## 1. Introduction

[2] The description of soil-water transport processes in unsaturated porous media often hinges on the adequacy of the highly nonlinear functions that represent water retention and hydraulic conductivity. The literature abounds with parametric models that describe both water retention and hydraulic conductivity data. These models range from simple mathematical expressions for unstructured media, with unimodal pore size distributions [*Leij et al.*, 1997], to more complex formulations for structured media with multimodal pore size distributions [*Othmer et al.*, 1991; *Ross and Smettem*, 1993; *Durner*, 1994]. Models have also been proposed to describe water retention [*Khlosi et al.*, 2008] and hydraulic conductivity [*Peters and Durner*, 2008] over the entire range of matric head. In many cases, however, measurements of unsaturated hydraulic conductivity are unavailable. As an alternative to direct measurements, models have been proposed to estimate hydraulic conductivity from water retention data, which is more easily measured. In general, these models may be classified as those based on (1) similarity between Darcy-Buckingham's law and other viscous flow equations such as those of Navier-Stokes and (2) statistical representations of pore space [*Brutsaert*, 1967; *Mualem*, 1986]. It follows that the models either conceptualize the pore space as cylindrical tubes of uniform size, with coexisting wetting and nonwetting fluids, or as assemblies of cylindrical capillary tubes of various sizes, which are either completely filled with wetting or nonwetting fluid. As such, these models have assumed that liquid configuration is the same in both the wet and dry ranges. Although it has generally been found that these models are successful in the wet to moderately wet range (high values of matric head), where water is mostly held by capillary forces, they have been found to underestimate hydraulic conductivity under dry conditions [*Goss and Madliger*, 2007; *Jansik*, 2009]. As pointed out by *Tuller and Or* [2001], this shortcoming may be attributed to the lack of consideration of soil-water flow in thin films, likely to become important at low values of matric head.

[3] Motivated by this and other limitations, such as the lack of consideration of adsorptive forces and unrealistic cylindrical representation of pore space, *Tuller and Or* [2001] proposed an alternative approach for the derivation of hydraulic conductivity functions for homogeneous porous media. Their model is based on liquid configurations in both angular and slit-shaped spaces, and accounts for capillary, corner and thin film flows. Although of great scientific interest, the model is mathematically complex and must be used in conjunction with the water retention model of *Or and Tuller* [1999], which often fails to describe experimental data in the intermediate saturation range because of the limited flexibility of the probability distribution function used to characterize the pore size distribution. *Tokunaga* [2009] also used the principles of interfacial physical chemistry and the concept of flow in thin films to predict water transport in dry monodisperse porous media.

[4] In this study, theoretical considerations and experimental observations are used to partition water retention into capillary and adsorptive components. These components are associated with two different liquid configurations. The first is an assembly of water or air filled capillary tubes of various diameters whereas the second is a thin film of water stretched over the surface of the solid particles. A conventional statistically based model is used to predict hydraulic conductivity within the water filled capillaries while a theoretical model of soil-water flow in thin films is used to predict hydraulic conductivity within the thin liquid films. In so doing, the model accounts for both capillary and adsorptive forces as well as for dual occupancy of wetting and nonwetting phases.

[5] The paper shows the effectiveness of the new model in using water retention data to predict hydraulic conductivity over the entire range of matric head. Model performance is subsequently evaluated by comparing results with those of other well-supported models.