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Soil Science Society of America Journal Abstract -

Equation for Extending Water-Retention Curves to Dryness


This article in SSSAJ

  1. Vol. 55 No. 4, p. 923-927
    Received: Oct 30, 1990

    * Corresponding author(s):
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  1. P. J. Ross ,
  2. J. Williams and
  3. K. L. Bristow
  1. Division of Soils, CSIRO, Davies Laboratory, Townsville, Qld 4814



Simple water-retention functions in current use do not predict zero water content at oven dryness. Using the Campbell equation θ = a ψ-c = θs-c − ψ-c0) relating volumetric water content, θ, to matric suction, ψ, as an example, we examined a modification to ensure that the curve terminates in a suction ψ0 = 1000 MPa, approximating oven dryness. The new equation is θ = a-c − ψ-c0) = θs-c − ψ-c0)/(ψ-cc − ψ-c0. It was compared with the unmodified equation by fitting to water-retention data from Australia, the UK, and the USA. Results showed not only that the new equation fit the data slightly better than the original equation, with a mean standard deviation of 0.014 m3 m−3 about the regression, but also that its parameters could be calculated from those of the unmodified equation if original water-content data were unavailable for fitting. Although lack of data hampered assessment of its predictions of water content in dry soils, its use appears preferable to extrapolating the unmodified equation.

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