TENSIOMETRIC MEASUREMENT AND METASTABLE STATE OF WATER UNDER TENSION

Nonosmotic tensiometers are widely used for measuring soil water poten tial, but they cannot usually be used for pressure potentials below -9 8 kPa. This study investigated the possibility of measuring pressure p otential below -98 kPa with a tensiometer. The measuring system was a microtensiometer connected to a pressure transducer; it was purged car efully and calibrated in the range 0 to -90 kPa. Two laboratory experi ments were performed. In the first one, the porous cups of several ten siometers were subjected to evaporating conditions, and measurements w ere made every minute. In the second one, tensiometers were installed at four depths of an evaporating soil column (silt loam); water conten t profiles in the soil were also measured with a gamma radiation atten uation method, and soil hydraulic properties were determined at differ ent depths of the sample. In both experiments, pressure potentials com puted from an extrapolation of the calibration lines of the tensiomete rs could be temporarily smaller than -98 kPa (during 10 h in the secon d experiment, and down to -140 kPa in the first experiment). In agreem ent with old published results and from a simple analysis of the vapor ization in a liquid, it was concluded that these results were physical ly realistic. These results also stress the purge quality to improve t he response time of tensiometers during transient water now experiment s.