The Green‐Ampt and Philip infiltration models are simplified with a capillary tube representation of porous media that can predict wetting front movement during horizontal and vertical infiltration into dry or initially moist soil. The simple model captures the fundamental physics of unsaturated fluid flow and reduces the number of parameters that must be measured. The relevant parameters are the air‐liquid interfacial tension, the density and viscosity of the liquid, the initial saturation, and the permeability. Comparisons with published data show good predictions for infiltration under nonnegative source pressures but are less successful under negative source pressures. The model applies to imbibition of both water and nonaqueous phase liquids.

Horizontal infiltration experiments were performed to validate a plug flow model that minimizes the number of parameters that must be measured. Water and silicone oil at three different viscosities were infiltrated into glass beads, desert alluvium, and silica powder. Experiments were also performed with negative inlet heads on air-dried silica powder, and with water and oil infiltrating into initially water moist silica powder. Comparisons between the data and model were favorable in most cases, with predictions usually within 40% of the measured data. The model is extended to a line source and small areal source at the ground surface to analytically predict the shape of two-dimensional wetting fronts. Furthermore, a plug flow model for constant flux infiltration agrees well with field data and suggests that the proposed model for a constant-head boundary condition can be effectively used to predict wetting front movement at heterogeneous field sites if averaged parameter values are used.