Relationship between Anisotropy in Soil Hydraulic Conductivity and Saturation

Anisotropy in unsaturated hydraulic conductivity is saturation-dependent. Accurate characterization of soil anisotropy is very important in simulating flow and contaminant transport (e.g., radioactive nuclides at the U.S. Department of Energy's Hanford Site). A tensorial connectivity-tortuosity (TCT) concept describes the hydraulic conductivity tensor of the unsaturated anisotropic soils as the product of a scalar variable, the symmetric connectivity tortuosity tensor, and the saturated hydraulic conductivity tensor. In this study, a model based on the TCT concept is used to quantify soil anisotropy in unsaturated hydraulic conductivity. The TCT model can describe different types of soil anisotropy; for example, the anisotropy coefficient can monotonically increase or decrease with saturation and can vary from greater than unity to less than unity and vice versa. Soil anisotropy is independent of soil water retention properties and can be characterized by the ratio of the saturated hydraulic conductivities and the difference of the tortuosity-connectivity coefficients in two directions. The anisotropy coefficient is log-linearly proportional to the effective saturation. This log-linear relationship allows the saturation-dependent anisotropy to be determined using regression with the measurements of the directional hydraulic conductivities at a minimum of two water content levels, one of which may be at full saturation. The model was tested using measured or simulated directional hydraulic conductivities.