Virtual Soil Simulator—Finite Volume Method Implementation for the Unsaturated Porous Media Water Transport Model Including Film Flow and Isothermal Vapor Transport Phenomena

Abstract Modeling unsaturated water transport in porous media traditionally relies on the Richards equation (RE) with hydraulic conductivity formulations to consider only capillary liquid water flow. However, this simplification neglects thin water films adsorbed onto particle surfaces, which can contribute significantly to flow under medium to dry conditions. Recent developments, notably the Peters–Durner–Iden (PDI) model, allow the consistent inclusion of film flow and isothermal vapor transport in unsaturated hydraulic conductivity formulations. In this study, we present Virtual Soil Simulator, an OpenFOAM‐based finite volume implementation of the RE enhanced with the PDI model to incorporate capillary, film, and vapor flow mechanisms. Model verification employed benchmark problems with analytical and established numerical solutions, including 1D infiltration, high‐gradient infiltration, and 2D nonlinear infiltration tests, demonstrating high accuracy and accurate mass‐balance behavior. The capabilities of the model are further illustrated by two specific cases. First, inverse modeling of a 12‐day soil core drying experiment revealed that the classical RE model matched observations only during the initial wet phase, whereas the PDI‐based model maintained agreement throughout and accurately captured the drying in film‐ and vapor‐dominated regimes. Second, a synthetic desaturation experiment over 467 soils revealed that film flow substantially accelerates drainage, with pronounced effects even at relatively high pressure heads (−10 m). Results confirm that omitting film flow can lead to substantial underestimation of unsaturated hydraulic conductivity and a misrepresentation of drying or drainage dynamics. Furthermore, simulations indicate that at, very low pressure heads, accurate modeling requires the inclusion of both film and vapor transport components.