Equation for Extending Water‐Retention Curves to Dryness

Abstract Simple water‐retention functions in current use do not predict zero water content at oven dryness. Using the Campbell equation θ = a ψ ‐c = θ s (ψ ‐c − ψ ‐c 0 ) 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 ψ 0 = 1000 MPa, approximating oven dryness. The new equation is θ = a (ψ ‐c − ψ ‐c 0 ) = θ s (ψ ‐c − ψ ‐c 0 )/(ψ ‐c c − ψ ‐c 0 . 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 m 3 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.