Modeling Water Movement in Heterogeneous Water‐Repellent Soil: 1. Development of a Contact Angle–Dependent Water‐Retention Model

In the past, hydrophobic soils have been associated mostly with preferential flow phenomena. It has become increasingly evident that besides the phenomena of extremely (hydrophobic) water‐repellent soils, soils with reduced wettability are more the rule than the exception. Despite the extensive literature on the hydraulic behavior of water‐repellent soil, a conceptual model flexible enough to describe typical behavior of wettable and hydrophobic soils as well as for soils with intermediate wetting properties is still missing. We propose a water‐content and time‐dependent contact angle (CA) model that was used as an extension of the van Genuchten equation for the capillary pressure–saturation (CPS) relationship. This model is based on conventional retention parameterizations; that is, the hydrophilic soil is considered as a special case of the general wetting model. Hydrophobic soils as well as soils with subcritical (reduced wettability, not hydrophobic) water repellency are represented by this model. Conceptually, the proposed model links microscopic interfacial properties, indicated by the CA, with the macroscopic hydraulic model mainly by modifying the α of the van Genuchten equation. The modification basically accounts for hysteresis of the main drainage–main wetting branch of the CPS relationship. Compared with conventional hydraulic models, only a few more parameters are needed to describe the wettability extension of the model: mean maximum and minimum CAs and their autocorrelation functions. Additionally, characteristic rewetting time and a breakthrough pressure function are needed for a complete description of the hydraulic properties of the soil. This extended hydraulic model serves as a base for a simulation study in unsaturated soil with reduced wettability.