TENSIOMETRIC MEASUREMENT AND METASTABLE STATE OF WATER UNDER TENSION

Abstract

Nonosmotic tensiometers are widely used for measuring soil water potential, but they cannot usually be used for pressure potentials below −98 kPa. This study investigated the possibility of measuring pressure potential below −98 kPa with a tensiometer. The measuring system was a microtensiometer connected to a pressure transducer; it was purged carefully and calibrated in the range 0 to −90 kPa. Two laboratory experiments were performed. In the first one, the porous cups of several tensiometers were subjected to evaporating conditions, and measurements were made every minute. In the second one, tensiometers were installed at four depths of an evaporating soil column (silt loam); water content profiles in the soil were also measured with a gamma radiation attenuation method, and soil hydraulic properties were determined at different depths of the sample. In both experiments, pressure potentials computed from an extrapolation of the calibration lines of the tensiometers could be temporarily smaller than −98 kPa (during 10 h in the second experiment, and down to - 140 kPa in the first experiment). In agreement with old published results and from a simple analysis of the vaporization in a liquid, it was concluded that these results were physically realistic. These results also stress the purge quality to improve the response time of tensiometers during transient water flow experiments.