Disc permeameters are designed to measure hydraulic properties of field soils containing macropores and preferential flow paths and are particularly useful in soil management studies. We present here designs for disc permeameters for both positive and negative water supply heads. The effects of the water supply membrane and soil contact material on permeameter performance are examined using approximate quasi‐analytic solutions to the flow equation. This analysis provides approximate criteria for the selection of membrane and soil contact materials. Limitations to performance caused by restricted air entry are considered and design criteria are given also. We present in situ tests of the disc permeameter for the early stages of one‐dimensional infiltration and an example of the deterministic variation of sorptivity of a field soil with supply potential. Finally, we use ponded and unsaturated sorptivities measured in situ with disc permeameters to find the saturated hydraulic conductivity and flow‐weighted mean characteristic pore dimension of a field soil.

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Convenient and reliable techniques for estimating intact soil hydraulic properties are required for predictions of soil‐water flow in the environment. The dependence of sorptivity, S, on water supply potential, Ψ0, is used here to find the dependence of the intact field soil hydraulic properties soil‐water diffusivity, D(θ), hydraulic conductivity, K(θ), and soil‐water characteristic Ψ(θ) on water content, θ. The approximations used in deconvoluting sorptivity are examined critically using a simple parametric D(θ) that gives analytic solutions applicable to most known soil behavior. We show that the Parlange approximation differs from the exact solution by <5% over a very wide range of D(θ) and over the entire water content range. A rapidly convergent iterative scheme in which this approximation forms the initial estimate is introduced for situations where greater accuracy is required. The technique is tested for a repacked soil, Ustochrept, and the D(θ) derived from S(Ψ0) agrees excellently with conventional measurements. We use the disc permeameter to find S(Ψ0) at 20 sites for an intact field soil, Haplustalf. The D(θ) derived from these measurements is not strongly θ‐dependent. This is attributed to the shape of Ψ(θ), which, in contrast to repacked samples, shows that as θ approaches saturation, Ψ approaches zero gradually. The capillary length found from the K(Ψ) measurements is only 23 mm. Both findings are attributed to the presence of biopores in the field soil. Sorptivity may be used to determine reliably intact soil hydraulic properties whose magnitudes reflect the intricacies of field soil structure.

Absorption of water into soil as the result of a constant flux condition at the soil surface is examined.Experiments for a fine sand show that the surface water content, movement of the wetting front, and the water content profiles may be predicted from the soil water diffusivity function using the notion of the flux‐concentration relation of Philip (1973).Reduced space and time variables X = Vox and T = Vost are introduced. Use of these variables greatly simplifies treatment of the system and reveals that the surface water content and the reduced position of the wetting front are uniquely defined by T while the water content profiles at any value of T are unique in terms of X.

An analysis of hydrodynamic dispersion accompanying constant flux absorption of KCl solution by an initially relatively dry soil, is developed for the case when the hydrodynamic dispersion coefficient is pore water velocity‐independent. It is shown that in this process both the water content and the soil water salt concentration are uniquely defined by θ(X,T) and C(X,T), where X = vox and T = vo2t are space and time‐like coordinates, and vo is the constant surface flux of water.Quasi‐analytical methods based on the flux‐concentration relation predict θ(X,T) while an error‐function solution, based on a material coordinate Q labeling parcels of water, predicts the salt profile.The analysis is demonstrated using a chemically inert sandy soil. The results show that during transient, unsaturated flow a simple piston‐flow model described the process over a range of water contents. The method may be extended to explore dispersion in structured and chemically reactive soils.