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

Abstract

The hydraulic properties of soil and their spatial structures are important for understanding soil moisture dynamics, land surface and subsurface hydrology, and contaminant transport. We investigated whether landscape features, 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 (Ksat) and soil water retention 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% slope on the hilltop position and Clarion loam with 2–5% slope on the shoulder position). The van Genuchten‐Mualem model was fitted to the experimental data using the RETC optimization computer code. At the 15 cm depth a statistical comparison indicated significant differences in Ksat, saturated water content (θs), water content at permanent wilting point (θ15,000) and van Genuchten fitting parameters (α and n) between soil types and landscape positions. At the 30 cm depth, θs, θ15,000, and residual water content (θr) were found to be significantly different across the soil‐slope transition. Available water content (θ333–15,000) did not show any significant difference across the soil‐slope transition for either depth. No clear directional trend was observed, with some exceptions for Ksat, θs, and α on specific transect limbs and depths. Drifts in the soil hydraulic parameters due to soil‐slope transition were removed using a mean‐polishing approach. Geostatistical analyses of these parameters showed several important characteristics including the following: (1) The spatial correlation lengths and semivariogram patterns of the independently measured (or estimated) loge Ksat and θs at 30‐cm depth matched extremely well; (2) better spatial structures with large correlation lengths were observed for (macro and micro) porosity‐related loge Ksat, θs, and loge α than for texture‐related loge θ333–15,000, loge θ15,000, θr and loge n at 30‐cm depth; and (3) a higher nugget effect at 15‐cm depth was evident for most soil hydraulic parameters, indicating tillage and other surface disturbances. These novel findings may prove to be critical for modeling and interpreting field‐scale or larger‐scale soil moisture dynamics, surface and subsurface flow, and solute transport.