Principles and Applications of a New Class of Soil Water Matric Potential Sensors: The Dihedral Tensiometer

Abstract

Soil tensiometers are instruments to measure directly, without calibration, the soil matric potential between zero and the barometric pressure, but in practice it is functional up to about 85kPa, allowing monitoring continuously the soil water status for irrigation scheduling and other hydrological applications. The major drawback of these instruments is water cavitation, which causes interruption of measurements (tension breakdown) and requires instrument re-saturation. In order to avoid the major problems related with common tensiometers, a new class of instruments named dihedral tensiometers was developed at the Brazilian Agricultural Research Corporation. In this new class of instruments two rectangular hydrophilic flat glass plates or one glass plate with a fine porous flat plate are fixed in angle, defining the dihedral angle. The distance from the vertex to the water meniscus (L) formed after equilibration is linearly proportional to the soil matric potential. Using different spacer thickness between the two flat plates allows constructing sensors for different matric potential ranges and applications. Several dihedral tensiometer prototypes have been designed, constructed and tested for matric potential measurements in pot substrates and soils. Results shows linear responses between the applied water potential, using a Richards chamber as reference, and the distance L. Time responses was of few minutes to about 40min, depending on the range of measurement, on the spacer thickness, and on the porous element properties and length. Results of experiments performed in laboratory are presented and discussed as well as the basic principles and construction details of the dihedral tensiometer are described.