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Two Indices of Soil Structure Based on Prediction of Soil Water Processes

Dep. of Land and Water Conservation, P.O. Box 445, Cowra, NSW 2794, Australia

H. P. Cresswell^*

CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia

*Corresponding author (hamish.cresswell{at}cbr.clw.csiro.au).

ABSTRACT

Evaluation of soil structure should reflect the nature and degree of soil physical limitations to land use for local climatic conditions. This can be achieved through mechanistic simulation of soil-plant-atmosphere processes. We propose two alternative indices of soil structure, based on predicted infiltration. The potential runoff index (PRI) is the runoff predicted from a 1-in-20-yr average recurrence interval storm event of 30-min duration using an event-based soil water infiltration model that utilizes Richards' equation. The runoff recurrence index (RRI) is the average recurrence interval of a 30-min duration storm of intensity just sufficient to result in runoff. These indices were applied to data from 37 sites (mainly Palexeralfs, but including Natrixeralfs, Rhodoxeralfs, and Haploxeralfs) in southeastern Australia, where rainfall partitioning between infiltration and runoff can significantly affect agricultural production. Sites with different land use histories were ranked on the basis of the RRI. Woodland sites have a significantly greater mean ranking (mean ranking [RRI] = 33.5), indicating more favorable soil structure, compared with less conservative agricultural land uses such as heavily grazed pasture (mean ranking = 11.7) or intensively cultivated cropping where stubble was not retained (mean ranking = 12.4). Both indices integrate the effects of soil structure, as represented through soil hydraulic properties, with local rainfall characteristics. They also account for soil horizon interactions influencing infiltration. The saturation-excess runoff generation predicted for 12 of the 37 sites indicates that structural amelioration of B horizon soil should be a high priority at these sites.

Received for publication November 15, 1996.