Measurement of Low Matric Potentials with Porous Matrix Sensors and Water‐Filled Tensiometers

Abstract

Water‐filled tensiometers are widely used to measure the matric potential of soil water. It is often assumed that, because these give a direct reading, they are accurate. With a series of laboratory tests with model laboratory systems of increasing complexity we show that the output of water‐filled tensiometers can, particularly in drying soils, be in serious error. Specifically, we demonstrated that water‐filled tensiometers can indicate a steady matric potential, typically between −60 and −90 kPa, when the soil is much drier. We demonstrate the use of water‐filled tensiometers that can measure matric potentials smaller than −100 kPa in the laboratory and in the field. The physics of the failure of water‐filled tensiometers is discussed. When the matric potential was greater than −60 kPa, in laboratory and field tests water‐filled and porous matrix sensors were in good agreement. In the field environment the porous matrix sensor was useful because it allowed early detection of the failure of water‐filled tensiometers. In dry soils (matric potential < −60 kPa) the porous matrix sensor was more reliable and accurate than the water‐filled tensiometer.

The velocity of shear waves in unsaturated soil

“…Matric potential would be the most easily measured (e.g. Whalley et al, 2009) and then void ratio could be estimated. Methods to measure void ratio are either invasive or use buried sensors combining TDR and thermal methods (Liu et al, 2008).…”

Measurement of the matric potential of soil water in the rhizosphere

“…7. Laboratory studies of tensiometer failure using a high capacity tensiometer (taken from Whalley et al, 2009; used by permission of the Soil Science Society of America, Inc.) The tensiometer had a ceramic with an air entry pressure of 300 kPa. In these panels, the applied pressure is always shown by the dashed line and the output recorded by the tensiometer is shown by the solid line.…”

“…Developments in porous matrix sensors have focused on the identification of more appropriate porous materials and the use of improved methods to measure their water content (e.g. Whalley et al, 2001Whalley et al, , 2007Whalley et al, , 2009Malazian et al, 2011), but this has produced mixed results. Wraith and Or (2001) described the use of calibrated soil as a reference material for use with TDR (time domain reflectrometry) probes to infer the matric potential of soil from its water content.…”

“…Tensiometers designed for this purpose have become known as 'high-capacity tensiometers' (Take and Bolton, 2003;Marinho et al, 2008). Careful saturation of commercially available tensiometers can allow matric potentials as low as -200 kPa to be measured (Whalley et al, 2009). In this review, we explore the possibility of using the high-capacity tensiometer in plant sciences.…”

“…However, Whalley et al (2001) showed how hysteresis could be taken into account in the calibration of a ceramic-based porous matrix sensor between matric potentials of 0 and −60 kPa. Recently, Whalley et al (2007Whalley et al ( , 2009) have designed dielectric tensiometers that work over a much wider range of matric potentials.…”