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A Simple Method for Combining Soil Properties that Show Variability¹

ABSTRACT

A method is shown by which variable soil properties expressed by distributions may be combined to estimate a variate that is derived from them through some functional relationship. The probability distribution (normal, log-normal, or any other form) of the input for each property is divided into sections, each having the same number of observations. The section medians for each property are then combined through the functional relationship in all combinations of section and property to generate a population of values for the required variate without presupposing any particular type of distribution for it. The method gave estimates of unsaturated hydraulic conductivity, K(), that agreed with those from the Taylor series method (Rao et al., 1977) and additionally showed a marked increase in the variance of K() as the volumetric moisture content, , decreased. Because the method generates a population of values, it could be further shown that as decreased, K() became less well described by a log-normal distribution. It can be used for nondifferentiable functions and, as is demonstrated for water flux calculations, functions whose inputs are log-normally distributed but which cannot be expressed in logarithmic form. Although the method is not equivalent to the Monte Carlo procedure, it gave estimates of water flux that differed little from those given by Monte Carlo simulations.

¹ Joint contribution from Rothamsted Experimental Station, England and New York State Agricultural Experiment Station, Cornell Univ.

² Soil Scientist, Soils and Plant Nutrition Dep., Rothamsted and Associate Professor, Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853.

Received for publication March 20, 1985. Accepted for publication July 9, 1985.

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