As a result of long-term groundwater overexploitation, the thickness of the vadose zone in the NCP has significantly increased, leading to changes in moisture transport patterns and groundwater recharge processes. This research gathers data on soil water potential and moisture content by conducting in situ profile monitoring of a 30.4 m thick vadose zone. A 44.5 m geological borehole was drilled for the purpose of measuring the hydraulic parameters of undisturbed soil samples, collecting 36Cl isotope tracer samples, and constructing a coupling model of the unsaturated–saturated zone with a depth of 47 m. The research objectives were to examine the moisture transport law and infiltration recharge mechanisms in deep vadose zones. Comprehensive analysis shows that the average infiltration velocity is 0.661–0.743 m/a and the average recharge intensity is 103.1–115.9 mm/a. The depth and silty clay play an important role in affecting the infiltration process. The characteristics of infiltration can be divided into three segments: rapid, slow, and stagnant. The residual pore gases in the clay strata have a certain inhibitory effect on moisture transport. The time required for precipitation infiltration is 75.14 years for a 44.5 m thick vadose zone; thereafter, new water replaces old water to continue recharging the aquifer. In recent years, the government has taken multiple actions to alleviate this continuous downward trend in groundwater levels, including river ecological flow replenishment and groundwater extraction reduction. Additionally, increased precipitation since 2021 has objectively halted the previous thickening trend of the vadose zone. It is recommended to further strengthen groundwater resource management and enhance groundwater-level monitoring and warning to prevent further declines. This research holds significant implications for the evaluation and sustainable management of groundwater resources in large-scale plains in semi-humid areas.
Yang et al. (2026) studied this question.