Anisotropy of soil water diffusivity of hillslope soil under spruce forest derived by X-ray CT and lab experiments

The idea of the study is to indicate direction-dependent differences in hydraulic conductivity, K (Se), and soil water diffusivity, D ( θ ), as function of the volume fraction related to the fractional capillary potential for each of the characteristic pore size classes by extended anisotropy factors. The study is exemplary focused on a BwC horizon of a Dystric Cambisol under spruce forest formed on the weathered and fractured granite bedrock in the mountainous hillslopes Uhlirska catchment (Czech Republic). Thus, undisturbed soil samples were taken in vertical (0°, y = x -axis) and horizontal (90°, z -axis) direction. The D ( θ ) values and especially the D ( θ )-weighted anisotropy ratios showed that anisotropy increases with the volume fraction of macropores, MaP ( d > 0.03 mm), with r 2 between 0.89 and 0.92. The X-ray computer tomography (CT) based anisotropy ratio ( A CT ) is larger for the horizontal sampled soil core with 0.31 than for the vertical with 0.09. This underlines the existence of a predominantly horizontally oriented pore network and the fact that weathered bedrock strata can initiate lateral preferential flow. The study results suggest that combining the hydraulic conductivity as intensity and the capacity parameter by means of diffusivity results in an extended anisotropy ratio which unveils the role of the soil hydraulic characteristics in generation of small-scale lateral preferential flow. In future, the small-scale direction-dependent differences in the soil hydraulic capacity and intensity parameter will be used for model-based upscaling for better understanding of preferential flow at the catchment scale.