How to Measure Low Matric Potentials with a Combination of Water-Filled Tensiometers and Porous-Matrix Sensors.

Measurement of the matric potential of soil water is central to almost all studies of water movement and plant stress responses. However, in comparison with advances made in the measurement of soil water content, there has been slow progress in the development of improved methods to measure matric potential. A constraint on the improved measurement of matric potential has been the physical limits to the use of the water-filled tensiometer (typically -100kPa), the sensor preferred by many scientists. However, recently there have been two exciting developments. Firstly, the extension of the range of the water-filled tensiometer is starting to be implemented in commercially available sensors. Take and Bolton (2003) have shown how the complete saturation of a water-filled tensiometer will allow matric potentials more negative than -100kPa to be measured. In a second and unrelated development, more reliable porous matrix sensors have been designed and implemented in a research environment. This paper has two objectives: 1) to explore the use of water-filled tensiometers with an extended range (i.e. matric potentials more negative than -100 kPa), and 2) to compare the use of porous matrix sensors with water-filled tensiometers. We present a number of important conclusions relevant to both the use of water-filled and porous matrix sensors. We show that the practice of refilling water-filled tensiometers, in drying soils, will lead to poor data sets and a loss in information. We also show that porous matrix sensors have the capability to give considerably more reliable information on matric potentials in the root zone.

This work was part funded by the European Union project 036958 “FlowAid”.

Take, W.A. & Bolton, M.D. 2003. Géotechnique, 53, 159–172.