Estimation of unsaturated hydraulic conductivity from field sorptivity measurements

Approximations are derived that relate unsaturated hydraulic conductivity K(Ψ) to sorptivity, S₀, measured over a range of negative supply pressures. Approximations are tested against the Green-Ampt, Knight, and Broadbridge-White models of soil hydraulic properties. The last model covers the range from a Green-Ampt like model to the weakly nonlinear Knight model. One of the derived approximations is found to give K(Ψ) accurate to within 5% over the entire range of soil-water potential and the full range of the single Broadbridge-White model parameter. Comparison of estimates of K(Ψ) using this approximation with conventionally determined K(Ψ) for repacked and intact soil samples shows good agreement. In an application of the technique measurement of S₀ are used to identify the effect of rain, falling at the end of a severe drought, on K(Ψ) of the surface soil.