## 4 Results and Discussion
### Soil Water--Groundwater Recharge Mode Analysis
#### 4.1.3 Vertical Water Movement Conceptual Model of Thick Cohesive Soil
The closer to the roof of the groundwater aquifer it was, the richer were the \(\delta\)D and \(\delta^{18}\)O values of the soil water that was affected by groundwater in general (Figure 6). There was an obvious depleted isotope value part of the third zone from 1060 cm to 1230 cm that was affected by a higher soil clay content, which was the same as in the second part (440-670 cm) of the second zone in the field test site. The \(\delta\)D and \(\delta^{18}\)O values of soil water from 1300 cm to 1430 cm were relatively stable in the ranges of \(-45.47\%\) to \(-42.76\%\) and \(-5.94\%\) to \(-5.29\%\), close to the groundwater, because of the almost direct influence of the groundwater level change. According to the variation characteristics of stable hydrogen and oxygen isotopes in the thick clay soil profile and factors influencing soil moisture migration, the thick cohesive  
Figure 6: Vertical variations of stable isotope values (\(\delta\)D, \(\delta^{18}\)O) and soil volume water content in the test site on 5 July 2020. The red dashed lines are dividing lines of different zones. soil profile in the test site can be divided into three zones (Figure 7): the shallow mixing zone, the steady zone, and the deep mixing zone [44]. In the first zone, named the shallow mixing zone, the liquid soil water migrated downward, and the gaseous soil water migrated upward in the form of water vapor. The vaporization front was located at a depth of 30 cm. The frequent alternation of rainfall-evaporation caused the \(\delta\)D and \(\delta^{18}\)O values of the soil water below the vaporization front to vary greatly, showing the characteristics of vacillation. The depth of the bottom of the shallow mixed zone was affected by evaporation, with some differences in different periods, but the maximum depth was not deeper than the maximum evaporation depth of 200 cm [13]. The second zone, named the steady zone, ranged from the bottom of the shallow mixing zone to the top of the capillary saturation zone without evaporation. Soil water content was not affected by evaporation but by precipitation infiltration, which caused stable effective recharge to soil layers below by means of piston flow. Peaks and troughs of \(\delta\)D and \(\delta^{18}\)O values can indicate climate change characteristics during the historical infiltration period to a certain extent, but this indication is only applicable to soil layers with small changes in soil texture. The third zone, named the deep mixing zone, ranged from the top of the capillary saturation zone to 1500 cm. Only piston flow existed in the whole zone. Groundwater and  
Figure 7: Conceptual model of vertical movement of cohesive soil water in the field test site (modified from [PERSON] et al., 2012). The blue dashed line stands for the micro-confined groundwater level with certain fluctuations within 530 cm. The deeper the blue color is, the higher was the soil moisture saturation of this layer in the profile. The red dashed line stands for the elevation of the capillary saturation roof with certain fluctuations caused by groundwater level changes. soil water above recharged together to this zone when the groundwater level increased, whereas the soil water of this zone recharged to groundwater.