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Use of Sorptivity to Determine Field Soil Hydraulic Properties¹

ABSTRACT

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, ₀, is used here to find the dependence of the intact field soil hydraulic properties soil-water diffusivity, D(), hydraulic conductivity, K(), and soil-water characteristic () on water content, . The approximations used in deconvoluting sorptivity are examined critically using a simple parametric D() 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() 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() derived from S(₀) agrees excellently with conventional measurements. We use the disc permeameter to find S(₀) at 20 sites for an intact field soil, Haplustalf. The D() derived from these measurements is not strongly -dependent. This is attributed to the shape of (), which, in contrast to repacked samples, shows that as approaches saturation, approaches zero gradually. The capillary length found from the K() 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.

¹ Contribution from CSIRO, Australia.

² Principal Research Scientist, CSIRO, and Experimental Scientist, retired, Div. of Environmental Mechanics, GPO Box 821, Canberra, A.C.T. 2601, Australia, respectively.

Received for publication December 1, 1986.