An investigation of groundwater table fluctuations induced by rainfall should consider interactions between the liquid and gas phases in soils. In this study, a water-air two-phase flow model was initially verified by simulating an infiltration experiment. It was then employed to model the interactions between liquid and gas phases regarding actions of airflow on the groundwater table and the fluctuations of the phreatic level and water level in the well induced by rainfall. The effects of airflo7w caused by rainfall on phreatic level fluctuations were also studied quantitatively by comparing the results obtained using the proposed model with those obtained from a water single-phase flow model. The simulation results show that in addition to actual recharge, compressed airflow in unsaturated zones causes the phreatic level to increase, but the rise in the phreatic level is lower than that in the pore-air pressure head in unsaturated zones due to the mitigation of capillary fringe. The existence of airflow enhances the phreatic level rise during and after rainfall. In addition, the water level in the well, pushed by the phreatic level fluctuations, varies similarly to the phreatic level, but it experiences somewhat delayed and slightly attenuated. The Lisse effect precisely reflects the phreatic level fluctuations before actual recharge. Furthermore, the fluctuations in the phreatic level and water level in the well and the contributions of airflow to phreatic level fluctuations are affected by many factors: rain intensity, initial moisture, overlying aquitard, groundwater table depths, and screen depths of the well.

In this study, a water-air two-phase flow model was employed to investigate the formation, extension, and dissipation of groundwater ridging induced by recharge events in a hypothetical hillslope-riparian zone, considering interactions between the liquid and gas phases in soil voids. The simulation results show that, after a rain begins, the groundwater table near the stream is elevated instantaneously and significantly, thereby generating a pressure gradient driving water toward both the stream (the discharge of groundwater to the stream) and upslope (the extension of groundwater ridging into upslope). Meanwhile, the airflow upslope triggered by the advancing wetting front moves downward gradually. Therefore, the extension of groundwater ridging into upslope and the downward airflow interact within a certain region. After the rain stops, groundwater ridging near the stream declines quickly while the airflow in the lower part of upslope is still moving into the hillslope. Thus, the airflow upslope mitigates the dissipation of groundwater ridging. Additionally, the development of groundwater ridging under different conditions, including rain intensity, intrinsic permeability, capillary fringe height, and initial groundwater table, was analyzed. Changes in intrinsic permeability affect the magnitude of groundwater ridging near the stream, as well as the downward speed of airflow, thereby generating highly complex responses. The capillary fringe is not a controlling factor but an influence factor on the formation of groundwater ridging, which is mainly related to the antecedent moisture. It was demonstrated that groundwater ridging also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.

The Yongding River (Beijing, China) was dry most times of the year, and groundwater storage was severely depleted. To address this issue, a river rehabilitation project was initiated. A downstream environmental flow release (EFR) project from upstream reservoirs has been implemented since 2019. This study evaluated the impact of EFR by constructing transient groundwater-flow and numerical tracer transport models to simulate the hydrogeological responses to the water release events in 2019–2020. The study identified two factors that significantly influence the river leakage rate, which are operational factors (i.e., water release rate and duration) and physical factors (i.e., hydraulic properties of the riverbed, regional hydraulic gradients, and groundwater depth) that determine the maximum water availability for groundwater recharge and maximum infiltration capacity, respectively. Predictive modelling was performed to assess the long-term effects of the proposed EFR scheme from 2021 to 2050, which showed that groundwater levels along the river will increase by 10–20 m by 2050. Groundwater storage is expected to be largely recovered and groundwater/surface-water connectivity in the middle reach of the river will be restored. This restoration will not only maintain the environmental flow for the benefit of ecosystems but also enhance groundwater recharge, promoting sustainable groundwater development in the region. Overall, this study provides valuable insights into the effectiveness of the proposed EFR scheme in achieving sustainable groundwater development in the region.

This study employed a coupled water‐air two‐phase flow and salt water transport model to analyze the behaviors of generated airflow in unsaturated zones and the fluctuations of salinity at the salt–fresh water interface in a two‐layered unconfined aquifer with a sloping beach surface subjected to tidal oscillations. The simulation results show that as the new dynamic steady state including effects of tidal fluctuations is reached through multiple tidal cycles, the dispersion zone in the lower salt water wedge is broadened because fresh water/salt water therein flows continuously landward or seaward during tidal cycles. The upper salt–fresh water interface exhibits more vulnerable to the tidal fluctuations, and the variation of salinity therein is periodic, which is irrelevant to the hydraulic head but is influenced by the direction and velocity of surrounding water‐flow. With the tidal level fluctuating, airflow is mainly concentrated in the lower permeable layer due to the restraint of the upper semi‐permeable layer, and the time‐lag between the pore‐air pressure and the tidal level increases with distance from the coastline. The effect of airflow in unsaturated zones can be transmitted downward, causing both the magnitude of salinity and its amplitude in the upper salt–fresh water interface to be smaller for the case with airflow than without airflow due to the resistance of airflow to water‐flow. Sensitivity analysis reveal that distributions of airflow in unsaturated zones are affected by the permeability of the upper/lower layer and the van Genuchten parameter of the lower layer, not by the van Genuchten parameter of the upper layer, whereas the salinity fluctuations in the salt–fresh water interface are affected only by soil parameters of the lower layer.

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