Hydraulic modeling of water flow in the thick vadose zone under precipitation

Data from field monitoring and artificial rainfall experiments suggest that the thick vadose zone can be divided into two sub-zones based on soil water variation, namely the active and steady zones. The soil water content of the top active zone (2-5 m depth) is sensitive to precipitation and evaporation and dominated by transient water flow. Soil water content of the underlying steady zone remains constant over time and there is a steady flow under the force of gravity. However, since the transition from transient flow to steady flow is difficult to observe in nature, the physical mechanism of this transition remains poorly understood. This study establishes a hydraulic model to visually demonstrate water flow in the entire vadose zone under multiple infiltration events. The model comprises of a series vertically aligned water tanks, each with a small outlet at the bottom, and each representing a soil unit. The water level in a tank represents the water content and the related permeability of the soil unit. The results of an experiment conducted with the model clearly show that transient flow in the upper active zone will transfer to steady flow. A zoomed out data with an annual rainfall record at a site in the central Chinese Loess Plateau is applied in the model to simulate the water content and the flow state of the vertical profile, and the results are in accordance with in-situ monitoring data. The outcomes of this study suggest that although water content in the steady zone remains unchanged, there is a constant steady flow seeping downward through the zone, acting as a typical source of groundwater recharge in the loess region.