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Statistical and Autoregressive Analysis of Soil Physical Properties of Portsmouth Sandy Loam¹

ABSTRACT

Reliable estimates of soil physical and hydraulic properties are needed for designing drainage systems for soils with limited natural drainage. The objective of the study was to determine the autocor-relation structure of soil physical properties in a poorly drained soil. Saturated hydraulic conductivity, soil-water characteristic, soil-water diffusivity, texture, and bulk density were measured on undisturbed soil cores sampled from the A, Btg, and Bg soil horizons at 150 points on two transects in a field of Portsmouth sandy loam (Typic Umbraquults). Unsaturated hydraulic conductivity for each of the 450 cores was computed using a quasi-analytical expression utilizing measured soil-water diffusivity data and the derivative of the inverse of the van Genuchten equation for the measured water characteristic. Hydraulic conductivity values at a given soil-water pressure had a high coefficient of variation, ranging from 130 to 3300%, and were lognormally distributed. Evidence of significant short-range autocorrelation, usually 2 m, was found for hydraulic conductivity, soil-water characteristic, texture, and bulk density. First-order coefficients for autoregressive equations indicated a decrease in predictability of the observed variation with increasing soil depth. The number of measurements required to estimate the mean of a soil property of the A horizon to within ±10%, ranged from four for sand content to 1985 for saturated hydraulic conductivity. The means for most soil properties in the A horizon, however, can be estimated to within ±10% with 35 observations.

¹ Contribution from the Dep. of Soil Science, North Carolina State Univ., Paper no. 10145 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601.

² Former Graduate Research Assistant and Professor of Soil Science, North Carolina State Univ. Senior author is now Assistant Professor of Agronomy, Univ. of Missouri, Columbia, MO 65211.

Received for publication October 15, 1985.

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