Quantitative Relationships between Soil Macropore Characteristics and Preferential Flow and Transport

Quantitative relationships between soil structure, especially macropore characteristics, and soil hydraulic properties are essential to improving our ability to predict flow and transport in structured soils. The objectives of this study were to quantitatively relate macropore characteristics to saturated hydraulic conductivity (K ₛₐₜ) and dispersivity (λ) and to identify major macropore characteristics useful for estimating soil hydraulic properties under saturated condition. Large intact soil columns were taken from two land uses (cropland and pasture) of the same soil type (a Typic Hapludalf), with four replicates for each land use. The soil columns were scanned using X-ray computed tomography (CT) to obtain macropore parameters including macroporosity, length density, mean tortuosity, network density, hydraulic radius, path number, node density, and mean angle. The K ₛₐₜ of the whole soil column, as well as each soil horizon within the column, and solute breakthrough curve (BTC) of CaBr₂ were determined for each column. For all eight soil columns studied, macroporosity and path number (the number of independent macropore paths between two boundaries) explained 71 to 75% of the variability in the natural logarithm of K ₛₐₜ values of the whole soil columns as well as of individual soil horizons. The traditional convection–dispersion equation (equilibrium model) simulated the BTCs well for all soil columns except one with an earthworm hole passing through the entire column, for which the two-region model (non-equilibrium model) was required. The path number, hydraulic radius, and macropore angle were the best predictors for λ, explaining 97% of its variability. Correlation between λ of the whole soil columns and K ₛₐₜ values of the Bt horizons (but not A horizons) implied that the dispersivity was mainly controlled by the horizon with the lowest K ₛₐₜ in the soil columns. These results indicate that the most useful macropore parameters for predicting flow and transport under saturated condition in structured soils included macroporosity, path number, hydraulic radius, and macropore angle.