Soil hydraulic conductivity function based on specific liquid–vapor interfacial area around the soil particles Sh. Zand-Parsa, A.R. Sepaskhah * Department of Irrigation, College of Agriculture, Shiraz University, Shiraz 71365 I.R., Iran Received 15 November 2000; accepted 17 July 2003 Abstract Water in soils flows through a tortuous path which depends on the specific liquid –vapor interfacial area around the soil particles (A ws ) and tortuosity factor (n). 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 A ws . The A ws 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 A ws 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 – h) was derived based on soil water retention curve, A ws and n 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 – h by derived equations were compared with experimental K – h data for 19 different soils from sandy to clayey textures obtained from literature. The results of comparison show that the estimated K – h and D – h (D is soil hydraulic diffusivity) relationships are very close to those reported in literature for the 19 selected soils. D 2003 Elsevier B.V. All rights reserved. Keywords: Unsaturated hydraulic conductivity; Hydraulic diffusivity; Soil water retention curve; Tortuosity; Liquid– vapor interfacial area 1. Introduction Unsaturated water flow in soils is described by partial differential equations in which unsaturated hydraulic conductivity varies with soil water content, h, or soil water pressure head, h [i.e., K(h) or K(h)]. Before any water flow problem can be solved math- ematically, the relationship between K and h or h must be known. In the past, various models have been used to predict the unsaturated hydraulic conductivity based on h or h. These models can be divided into two groups. The first group is based on a generaliza- tion of the Kozeny (1927) approach for saturated and unsaturated porous media, according to which the relative hydraulic conductivity, K r [the ratio of K(h) to saturated hydraulic conductivity] is a power func- tion of saturation degree (h/h s ), where h s is the soil saturated water content (Hillel, 1998; Brutsaert, 1967). The second group uses h – h relationship (i.e., 0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0016-7061(03)00258-1 * Corresponding author. Tel.: +98-71-20101-2; fax: +98-71- 48093. E-mail addresses: zandparsa@yahoo.com (Sh. Zand-Parsa), sepas@hafez.shirazu.ac.ir (A.R. Sepaskhah). www.elsevier.com/locate/geoderma Geoderma 119 (2004) 143 – 157