Hydraulic conductivity of stratified unsaturated soils: Effects of random variability and layering

For simulating flow in heterogeneous porous media it is computationally more efficient to define an equivalent effective (i.e., upscaled) medium rather than considering detailed spatial heterogeneities. In this paper, the effective unsaturated hydraulic conductivity (K) of soils exhibiting random variability, layering, or both is calculated based on numerical simulations of steady-state evaporation from a shallow water table. It is demonstrated that the effective K of randomly-varied coarse-textured soils generally falls between the harmonic and geometric means of the unsaturated hydraulic conductivities of the constituting soils. Layering and random variability when occurring concurrently magnify each other's effects on effective K. As a result, the higher the degree of heterogeneity, the lower the effective K. Therefore, neglecting either random spatial variability or layering in numerical simulations can lead to significant overestimation of water flow in soils.