MEASUREMENT OF UNSATURATED SOIL HYDRAULIC CONDUCTIVITIES USING A CERAMIC CUP TENSIOMETER

Abstract

The objective of this study was to develop and evaluate a simple field method to determine unsaturated hydraulic conductivities using measurements of water flux into a tensiometer. The tensiometer consists of a ceramic cup glued to one end of a piece of plastic tubing. A suction is first applied to the inside of the tensiometer, which is closed to the atmosphere. The reduced pressure in the tensiometer causes water to flow into the tensiometer from the soil. As the water flows into the tensiometer, the volume of air in the tensiometer decreases, and pressure increases. The rate of water flow into the tensiometer, water flux, is calculated from the measured pressures using a form of the ideal gas equation, PV = Constant, and its full differential, PdV/dt+VdP/dt = 0, where P is the measured pressure, and V is volume. The water flux is obtained from the change in volume with time, -dV/dt. The parameters for the unsaturated conductivity equation are determined by using a two-dimensional finite element soil model (2DSOIL) coupled with a Marquardt-Levenberg algorithm to fit calculated fluxes to measured ones. For comparison purposes, unsaturated hydraulic conductivities were also determined 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 conductivities obtained from the tensiometer inflow data, however, were much less than unsaturated hydraulic conductivities measured during drainage. We attributed the differences to anisotropy and scale effects although clogging of the tensiometer pores by fine soil material could also be a contributing factor. The method is relatively quick, uses inexpensive materials, provides consistent results and is not limited greatly by the conductivity of the cup.