Estimation of Unsaturated Hydraulic Conductivity from Field Sorptivity Measurements

Abstract

Abstract Measurement or estimation of the unsaturated hydraulic conductivity of soils, K (Ψ), is of central importance to the understanding of soil‐water flows in many areas of hydrology and agriculture. Here we derive approximations that relate K (Ψ) to sorptivity, S 0 , measured over a range of negative supply pressures. We test the approximations 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 we use measurements of S 0 to identify the effect of rain, falling at the end of a severe drought, on K (Ψ) of the surface soil. We adapt these measurements to pinpoint the changes in characteristic pore size brought about by the rain. In addition, we show how the macroscopic capillary length of a soil may be estimated from sorptivity measurements at any supply pressure. One of the principal advantages of the technique is that measurements of sorptivity sample shallow soil depths. The technique therefore is suited to layered soils, shallow surface soils and soil tillage studies.