## 3 Results and Discussion
### Wetting Front Under Unsaturated Infiltration  
When studying 1D water transport in unsaturated soil in the vertical or horizontal direction, the migration characteristics of the wet and dry interface in the soil, i.e., the wetting front, intuitively reflect the speed of water infiltration in the soil and are an important aspect of analyzing water transport characteristics in unsaturated soils. The migration rate is controlled by the soil properties, bulk density, water supply flow rate, and time. In this section, the migration rates of the wetting fronts of the C1 soil column (density of 1.31 g/cm\({}^{3}\)) and C2 soil column (density of 1.39 g/cm\({}^{3}\)) were comparatively analyzed to investigate the effect of the initial structure of homogeneous soil on soil water infiltration. By comparing the infiltration processes of the C2 and C3 soil columns, the effect of the presence of surface ponding water infiltration on the migration displacement and rate of the wetting front was investigated.  
As the vertical soil column infiltration test progressed, the wetting front continuously moved forward, and the infiltration displacement of each soil column obeyed the available power function (Figure 6). Based on the derivative of the power function or the migration rate curve of the wetting front, the wetting front advancing rate-time curve gradually changed from relatively steep to relatively gentle (Figure 7). In general, the three unsaturated soil columns are relatively dry at the initial stage of infiltration, so the infiltration capacity of the soil is large, and the downward migration rate of the wetting front is relatively high during the water infiltration process. As infiltration progresses, the upper soil is gradually saturated, and the pores in the upper soil are gradually filled with water, so the infiltration capacity gradually decreases, thus affecting the migration of the wetting front, and the migration rate gradually decreases (Figure 7). During this stage, the main factors affecting the migration of the wetting front are the properties of the soil column, such as the initial dry density, initial porosity, and soil properties, rather than the rainfall intensity.  
Figure 5: Relationship between the saturated permeability and dry density.  
A comparison shows that the advancing rate of the wetting front of the C1 soil column is significantly higher than that of the C2 soil column (Figure 6). An increase in the initial dry density or a decrease in the initial porosity reduces the effective cross-section of the soil for water flow, so the infiltration capacity and migration ability of the wetting front are reduced (Figure 7). A comparison of the wetting front-time curves of the C2 and C3 soil columns revealed that the displacements of the two wetting fronts are the same before 120 min, the infiltration process at this stage is free infiltration, the infiltration rate mainly depends on the rainfall intensity, and all water migrates into the soil column. As rainfall infiltration progresses, ponding water gradually appears on the upper surface of the C2 soil column, and the C2 soil column enters the ponding water infiltration stage; the migration rate of the wetting front becomes gradually higher than that of the C3 soil column, and the difference in the wetting front displacement gradually becomes more pronounced. Therefore, the infiltration process of the C2 soil column at this stage is referred to as pressure infiltration.  
Figure 6: Wetting front displacement of the soil column.  
Figure 7: Advancing rate of the wetting front of the soil column.