When the one‐dimensional moisture flow equation is simplified by applying the unit gradient approximation, a first‐order partial differential equation results. The first‐order equation is hyperbolic and easily solved by the method of P. D. Lax. Three published K(θ) relationships were used to generate three analytical solutions for the drainage phase following infiltration. All three solutions produced straight lines or nearly straight lines when log of total water above a depth was plotted versus log of time. Several suggestions for obtaining the required parameters are presented and two example problems are included to demonstrate the accuracy and applicability of the method.

Steady‐state infiltration rates were measured in a field plot with infiltrometers of 5‐, 25‐, and 127‐cm inside diameter. Measurements were made with the three ringsizes along five parallel transects, spaced 125 cm apart. The infiltrometers were placed adjacent to each other along each transect (i.e., one hundred twenty‐five 5‐cm rings, twenty‐five 25‐cm rings, and five 127‐cm rings per transect). Infiltration rates were found to be lognormally distributed for all ringsizes and autocorrelated for the 5‐cm ringsize. The 5‐cm data showed that a large fraction of the water infiltrated through a small fraction of the plot area. Autocorrelation functions were determined and power spectrums computed. A first‐order autoregressive process was found to describe the 5‐cm data, and variances of samples composited along a transect agreed with sample variances. The variances for the 25‐ and 127‐cm infiltrometers were compared with sample variances, assuming a simple two‐dimensional autocovariance function. Agreement was found for the 127‐cm rings, but not for the 25‐cm rings.

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