Equation for Extending Water-Retention Curves to Dryness
1991
Ross, P. J. | Williams, J. | Bristow, K. L.
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 (ψ⁻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(ψ⁻ᶜ − ψ⁻ᶜ₀) = θₛ (ψ⁻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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