A Soil Hydraulic Conductivity Equation Incorporating Adsorption and Capillarity

A Simple Model for Estimating the Hydraulic Conductivity of Unsaturated Soil

“…As the suction further increases, the filled water in the large pores becomes discharged and replaced by air, a large amount of pendular water appears between the soil particles, and the soil enters the transition zone. Many previous studies have shown that the hydraulic conductivity in the transition zone decreases almost linearly with the suction in the log-log scale [11,25,31]. As the suction exceeds the residual one, the water in the soil mainly exists in the form of an adsorptive water film, and the adsorption will dominate the soil-water interactions in this range.…”

“…In this adsorptive dominant region, the thickness of the water film decreases with increasing suction, and the hydraulic conductivity is recognized to be decreasing linearly with increasing suction in the log-log scale as well [26,29,30]. In addition, when the suction is smaller than the transition suction Ψ f , the hydraulic conductivity is entirely dependent on the capillary flow [31]. [25,31,33]).…”

“…Two distinct soil-water interaction mechanisms exist in unsaturated soil: capillarity and adsorption. The capillarity results from the difference between the inner water pressure and the outer air pressure (as caused by the surface tension), and the adsorption stems from four interactions involving the water film and the soil particles: electric double layer, van der Waals, and the surface and cation hydration potentials [31]. The electric double layer and van der Waals potential are commonly known as ionic-electrostatic and molecular potential, respectively [25].…”

“…To improve the estimation of the hydraulic conductivity, which is dominated by an adsorptive water film flow in the high suction range, Tokunaga [23,24] derived a hydraulic conductivity model based on the planar thin film dynamics. Since this model has explicit physical meaning, many studies have combined it with statistical models to predict the hydraulic conductivity over a wide suction range [25][26][27][28][29][30][31]. Lebeau and Konrad [25] adopted the Mualem model to account for the capillary flow, and constructed a new equation for calculating the adsorptive water flow which included the effects of ionic-electrostatic forces and molecular forces.…”

“…Wang et al [26][27][28] used the hydraulic conductivity at the critical suction point as a reference in simplifying the model of water film flow, and they also used the Mualem model to calculate the capillary flow. Gou et al [31] proposed a more precise model accounting for the adsorptive water film based on micromechanics, and they used the Mualem model to predict the capillary flow as well. These models have a good ability to estimate the hydraulic conductivity over a wide suction range.…”