MEASUREMENT OF UNSATURATED SOIL HYDRAULIC CONDUCTIVITIES USING A CERAMIC CUP TENSIOMETER

The objective of this study was to develop and evaluate a simple field method to determine unsaturated hydraulic conductivities using measur ements of water nux into a tensiometer. The tensiometer consists of a ceramic cup glued to one end of a piece of plastic tubing. A suction i s first applied to the inside of the tensiometer, which is closed to t he atmosphere. The reduced pressure in the tensiometer causes water to now into the tensiometer from the soil. As the water flows into the t ensiometer, the volume of air in the tensiometer decreases, and pressu re increases. The rate of water now into the tensiometer, water nux, i s calculated from the measured pressures using a form of the ideal. ga s equation, PV = Constant, and its full differential, PdV/dt+VdP/dt = 0, where P is the measured pressure, and V is volume. The water nux is obtained from the change in volume with time, -dV/dt. The parameters for the unsaturated conductivity equation are determined by using a tw o-dimensional finite element soil model (2DSOIL) coupled with a Marqua rdt-Levenberg algorithm to fit calculated fluxes to measured ones. For comparison purposes, unsaturated hydraulic conductivities were also d etermined for the same soil within 25-cm-diameter rings from measured water contents and matric potentials during drainage for two locations . Fitted and measured fluxes agreed well. Unsaturated hydraulic conduc tivities obtained from the tensiometer in flow data, however, were muc h less than unsaturated hydraulic conductivities measured during drain age. We attributed the differences to anisotropy and scale effects alt hough clogging of the tensiometer pores by fine soil material could al so be a contributing factor. The method is relatively quick, uses inex pensive materials, provides consistent results and is not limited grea tly by the conductivity of the cup.