Anisotropic Variation of Profile Characteristics and Saturated Hydraulic Conductivity in an Ultisol Landscape

Oriented soil horizons and macropore features may preferentially deflect water flow into horizontal or vertical paths that could short circuit the conventionally modeled matrix flow process and lead to an underestimation of water flow velocities and discharge rates. The objective of this study was to evaluate the anisotropy of soil physical and hydraulic properties of an Ultisol polypedon with respect to landscape position and horizonation in the dissected piedmont of the southeastern USA. Vertical and horizontal saturated soil hydraulic conductivity (KV and KH, respectively) were measured using undisturbed cores (76 mm long by 76 mm diam.) taken within soil horizons and at horizon boundaries in profiles at the interfluve, shoulder, linear, and footslope landscape positions. The field was mapped as Cecil (clayey, kaolinitic, thermic Typic Kanhapludult). The observed decreases in both KV and KH with soil depth at all landscape positions were associated with increases in bulk density, total porosity, and clay content and decreases in macroporosity, sand content, and percent soil solids >2 mm diam. Macroporosity had a strong direct effect on KV and KH in all multiple regression equations, but the influence of all soil properties on KV and KH varied with soil core orientation and landscape position. Maximum reductions in KV and KH, with respect to KV and KH of the horizon immediately above it, occurred in the soil profile where the most abrupt changes in soil physical properties occurred. The KV/KH ratio was <1.0 at the A horizon boundary at the interfluve and for all horizons at the linear slope position, indicating the potential for water flow in the horizontal direction when the soil is saturated. Models of water movement in complex landscapes must account for lateral flow due to differences in vertical and horizontal hydraulic conductivity. Contribution from the Soil Science Dep., North Carolina State Univ. Research supported by the North Carolina Agric. Res. Serv.