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

Near-saturated hydraulic conductivity K-ns reflects the presence of structu ral 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 pref erential flow in dual or multiple pore domains. Thus, the aim of this study was to investigate the spatial dependence of K-ns. Steady state infiltrati on rates were measured at supply pressure heads ranging from -9.1 to -0.4 c m using tension infiltrometers, Twenty five measurement locations were site d on a regular 10 x 10-m grid with a 2-m spacing. An additional 12 measurem ent locations were placed in the center of the plot with the shortest lag b eing 0.5 m. Variability was expressed by a single parameter, the scale fact or, strictly following the Miller and Miller similar media theory, and with the reference function defined by the Mualem-van Genuchten 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 m acropores on conductivity near saturation. However, the spatial variability of K-ns did not depend on the soil water pressure head across the range me asured. The scale factors were approximately lognormally distributed, with a coefficient of variation (CV) of 53%. Variogram analysis showed a dear sp atial dependence of the scale factors within distances of at least 8 to 10 m and an uncorrelated variance (nugget) of approximate to 13% of the total variability. It was concluded that the macropores and mesopores, responsibl e for preferential water flow and solute transport near saturation, were no t randomly distributed across the plot but showed a well defined spatial st ructure.