A fractal hydraulic model for water retention and hydraulic conductivity considering adsorption and capillarity

Commonly used hydraulic models that describe the water retention and transport properties of variably saturated soils mainly account for the capillary component of matric potential but ignore the adsorption component. Experimental evidence suggests that soil hydraulic conductivity may be underestimated by several orders of magnitude in the low saturation range for models ignoring the adsorbed film flow. Thus, this paper introduces a fractal method to characterise the effects of capillary and adsorption components of matric potential on soil hydraulic properties considering a simplified pore water transport process. Furthermore, the concept of specific thickness for the adsorbed film was defined to describe the adsorption and adsorption capacity of the soil. Based on these, a complete hydraulic model, including the soil water retention curve (SWRC) and relative hydraulic conductivity curve (RHCC), was derived to describe the water retention and transport properties of the soil in the entire matric head range. The performance of the proposed model was then evaluated using 13 sets of test data, from clean sand to bentonite, showing that it can capture the characteristics of water retention and transport properties well. The model parameters calibrated from the SWRC dataset could predict RHCC over the entire suction range without introducing any additional parameters. Additionally, the influences of the model parameters on soil hydraulic behaviours are also discussed in the paper. It was found that the pore size distribution of the soils mainly controlled the transport characteristics of capillary water, but had a slight influence on the hydraulic properties of adsorbed water, which was dominated by the type and content of minerals and specific surface area of the soil.