Although soil structure and pore geometry characteristics largely control flow and transport processes in soils, there is a general lack of experiments that study the effects of soil structure and pore‐space characteristics on air and water permeability. Our objective was to determine the dependency of soil permeability on fluid content for both water and air, and compare results for both disturbed (D) and undisturbed (UD) soils. For that purpose, we first measured the water permeability (kw) and air permeability (ka) for several intact UD soil samples. Subsequently, the same samples were crushed and repacked into the same soil cores to create the D equivalent for the same soil material. Measurements showed large differences between D and UD samples, confirming the enormous impact of soil structure and pore‐space characteristics on flow. The permeability of both fluid phases (air and water) was greatly reduced for the D samples, especially for soil air permeability due to its greater dependency on soil aggregation and structure. Soil water retention and permeability data were fitted to Campbell's and Mualem's pore‐size distribution model, respectively. Regardless of soil disturbance, we showed that the tortuosity–connectivity parameter, l, for the water permeability (l1) and air permeability (l2) were different. This is in contrast to the general practice of using the same parameter value for both functions. The relation between l1 and l2 was largely controlled by soil structure and associated macroporosity properties.

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Statistical analysis and interpretation of heterogeneous sediment hydraulic properties is important to produce reliable forecasts of water and solute transport dynamics in the unsaturated zone. Most field characterizations to date have focused on the shallow 2‐m root zone. We characterized the geologic and hydraulic properties of a 16‐m‐deep, alluvial vadose zone consisting of unconsolidated sediments typical of the alluvial fans of the eastern San Joaquin Valley, California. The thickness of individual beds varies from <5 cm for some clayey and silty floodplain material to >2.5 m for large sandy deposits associated with buried stream channels. Eight major geologic units (lithofacies) have been identified at the site. Unsaturated hydraulic properties were obtained from multistep outflow experiments on nearly 100 sediment cores. Multivariate analysis of variance and post hoc testing show that lithofacies and other visual‐ and texture‐based sediment classifications explain a significant amount of the spatial variability of hydraulic properties within the unsaturated zone. Geostatistical analysis of hydraulic parameters show spatial continuity of within‐lithofacies variability in the horizontal direction in the range of 5 to 8 m, which is approximately an order of magnitude larger than spatial continuity in the vertical direction. Low nugget/sill ratios suggest that 1‐ to 10‐m sampling intervals are adequate for detection of horizontal spatial structure. The existence of thin clay or silt layers within lithofacies units results in only moderate spatial continuity in the vertical direction, however, suggesting inadequate sampling frequency for hydraulic parameter variogram development in that direction.

Summary To improve the predictive capability of transport models in soils we need experimental data that improve their understanding of properties at the scale of pores, including the effect of degree of fluid saturation. All transport occurs in the same soil pore space, so that one may intuitively expect a link between the different transport coefficients and key geometrical characteristics of the pores such as tortuosity and connectivity, and pore‐size distribution. To understand the combined effects of pore geometry and pore‐size distribution better, we measured the effect of degree of water saturation on hydraulic conductivity and bulk soil electrical conductivity, and of degree of air saturation on air conductivity and gaseous diffusion for a fine sand and a sandy loam soil. To all measured data were fitted a general transport model that includes both pore geometry and pore‐size distribution parameters. The results show that both pore geometry and pore‐size distribution determine the functional relations between degree of saturation, hydraulic conductivity and air conductivity. The control of pore size on convective transport is more for soils with a wider pore‐size distribution. However, the relative contribution of pore‐size distribution is much larger for the unsaturated hydraulic conductivity than for gaseous phase transport. For the other transport coefficients, their saturation dependency could be described solely by the pore‐geometry term. The contribution of the latter to transport was much larger for transport in the air phase than in the water phase, supporting the view that connectivity dominates gaseous transport. Although the relation between effective fluid saturation and all four relative transport coefficients for the sand could be described by a single functional relation, the presence of a universal relationship between fluid saturation and transport for all soils is doubtful.

[1] The heat pulse probe (HPP) technique has been successfully applied for estimating water flux density (WFD)

whose two most important attributes are finger diameter and the fraction of the total cross-sectional area occu-A conceptual model is presented that represents the development pied by fingers (hereafter called the finger flow fraction). of unstable flow in uniform soils during redistribution. The flow insta-Various attempts have been made to predict the finger bility results in the propagation of fingers that drain water from the diameter from soil properties and the water flow characwetted soil matrix until equilibrium is reached. The model uses soil retention and hydraulic functions, plus relationships describing finger teristics. They either involve an approximate equation size and spatial frequency. The model assumes that all soils are unsta-that is a function of the sorptivity (Parlange and Hill, ble during redistribution, but shows that only coarse-textured soils will 1976) or include a macroscopic surface tension associform fingers capable of moving appreciable distances. Once it forms, a ated with the large-scale curvature induced by the radius finger moves downward at a rate governed by the rate of loss of water of the finger (Chuoke et al. , 1959; Glass et al., 1989a). from the soil matrix, which can be predicted from the hydraulic con-Hydrostatic balance equations written at the wettingductivity function. Fingers are assumed to stay narrow as a result of front interface have periodic solutions that have a maxihysteresis, which prevents lateral diffusion. The draining front in the mum unstable wavelength, and this is used to calculate soil matrix between the fingers is assumed to cease downward movethe finger diameter. Both approaches involve associating ment because water pressure drops below the threshold water-entry one-half of the wavelength with the finger diameter. matric potential. The threshold potential is not present in the conven-

Accurate modeling of water and air flow in porous media requires the definition of the relevant hydraulic properties, namely, the water retention curve (WRC) and the relative hydraulic conductivity function (RHC), as well as the definition of the relative air permeability function (RAP). Capitalizing on the approach developed previously to represent the RHC, a new model allowing the prediction of RAP based on information resulting from the WRC is proposed. The power value h a in the model is a decreasing exponential function of the coefficient of variation, E, characterizing the pore size distribution of the porous medium, and derived from its WRC. The model was calibrated using data from 22 disturbed and undisturbed soil samples and was validated using data from eight soil types ranging from quartz sand to silty clay loam. The proposed model provided accurate prediction of the soil RAP and performed in some cases (sandy loam and silty clay loam soils) better than available alternative models.