Measurement of Sorptivity and Soil Water Diffusivity in the Field

Abstract

An analysis for scaling the exponential soil water diffusivity function, D(θ), from concurrent measurements of sorptivity and wet front advance is presented. This analysis provides a field method for measuring β, the slope of the exponential D(θ). For Bungendore fine sand in the field it was found that β = 3. This is contrary to absorption experiments on a variety of repacked soils that have consistently found β = 8 and led to the suggestion that β = 8 may be the “universal” value applicable to all soils. Although the exponential function with β = 3 provided a usable approximation over the entire θ range, D(θ) data from undisturbed cores showed that at high water contents D was effectively constant, and for lower water contents the undisturbed data were well described by an exponential function with β = 8. Measurements of sorptivity, wet front advance, and hydraulic conductivity were made in the field using a device that supplied water to the soil surface at the small tension of 4 cm. This gives the physical properties of the soil matrix, since larger channels and voids (>0.75 mm in diam) do not affect the measurements made at a small tension. Field heterogeneity due to such macropores is consequently eliminated. This sorptivity of Bungendore fine sand is about 0.4 that from ponded infiltration affected by the macropores.