Measurement of Sorptivity and Soil Water Diffusivity in the Field

Abstract

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.