Soil hydraulic conductivity function based on specific liquid–vapor interfacial area around the soil particles

Water in soils flows through a tortuous path which depends on the specific liquid–vapor interfacial area around the soil particles (Aws) and tortuosity factor (ξ). This specific area is defined as the area of liquid–vapor interfacial around the soil particles per unit bulk volume of soil. The objective of this study was to develop a new approach for determining unsaturated hydraulic conductivity using soil water characteristic function based on the specific liquid–vapor interfacial area around the soil particles. In this study, the capillary rise equation has been modified by Aws. The Aws was determined from the soil water retention curve. Tortuosity factor or the ratio of actual path length to the straight path length of flow was estimated based on Aws by trail and error procedure from soil water retention curve and measurement of saturated soil hydraulic conductivity. Therefore, a new soil hydraulic conductivity function (K–θ) was derived based on soil water retention curve, Aws and ξ in laminar flow conditions. In this approach, variable liquid viscosity near the soil particles was considered and a tortuous undulating wide-open channel with small and variable water depth was assumed. The resulted curves of K–θ by derived equations were compared with experimental K–θ data for 19 different soils from sandy to clayey textures obtained from literature. The results of comparison show that the estimated K–θ and D–θ (D is soil hydraulic diffusivity) relationships are very close to those reported in literature for the 19 selected soils.