Analytical solutions on three subdivided stages indicate the permeability and thickness of the cap layer, as well as permeability and porosity of the media in the vadose zone are the key controlling factors on the process of coupling of air flow and water flow. In each experiment, NP or PP increases rapidly to reach a maximum in the early stage, and gradually becomes zero in the late stage. Negative pressure (NP)/positive pressure (PP) of the air in the vadose zone is observed in the drainage/injection experiments, with higher pressure in experiments of thicker cap layer. Different cap thickness (2 cm, 5 cm, 7.5 cm) and different thickness of the vadose zone (30 cm, 40 cm) are set for the experiments. Water injection and drainage experiments are conducted in a double-layered sand column with a thick layer (80.5 cm) of coarse sand and a thin layer of fine sand as a low-permeability cap. In this study, a conceptual model is established for coupling of air flow and water flow in the vadose zone in response to rapid fluctuations of the water table. When the unconfined aquifer is covered by a low-permeability media, the coupling of the water flow and the air flow is more obvious. In turn, the air flow can interact with the water flow.
The rise and fall of the groundwater level can drive air flow in the vadose zone.
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