A Statistical Exploration of the Relationships of Soil Moisture Characteristics to the Physical Properties of Soils

Abstract

Stochastic modeling of soil water fluxes in the absence of measured hydraulic parameters requires a knowledge of the expected distribution of the hydraulic parameters in different soil types. Predictive relationships describing the hydraulic parameter distributions must be developed based on the common descriptors of the physical properties of soils (e.g., texture, structure, particle size distribution). Covariation among the hydraulic parameters within these relationships must be identified. Data for 1448 soil samples were examined in an evaluation of the usefulness of qualitative descriptors as predictors of soil hydraulic behavior. Analysis of variance and multiple linear regression techniques were used to derive quantitative expressions for the moments of the hydraulic parameters as functions of the particle size distributions (percent sand, silt, and clay content) of soils. Discriminant analysis suggests that the covariation of the hydraulic parameters can be used to construct a classification scheme based on the hydraulic behavior of soils that is analogous to the textural classification scheme based on the sand, silt, and clay content of soils. INTRODUCTIONApplication of the classical theory of soil water movement requires knowledge of the relationships among matric potential, moisture content, and hydraulic conductivity. The physical attributes of the soil giving rise to these interrelationships are understood in a qualitative sense [e.g., Childs, 1969]. A comprehensive theory to allow derivation of the relationships from fundamental properties of the medium (e.g., grain size distribution) is, however, not yet fully developed, although recent work suggests that certain aspects of the hydraulics may be amenable to a theoretical treatment [Nakano, 1976;Arya and Paris, 1981]. In most cases, curves of matric potential versus moisture content (the moisture characteristic) and of hydraulic conductivity versus either matric potential or moisture content must be determined for a given soil by direct measurement. Statistical analyses can be used to identify what soil properties are important in describing the observed variation in these curves, thereby providing information of practical value as well as suggesting how theoretical exploration might proceed.One approach that has been used to define the moisture characteristic is the construction of regression equations to predict the moisture content at specified values of matric potential using properties such as bulk density, percent sand, and other measured properties such as organic matter content [Ghosh, 1980;Gupta and Larson, 1979;Rawls and Brakensiek, 1982]. Results from these studies indicate that reasonable predictions can be made when the necessary data are available.

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