Gravity-driven water flow into an initially dry porous medium can lead to a non-monotonic behavior. Instead of a uniform flow front, water infiltrates in finger patterns, which has been observed experimentally [1, 3, 4]. Fingering effect cannot be described by the Richards equation or standard two-phase flow system unless some additional terms are incorporated. One of the most effective approaches to model the observed saturation and pressure overshoot is to include the so-called non-equilibrium capillarity effect. Besides it, capillary or relative permeability hysteresis between imbibition and drainage processes also plays a key role in the degree of nonmonotonicity and its pattern.
In this work, we first performed a series of one-dimensional water infiltration experiments in dry soil. The experiments were conducted with different water flow rates and different initial saturations. Saturation was measured at one location using the gamma-transmission method, while water pressures were measured at multiple locations during the experiments. We employed two alternative approaches, referred to as the extended-standard (ESD) model and interfacial area (IFA) model, to simulate the experimental results.
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