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United States Department of Agriculture

Agricultural Research Service

Title: Measurement of Unsaturated Soil Hydraulic Conductivities Using a Ceramic Cup Tensiometer

Authors
item Timlin, Dennis
item Pachepsky, Yakov - DUKE UNIVERSITY

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 5, 1998
Publication Date: N/A

Interpretive Summary: Understanding of the process of water infiltration into soil and movement in soil is important to be able to predict soil erosion, runoff of water and chemicals from soil, water availability to plants, movement of chemicals to groundwater, salt leaching, and groundwater recharge. An important soil parameter that effects infiltration is the conductivity of soil to water, the soil hydraulic conductivity. Researchers and agricultural managers who need to be able to predict infiltration and water movement need to have estimates of hydraulic conductivity when the soil is saturated and when the soil water content is less than at saturation. Unfortunately, soil hydraulic conductivity can be very difficult and expensive to measure. Many of the current measurement methods also cannot be carried out in the field. This paper presents a novel method to utilize a commonly used instrument, a ceramic cup tensiometer, to measure the rates at which water moves in the soil when the soil is less than saturated with water. Use of this method can simplify the measurement of soil hydraulic conductivity when the soil is not fully wet. The theory employed to monitor water movement into the tensiometer tube through the ceramic cup was found to be sound. We found that the measurements obtained using this instrument reflected the relative soil conditions among the sites tested.

Technical 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 applied to reduce the pressure inside the tensiometer which is closed to the atmosphere. As the water flows into the tensiometer, the volume of air in the tensiometer decreases and pressure increases. 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 determined for soil within two 25 cm diameter rings from measured water contents and matric potentials during drainage. Predicted and measured fluxes agreed well. The relative differences between the optimized hydraulic conductivities and the conductivities obtained from drainage data were similar for the two rings. Hydraulic conductivities obtained from the tensiometer inflow data, however, were greatly less than hydraulic conductivities measured during drainage possibly due to differences in scale. The method is relatively quick, inexpensive materials and provides consistent results.

Last Modified: 7/31/2014
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