A new analysis of steady, ponded infiltration from within a single ring takes soil hydraulic properties, ring radius, depth of ring insertion, and depth of ponding into account. It also provides a means for determining the field‐saturated hydraulic conductivity (Kfs) and the matric flux potential (φm). The analysis employs numerically determined shape factors (G) that are found to depend significantly on ring radius (a) and depth of ring insertion (d), but only slightly on depth of ponding (H) and soil hydraulic properties. As a consequence, averaged G values (Ge) can be developed for specified d and a that apply to a wide range of ponded heads and soil types. Procedures for calculating Kfs and φm are based on G or Ge, and on the ponding of one, two, or multiple H levels in the ring. Test calculations based on Ge suggest that Kfs can be obtained with an accuracy of about ±20% for H = 0.05 to 0.25 m and α = 1 to 36 m−1, where α is the soil parameter of the exponential hydraulic conductivity‐pressure head relationship. A similar level of accuracy (using Ge) is obtained for φm when α is small (α ≤ 4 m−1), and when both α and H are large (α > 4 m−1, H ≥ 0.20 m). Significant errors in φm can occur, however, when α is large but H is small. Potentially important features of this single ring method include low sensitivity of the Kfs calculation to errors in Ge, reduced measurement errors resulting from small‐scale soil variability, and the ability to pond large heads in order to increase flow rates in low‐permeability materials.

A new procedure is presented for in situ determination of saturated and near‐saturated hydraulic conductivity [Kfs and K(ψ), respectively] from a sequence of steady infiltration measurements made at several tensions on a single infiltration surface. The method applies to tension infiltration from either a surface disk or from within a ring inserted a small distance into the soil. The analysis employs a modification of Wooding's solution for infiltration from a shallow pond, combined with numerically determined shape factors that account for the interaction effects between flow geometry and soil properties. The method is found, via numerical simulations, to have an overall accuracy within about ± 7%, regardless of whether the predicted K(ψ) function is flat with an indistinct air‐entry value, steep with a distinct air‐entry value, or very steep with no air‐entry value. Some important practical features of the method are that it does not require measurement of the often‐difficult square‐root‐of‐time infiltration behavior, no special considerations are required regarding the thickness of the contact layer, it minimizes the effect of local spatial heterogeneity by using only one infiltration surface per set of K(ψ) measurements, it avoids the use of potentially unstable simultaneous‐equations solution procedures, and it can be applied to tension infiltration from both disk and ring infiltrometers.

The constant‐head well permeameter has proven to be a useful and versatile instrument for determining the in situ hydraulic properties of soils in the unsaturated (vadose) zone. The flow measurements are obtained under conditions of saturated‐unsaturated, three‐dimensional flow in the unsaturated zone. As a consequence, the steady‐state flow rate out of the permeameter is determined by both the field‐saturated hydraulic conductivity (Kfs) and the matric flux potential (φm) of the unsaturated soil. Because both Kfs and φm contribute to the flow, calculation of these parameters from well permeameter data requires either the solution of two (or more) simultaneous equations, or reduction of the problem to one equation in one unknown if additional information is known or estimated. Use of the simultaneous‐equations approach in heterogeneous soils can result in a high percentage of invalid (i.e., negative) Kfs and φm values. Negative results can be avoided and good estimates obtained, however, by using an independent measurement or site estimate of the ratio α* = Kfs/φm.

Tension disk infiltrometer experiments are generally conducted until apparent steady state is reached because most of the methods of analysis are based on Wooding's solution for steady state flux. However, the time necessary to reach steady state may be a penalizing aspect for soils with low permeability and the information contained in the transient stages is not utilized. Moreover, these methods assume homogeneous soil and a uniform initial water content, which may be unrealistic when a large volume of soil is sampled. In this series, we propose and compare several new methods of analysis that are based on the transient stage of axisymmetric infiltration. In the first part, we show that a two‐parameter equation—one term linear in square root of time and one term linear in time—adequately describes the transient flow from the disk infiltrometer for both simulated and laboratory tests. The technique used for the determination of the two coefficients must meet two criteria; it must verify the validity of the two‐term equation throughout the duration of the experiment, and it must account for the early‐time perturbation that is induced by the sand‐contact layer placed between the disk and the soil. We show that the best technique consists in linearizing the data by differentiating cumulative infiltration with respect to the square root of time. Direct nonlinear fitting on cumulative infiltration or infiltration flux is likely to lead to unacceptable errors, either because of the undetected invalidity of the two‐parameter equation or arising from the influence of the contact layer.

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