## 3 Results and Discussion
### Hcf
Figure 15: Unsaturated permeability of the soil column. permeability curves predicted by the actual data points show that the permeability of the C1 soil column can change by multiple orders of magnitude in the wetting process of the soil column, and the permeability increases in stages with increasing moisture content or decreasing suction force. In the first stage, where the volumetric moisture content changes from 0% to the residual moisture content, the permeability increases rapidly, which is manifested as linear exponential growth of the \(k(\theta)\) curve. However, this stage was not obtained in this study because the initial moisture content of the soil column was not 0%. In the second stage, from the residual moisture content to the moisture content at air-entry value, the \(k(\theta)\) curve shows that the permeability increases slowly with increasing moisture content. At this time, the permeability slowly increases with decreasing matrix suction, showing straight line segments with a consistent slope in log-log \(k(\psi)\). The matric suction decreases from the air-entry value \(\psi_{a}\) to 0 kPa, and the \(k(\psi)\) curve shows that the permeability is basically unchanged with decreasing matric suction. Since the saturation permeability is basically consistent with the stable infiltration rate in Figure 11, this stage is referred to as the saturated infiltration stage. The dual-parameter Gardner model is different from the VG model since it cannot describe three-stage permeability curves. The \(k(\psi)\) curve derived by the Gardner model can be divided into a saturated segment with a constant permeability and a straight line segment with a steadily decreasing permeability, which is more suitable for coarse-grained soil. Figure 16 shows a comparison of the unsaturated permeabilities of the C1 and C2 soil columns, and Figure 17 shows a comparison of the unsaturated permeabilities of the C2 and C3 soil columns. The permeabilities of the soil columns with different initial dry densities differed. For all soil moisture contents, the permeability of the soil column with a low initial dry density is greater than that of the soil column with a high initial density, which is consistent with the pattern of the saturated permeability. The unsaturated permeabilities of C1 and C2 soil columns exhibited differences of several orders of magnitude, i.e., when the soil water content is high, the difference in the permeabilities of soil columns of different densities is small; when the moisture content is low, the difference between the two is large. On the other hand, the unsaturated permeabilities of C2 and C3 soil columns are basically the same. These results suggest that in this study, the unsaturated permeability is not strongly sensitive to the presence of runoff and rather is related to the properties of the soil column, such as dry density, moisture content, soil quality and external temperature. That is, the HCF calculated from the volumetric moisture content is more sensitive to changes in dry density than to changes in runoff or hydraulic head height. Figure 16: Unsaturated permeability of the C1 and C2 soil columns.