Spatial Variability of Wet-Range Soil Hydraulic Conductivity As Affected by Land Use.

A precise characterization of the spatial variability of saturated and unsaturated hydraulic conductivities within and across different land-use systems is required in hydrological models as well as site-specific transport modeling. This study was conducted to investigate the spatial behavior of soil hydraulic conductivities and their relationship to soil structure as affected by land use. A 4125m² field located in Lexington, KY, was divided into two established systems, cropland (wheat) and grassland. Hydraulic conductivities, K(h), at three soil water pressure heads of h=-1, -5 and -10cm were measured using tension infiltrometers along the periphery of a rectangle at every 5m; in the middle of each side, a 5-m nest was investigated at 1-m intervals. Saturated hydraulic conductivity, Ks, and the slope of lnK(h), α, were calculated based on Wooding’s equation. K(-10) and K(-5) did not differ significantly between the two land-use systems; while K(-1), Ks, α, and water aggregate stability (WAS) were higher in grassland than those in cropland. Additionally, K(-1) and Ks were significantly correlated with WAS. The typically denser root systems and higher organic matter in grassland were probably beneficial for the formation of aggregates, exerting higher macroporosity contributing to greater transport efficiency at or close to saturation. The spatial correlation ranges and the semivariances of K(-10) and K(-5) in grassland were larger than those in cropland. Other hydraulic properties, i.e., K(-1), Ks and α in cropland were spatially dependent, too, with a similar range of around 7.8m. The semivariances of WAS in the nests were much higher than those observed for the 5-m intervals, resulting in distinct scale-variant spatial structures. At the larger scale, a correlation range of 38.6m was observed. The longer range and the scale-variant distribution pattern of WAS may contribute to the lack of its spatial correlation with hydraulic properties.