## 4 Results and Discussion
### Numerical Simulation of Precipitation Infiltration in the Field Test Site
#### 4.2.6 Simulation Results  
The thick cohesive soil profile was divided into four mass balance zones according to the vertical hydrogeological conceptual model of the field test site (Figure 8). The depth range of the first mass balance zone was 0-200 cm, which belonged to the vadose zone. The change in cohesive soil moisture in this depth range was affected by precipitation and evaporation. Most of the water infiltrated into the vadose zone from precipitation returned back to the atmosphere by evaporation; only a small percentage of water was effective and recharged the cohesive soil moisture in the vadose zone [45]. The depth range of the second mass balance zone was 200-500 cm, which belonged to the vadose zone. Water flux at the depth of 200 cm was the effective infiltration recharge from precipitation to soil water in the vadose zone. Water flux at the depth of 500 cm was the sum of the effective recharge from precipitation infiltration and the water release of cohesive soil in the vadose zone at the depth range of 200-500 cm. The depth range of the third mass balance zone was 500-1100 cm, also called the unsaturated-saturated variable zone. It can be considered that the micro-confined groundwater had been recharged when it arrived at this zone, and the water flux at the bottom of this mass balance zone was the actual recharge to the micro-confined groundwater when the soil moisture migrated to the bottom of this zone. The depth range of the fourth mass balance zone was 1100-1500 cm, which was a stable saturated zone of micro-confined groundwater. Water flux at the bottom of this mass balance zone was the exchange capacity of soil water-groundwater. Surface runoff could be formed when the rainfall was greater than 25 mm/d, which was mainly formed in the wet season from June to July. The cumulative rainfall was 1684 mm during two hydrological years, from 1 July 2018 to 30 June 2020. The cumulative surface runoff which discharged out to surface water was 272 mm (16.15%). The cumulative rainfall infiltration was 1382 mm (82.07%), but most of the rainfall infiltration returned back to the atmosphere by evaporation, having accumulated to 1190 mm, accounting for 70.67% (Figure 10). The cumulative water flux of the clay layer at the depth of 200 cm reflected the effective precipitation infiltration recharge that could be totally absorbed in the cohesive soil and finally recharged to the groundwater after removing the water amount of evaporation. Due to the low permeability coefficient of cohesive soil in the test site and the long recharge path, there was an obvious lag effect on precipitation infiltration recharge to the micro-confined groundwater aquifer. We can see from Figure 8 that the soil moisture in the vadose zone at the depth range of 200-500 cm was only affected by infiltration from cohesive soil layers above 200 cm by piston flow. So, the cumulative water flux of the cohesive soil layer at the depth of 200 cm can be regarded as an effective recharge of precipitation for the vadose zone. The cumulative water flux at the depth of 200 cm found through numerical simulation was 234 mm, so the effective recharge of precipitation to the vadose zone was 234 mm, accounting for 13.89% of the cumulative rainfall amount (1684 mm) during two hydrological years. The precipitation infiltration recharge coefficient we obtained is 0.1389. Compared with the experienced value (0.10-0.15) of the precipitation infiltration recharge coefficient (\(R_{c}\)) of cohesive soil in Anhui listed (Table 7) in the _Handbook of Hydrogeology_[46], we found that an approximate result was given by our model. This comparison confirmed the correctness of the simulations. Hengshui and Luancheng are located in the North China Plain. The recharge coefficient (\(R_{c}\)) of cohesive soil in the Jianghan Plain we obtained is lower than that in the North China Plain; this was mainly caused by its lower permeability. The constant soil water saturation zone comprised cohesive soil layers below the depth of 1100 cm during the simulation period.