Hydraulic conductivity estimation for soils with heterogeneous pore structure

Abstract

The hydraulic conductivity function, which is required to solve the Richards equation, is difficult to measure. Therefore prediction methods are frequently used where the shape of the conductivity function is estimated from the more easily measured water retention characteristic. Errors in conductivity estimations can arise either from an invalidity of the prediction model for a given soil, or from an incorrect description of the retention data. This second error source is particularly important for soils with heterogeneous pore systems that cannot be adequately described by the usually used retention functions. To describe the retention characteristics of such soils, a flexible θ(ψ) function was formed by superimposing unimodal retention curves of the van Genuchten (1980) type. By combining this retention model with the conductivity prediction model of Mualem (1976), conductivity estimations for soils with heterogeneous pore systems are obtained. Estimated conductivities by this model and the classical van Genuchten‐Mualem method can differ by orders of magnitude. Thus reported disagreements between measured and estimated conductivities may in some cases be due to an inadequate description of the retention data rather than due to a failure of the prediction model.

A review of model applications for structured soils: a) Water flow and tracer transport

“…With model parameters estimated from the observed drainage hydrograph, the KDW model described the outflow hydrographs accurately (Di Pietro et al, 2003). In another example, the Richards model with bimodal functions (Durner, 1994) performed similarly well as the DPM in describing water outflow from the bulk soil (Köhne and Mohanty, 2005).…”

“…Variably saturated water flow through such a porous system is uniform and at local equilibrium, and is often described using the Richards equation. Bimodal or multimodal hydraulic functions (e.g., Othmer et al, 1991;Wilson et al, 1992;Ross and Smettem, 1993;Durner, 1994;Mohanty et al, 1997) are an indirect way of representing soil structure in such a uniform flow approach. A one-domain PNE concept of infiltration (Fig.…”

“…Parameter fitting of retention and hydraulic conductivity measurements on soil cores (Othmer et al, 1991;Ross and Smettem, 1993;Durner, 1994;Coppola, 2000;Köhne et al, 2002b, Priesack andDurner, 2006) UC θ(h) and K(h) of fractured porous media Upscaling of modelled liquid configurations of cross-sections of partially saturated fractured porous soil or rock (Tuller and Or, 2002;Or and Tuller, 2003) UC Moreover, it was suggested to direct more effort to derive pedotransfer functions accounting for soil structure effects (Merdun and Quisenberry, 2004;Pachepsky et al, 2006). The relation between lateral dispersivity, unsaturated hydraulic conductivity, and soil morphological properties, was studied in tracer experiments carried out in columns and field plots with key Belgian soil types (Vanderborght et al, 2001).…”

Migration of contaminants through the unsaturated zone overlying the Hesbaye chalky aquifer in Belgium: a field investigation

“…A new formulation, close to that of Ross and Smetten (1993) or Durner (1994), is used to model the unsaturated properties of the fissured chalk to consider with greater accuracy the bi-modal distribution of the chalk porosity (matrix porosity and fissure porosity). To account for the variation of effective porosity according to recharge conditions and saturation degree of the chalk, a partitioning algorithm similar to that proposed by Zurmühl and Durner (1996) is used.…”

Solute contaminant transport in variably saturated dual-porosity/dual permeability chalk: field tracer experiments and modelling

“…The retention curve of the chalk is modelled using the combination of two van Genuchten relationships, one defined for the matrix, one for the fissures. This model differs from that of Ross and Smettem (1993) or Durner (1994) by the fact that it forces a continuously derivable transition between the relations that describe the matrix and the fissure component respectively (Fig. 2a).…”