THE POWER-FUNCTION MODEL FOR THE SOIL-MOISTURE CHARACTERISTIC

The minimum number of parameters required to model the unsaturated soil moisture characteristic, relating volumetric water content (θ) and matric suction (χ), is shown to be two. A third parameter, θ=θs at saturation, is required to define its saturation limit. The popular power‐function χ/χe, = (θ/θs)b is the most general three‐parameter model, with χ normalized by a notional air‐entry potential, χe. When log‐transformed, e.g. as In χ= a + bln(θ/θs), it gives a good fit to observations over varying ranges of χ. We show, using US, Australian, UK and NZ data, that a,b and θs, in this formulation are uncorrelated across a wide range of textures, thus providing a ‘basis set’ of independent parameters. Gregson et al. (1987) used the alternative formulation In χ= a″+ bln(100θ), with χ rescaled to a percentage. Their reported correlation between a″ and b, which led to their ‘one‐parameter model’ of the characteristic, is shown to be a mathematical artefact, arising from absorption of the term – b[ln(100)+ ln θs] into a″. Copyright © 1990, Wiley Blackwell. All rights reserved