A mathematically continuous model for describing the hydraulic properties of unsaturated porous media over the entire range of matric suctions

Recent studies suggest that water flow in unsaturated porous media extends beyond the commonly known capillary-driven regime into the film regime. There is a need to develop the unsaturated hydraulic properties over the entire range of matric suctions to capture both flow regimes. In this study, Fredlund and Xing model is modified to represent the soil water retention curve from saturation to oven dryness. The new function is mathematically differentiable. The hydraulic conductivity function is composed of the capillary-driven term and film associated term, which is easy to apply. The new model has capacity to represent the bimodal hydraulic properties that are often present in structured and aggregated soils. Testing with the published data of sixteen soils shows good performance for both the water retention curve and the hydraulic conductivity function. For most soils, the new model results in a better agreement with observations than a published model. The result also indicates a possibility to improve the previously published film-associated hydraulic conductivity function.