Explaining Air and Water Transport in Undisturbed Soils By X-Ray CT Derived Macroporosity and CT- Number-Derived Matrix Density.

The characterization of soil pore space geometry is important to predict the fluxes of air, water and solutes through soil and understand soil hydrogeochemical functions. X-ray computed tomography (CT) -derived parameters were evaluated as predictors of water, air and solute transport through soil. Forty five soil columns (20-cm × 20-cm) were collected at an agricultural field in Estrup, Denmark. The soil columns were scanned in a medical CT-scanner. Subsequent to this, non-reactive tracer leaching experiments were performed in the laboratory together with measurements of air permeability (K_a), and saturated hydraulic conductivity (K_sat). The CT number of the matrix (CT_matrix) was obtained from the CT scans by calculating the average CT number of the voxels in the greyscale image excluding macropores and rocks. CT_matrix was the best predictor of the five percent arrival time (t_0.05) which is considered a robust indicator of preferential flow. Meanwhile, CT-derived limiting macro-porosity was the best predictor for K_a and log₁₀K_sat. A best subsets regression analysis was performed combining macroporosity, limiting macroporosity and CT_matrix. The predictions of water and air flow improved using CT_matrix and limiting macroporosity, while the best model for t_0.05 used only CT_matrix. The resolution of the scanning and the time for soil structure development after mechanical activities can be factors that increased the uncertainty of the proposed models. Nevertheless, the results confirmed the potential of X-ray CT visualization techniques for estimating fluxes through soil.