## 4 Results and Discussion
### Soil Water--Groundwater Recharge Mode Analysis
#### 4.1.2 Moisture Migration Rules of Thick Cohesive Soil  
Two infiltration pathways occur in cohesive soil layers: piston (or uniform) flow and preferential flow. Piston flow leads to stable wetting fronts parallel to the soil surface, whereas preferential flow results in irregular wetting. Preferential flow usually occurs in the shallow cohesive layer, which includes macropores, cracks, and roots. After reaching a certain depth, the preferential flow disappears, and only piston flow remains below this certain depth [34; 35; 36]. According to the dynamic change in soil volume water content (Figure 3), we could see the response of the soil volume water content in the shallow layers above 0.9 m to the rain almost immediately, which, by means of preferential flow, occurred within the depth of 0.9 m. Then, the cohesive soil water content change in soil layers below 0.9 m occurred more slowly, with a lag [37], to the rain than did shallow surface layers above 0.9 m. There was only piston flow remaining in cohesive soil layers below the depth of 0.9 m.  
Dynamic changes in soil volume water content in the profile of 3E showed an \"increase-decrease-decrease-decrease\" wave-cycle trend (Figure 2). We divided the wave-cycle trend of soil volume water content into a \"dehydration period\" and \"water absorption period\" during the monitoring period. This indicates that water recharge was absorbed in cohesive soil pores during the water absorption period, and water in cohesive soil pores was lost during the dehydration period. Two transition times between the dehydration period and the water absorption period in the cohesive soil layers in the field test site were included from June 2019 to April 2020. Information about transition time, lag time, response time of soil moisture, and groundwater level (GWL) is shown in Table 3 and Figure 4.  
The initial time when the soil moisture changed from the water absorption period to the dehydration period was the moment when the free infiltration process started without rain falling. The soil moisture of the cohesive soil layer near the surface began to decrease and continued to migrate downward when the transition started. According to the transition time of each soil layer (Table 3), we found that soil layers above the depth of 4.5 m (Figure 4) continued infiltrating, resulting in water loss from the surface to the bottom in the unsaturated zone during the dehydration period without rainfall. The groundwater level started to fall 1 day and 15 h after the rain stopped, followed by a moisture decrease in soil layers below 4.5 m, then the moisture decreased in soil layers in the depth range of 1.4-4.5 m. Water release from the deeper soil layer to groundwater in the field test site was caused by a pressure head decrease at the bottom of the soil layer [38; 39] due to the groundwater level falling. The capillary saturation zone roof moveddown, enhancing the soil water's decrease in the unsaturated zone. It can be seen from the dynamic changes in soil water content and groundwater level on 1 July 2019 that the micro-confined groundwater level's falling can affect the soil volume water content of the soil layer at the depth of 5.0 m and cause an earlier transition time for soil layers below 4.5 m than that of the soil layers in the depth range of 1.4-4.5 m.  
The initial time when the soil moisture changed from the dehydration period to the water absorption period was the moment when rainfall infiltration was greater than evaporation. In the early stage of infiltration, water was mainly absorbed by soil particles to increase soil moisture content and did not move down until a positive water potential gradient was reached. The soil moisture began to infiltrate and migrate downward after the early stage of infiltration to the lower soil layers. The lower layers repeated the soil moisture change process of the upper soil layer; the soil water moved downward gradually layer by layer.