Stochastic simulation of three-dimensional unsaturated flow in water repellent heterogeneous soil

It has been established that spatial variability of material properties can lead to distinct unstable hydraulic behaviour, and that this is prominent in soils due to their large spatial heterogeneity. This characteristic can lead to non-uniform wetting behaviour and is particularly prominent when the wettability of the medium is also non-uniform. In water repellent soil, its wettability is often spatially varying, such that a network of flow paths is created where fluid can move preferentially, leading to fingered flow patterns. In this study, the development of a model to represent moisture transport in hydrophobic soil is presented. Local spatial variations in material properties are represented by Gaussian random fields as part of a stochastic finite element based model. Key components of the model include an approach to represent the transition region between wettable and non-wettable layers, and the adoption of a suitable saturation–capillary pressure relationship. For wettable soil, this can be achieved with the standard van Genuchten relation. For hydrophobic soil, this is not applicable; thus, an alternative is employed. The model is then validated against field-scale experimental observations by Lipsius and Mooney (2006) , which examined the impact of soil heterogeneity on infiltration profiles. The results demonstrate the model’s ability to capture complex flow dynamics in hydrophobic soils, extending the understanding of moisture transport in heterogeneous soils by explicitly modelling the spatial variability of wettability and its impact on soil hydraulic response.