Estimating Hydraulic Properties of Soil Aggregate Skins from Sorptivity and Water Retention

Skins of soil aggregates often consist of clayey or clay‐organic coatings which may affect preferential flow in aggregated soils. The objective was to determine hydraulic properties of samples with intact and removed (cut) skins and interior/skin hydraulic conductivity ratios for estimating mass transfer parameters in dual‐permeability models. Soil aggregates from the C sd ‐horizon of a clay‐loam glacial till soil (Stagnic Calcaric Regosol) were analyzed. A tension‐imbibition apparatus was used for measuring water uptake of multiple aggregates at boundary matric potential heads of −1 and −5 cm. Sorptivities were used to calculate mean weighted water diffusivities and final water contents to fit wetting retention functions. Water retention and hydraulic conductivity functions for the skin layer were derived from differences in water contents and hydraulic resistances between intact and cut samples. Water absorption rates were generally smaller for intact than for cut aggregates. The water retention function of cut was shifted towards smaller water contents compared with intact samples. Mean water diffusivity of intact was 4.5 times smaller than that of cut samples. The interior/skin ratio in unsaturated hydraulic conductivity was about 12 in the measured matric potential head range. The ratio was up to 70 near water saturation and dropped below unity for soil water potentials smaller −1000 cm of water. Aggregate skins may be regarded as a separate porous domain whose hydraulic properties may control water transfer between inter‐ and intraaggregate pore domains in structured soils.