiltration stops as well as the slope lateral flow diminish. Therefore, the soil moisture and the supra-permafrost water flow are regarded in a downward trend. Furthermore, the precipitation in the thaw period and the soil water storage capacity, due to the thawing of the soil, will eventually increases. It should be mentioned that the soil water infiltration and slope lateral flow increase; consequently, the soil moisture and the supra-permafrost water flow are taken into account in an upward trend. ③ The thickness of an active layer on the upslope is higher than that one on the downslope because of the effect of terrain slope, leading to changing the freeze-thaw action on the upslope which its rate is higher than that one on the downslope. Additionally, the variation of the soil moisture and the supra-permafrost water flow on the upslope is higher than that one on the downslope. Consequently, the soil moisture content on the upslope is lower than that one on the downslope, while the supra-permafrost water flow on the downslope is relatively stable.