Use of similar media scaling to characterize spatial dependence of near-saturated hydraulic conductivity

Near-saturated hydraulic conductivity Kns reflects the presence of structural macropores and mesopores that, in turn, determine the pathways of water infiltration and solute fluxes in soil. Little is known about the spatial dependence of structural pore space in soil, although such information will be required by stochastic models of solute transport that account for preferential flow in dual or multiple pore domains. Thus, the aim of this study was to investigate the spatial dependence of Kns. Steady state infiltration rates were measured at supply pressure heads ranging from -9.1 to -0.4 cm using tension infiltrometers. Twenty five measurement locations were sited on a regular 10×10-m grid with a 2-m spacing. An additional 12 measurement locations were placed in the center of the plot with the shortest lag being 0.5 m. Variability was expressed by a single parameter, the scale factor, strictly following the Miller and Miller similar media theory, and with the reference function defined by the Mualem-van Genuch-ten model. Hydraulic conductivity increased about three orders of magnitude across the pressure head range from -9.1 to -0.4 cm, confirming the strong influence of soil macropores on conductivity near saturation. However, the spatial variability of Kns did not depend on the soil water pressure head across the range measured. The scale factors were approximately lognormally distributed, with a coefficient of variation (CV) of 53%. Variogram analysis showed a clear spatial dependence of the scale factors within distances of at least 8 to 10 m and an uncorrelated variance (nugget) of 13% of the total variability. It was concluded that the macropores and mesopores, responsible for preferential water flow and solute transport near saturation, were not randomly distributed across the plot but showed a well defined spatial structure.