EQUATION FOR EXTENDING WATER-RETENTION CURVES TO DRYNESS

Simple water-retention functions in current use do not predict zero water content at oven dryness. Using the Campbell equation theta = a psi-c = theta-s (psi/psi-e)-c relating volumetric water content, theta, to matric suction, psi, as an example, we examined a modification to ensure that the curve terminates in a suction psi-0 = 1000 MPa, approximating over dryness. The new equation is theta = a(psi-c - psi-0-c) = theta-s (psi-c - psi-0-c)/(psi-e-c - psi-0-c). 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.