Generalized hydraulic conductivity model for capillary and adsorbed film flow

Commonly used transport models of unsaturated flow assume that the movement of pore water is dominated mainly by capillary flow and they neglect adsorbed film flow. These models have been proven to be successful at high and intermediate saturations but typically underestimate the hydraulic conductivity in the dry range, where water movement in equilibrium conditions is dominated by adsorbed film flow. Given these considerations, this paper proposes a simplified configuration of pore water that accounts for the transport processes of both capillary and film flow. Based on the mechanisms of soil water retention, a conception of the specific thickness of the adsorbed film is defined to describe the adsorption strength and adsorption capacity of porous media. Furthermore, a statistical physically based model of relative hydraulic conductivity in the full range of suction is derived. Fractal and Monte Carlo methods are used to determine the pore size distribution of porous media and then the corresponding specific model of relative hydraulic conductivity is derived. The results show that the proposed model agrees well with the experimental data in the entire suction range. It is also found that the pore size distribution of porous media controls the transport characteristic of capillary water but not adsorption film flow which is only related to the mineral content, mineral species, and specific surface area. Additionally, the influences of the model parameters on the transport of porous media are also addressed.