Macroscopic Unsaturated Soil Hydraulic Conductivities.

This study investigates equivalent hydraulic conductivities for unsaturated vertical flows under two distinct soil layering scenarios, layers parallel to flows (i.e., parallel vertical slabs/columns) and layers perpendicular to flows (i.e., horizontal layers). The two scenarios are referred to as horizontal heterogeneity scenario and vertical heterogeneity scenario respectively. The main objectives of this study are to: (1) examine the appropriateness of using simple arithmetic and harmonic means of hydraulic conductivity for unsaturated flows for the horizontal heterogeneity and vertical heterogeneity scenarios, and (2) investigate the equivalent unsaturated hydraulic conductivities that are applicable to large scale hydrologic process modeling. The considered domain is typical for unsaturated flow which lies between the ground surface on the top and the groundwater table at the bottom. The heterogeneous soils consist of large numbers of either vertical slabs/columns for the horizontal heterogeneity scenario or horizontal layers for the vertical heterogeneity scenario. Results demonstrate that the arithmetic mean for horizontal heterogeneity scenario and the harmonic mean for vertical heterogeneity scenario, extended from the saturated flow situations, would introduce large relative errors in simulating the actual flux for coarser textured soils and for heterogeneous soils with large hydraulic parameter variances. The optimal equivalent hydraulic conductivities for both horizontal and vertical heterogeneity scenarios also depend on the capillary pressure head conditions at the ground surface. It is more challenging when the capillary pressure head at the ground surface is higher, which is typically the case for evaporation.