Saturated hydraulic conductivity and soil water retention properties across a soil-slope transition

The hydraulic properties of soil and their spatial structures are important for understanding soil moisture dynamics, land surface and subsurface hydr ology, and contaminant transport. We investigated whether landscape feature s, including relative position on a slope, contribute to the variability of soil hydraulic properties in a complex terrain of a glacial till material. Using 396 undisturbed soil cores collected along two orthogonal transects, we measured saturated hydraulic conductivity (K-sat) and soil water retent ion functions at two (15 and 30 cm) depths across a glacial till landscape in central Iowa that encompassed two soil types (Nicollet loam with 1-3% sl ope on the hilltop position and Clarion loam with 2-5% slope on the shoulde r position). The van Genuchten-Mualem model was fitted to the experimental data using the RETC optimization computer code. At the 15 cm depth a statis tical comparison indicated significant differences in K-sat, saturated wate r content (theta (s)), water content at permanent wilting point (theta (15, 000)), and van Genuchten fitting parameters (alpha and n) between soil type s and landscape positions. At the 30 cm depth, theta (s), theta (333-15,000 ), and residual water content (theta (s)) were found to be significantly di fferent across the soil slope transition. Available water Content (theta (3 33-15,000)) did not show any significant difference across the soil-slope t ransition for either depth. No clear directional trend was observed, with s ome exceptions for K-sat, theta (s), and OL On specific transect limbs and depths. Drifts in the soil hydraulic parameters due to soil-slope transitio n were removed using a mean-polishing approach. Geostatistical analyses of these parameters showed several important characteristics including the fol lowing: (1) The spatial correlation lengths and semivariogram patterns of t he independently measured (or estimated) log(e) K-sat and theta (s) at 30-c m depth matched extremely well; (2) better spatial structures with large co rrelation lengths were observed for (macro and micro) porosity-related log( e) K-sat theta (s), and log, oc than for texture-related log(e) theta (333- 15,000), log(e) theta (15,000), theta (r), and log(e) n at 30-cm depth; and (3) a higher nugget effect at 15-cm depth was evident for most soil hydrau lic parameters, indicating tillage and other surface disturbances. These no vel findings may prove to be critical for modeling and interpreting field-s cale or larger-scale soil moisture dynamics, surface and subsurface flow, a nd solute transport.