## 4 Results and Discussion
### Soil Water--Groundwater Recharge Mode Analysis
#### 4.1.3 Vertical Water Movement Conceptual Model of Thick Cohesive Soil  
In order to reveal the variation characteristic details of stable isotopes (D, \({}^{18}\)O) and soil volume water content in the thick cohesive soil profile to a large extent, the vertical interval of the soil samples was set to 10 cm. It was obvious that the thick cohesive soil profile in the test site can be divided into three zones (0-160 cm, 160-670 cm, and 670-1430 cm), with different variation characteristic details (Figure 6). The depth range of the first zone was 0-160 cm (Figure 6). Evaporation effects frequently can enrich \(\delta^{18}\)O and \(\delta\)D values, and frequent precipitation effects can impoverish \(\delta^{18}\)O and \(\delta\)D values [40]. There was abundant rain and strong evaporation because of the high temperature at the field test site. The \(\delta\)D and \(\delta^{18}\)O values of soil water changed greatly with no obvious trend because of the effects of the frequent alternation of precipitation and evaporation (Figure 6). The maximum depth of the evaporation effect reached 160 cm. The depth range of the second zone was 160-670 cm (Figure 6). Soil water content was not affected by evaporation but by precipitation infiltration, which caused stable effective recharge to soil layers below by the means of piston flow [41]. The second zone could be divided into two parts because of the \(\delta^{18}\)O and \(\delta\)D values' range caused by the difference in soil particle composition. The \(\delta\)D and \(\delta^{18}\)O values of the first part (160-440 cm) ranged from \(-49.9\%\) to \(-43.3\%\) and from \(-6.66\%\) to \(-5.99\%\), and the second part (440-670 cm) ranged from \(-51.7\%\) to \(-46.1\%\) and from \(-7.26\%\) to \(-6.60\%\). The higher the soil clay content, the more light water molecules with relatively rich \(\delta\)D and \(\delta^{18}\)O values the soil absorbed, replacing the heavy water molecules of bound water with relatively depleted \(\delta\)D and \(\delta^{18}\)O values, and the more obvious was the isotope fractionation [26,42]. The higher the soil clay content, the higher the bound water content percentage was and the lower the content percentages of capillary water and gravity water were. Because the soil moisture extracted by the LI-2100 automatic water extraction system included bound water, capillary water, and gravity water [43], soil water \(\delta\)D and \(\delta^{18}\)O values of the first part (160-440 cm) were much richer than those of the second part (440-670 cm). The soil water \(\delta\)D and \(\delta^{18}\)O values inside the same part changed a little. The \(\delta\)D and \(\delta^{18}\)O values represented the mixing results of multiple rainfalls and evaporation effects during the wet season or dry season in a year. Figure 5: Soil volume water content and total water potential of thick cohesive layers are shown in the test site from January to July 2020. The left side is the soil water volume content’s dynamic change in monitoring layers on the 15 th of each month from January to July 2020, and the right side is the total water potential. The depth range of the third zone was 670-1430 cm, and the \(\delta\)D and \(\delta^{18}\)O values increased, generally in the range of \(-50.28\%\) to \(-42.76\%\) and \(-7.31\%\) to \(-5.16\%\), with the increase in depth (Figure 6). The elevation range of the groundwater level was 22.10-27.40 m with a maximum amplitude of 5.30 m. Groundwater and the soil water above recharged together to the soil water when the groundwater level increased, whereas the soil water recharged to the groundwater. The \(\delta\)D and \(\delta^{18}\)O values of groundwater were richer compared to the soil water of cohesive soil in the test site. Both groundwater infiltration upward and soil water infiltration downward are slow-changing processes. The recharge from groundwater to soil water enriched the \(\delta\)D and \(\delta^{18}\)O values of the soil water after mixing.