Measurement of plant xylem water pressure using the high-capacity tensiometer and implications for the modelling of soil-atmosphere interaction

The response of shallow geotechnical structures is affected by interaction with the atmosphere. Since the ground surface is very often vegetated, plant transpiration plays a major role in such an interaction. Transpiration in geotechnical applications is generally modelled by way of a transpiration reduction function (e.g. the Feddes function). However, its parameters are generally borrowed from the agricultural literature, where the focus is on crop species and often loosely compacted organic agricultural soils. For the non-crop species in denser soils typically encountered in geotechnical applications, monitoring of the flow taking place in the soil through the xylem up to the leaves can potentially be exploited to characterise the transpiration reduction function. The main challenge is the measurement of the water pressure in the xylem. Techniques currently used include the pressure chamber and thermocouple psychrometer. The pressure chamber is destructive and thus not suitable for continuous monitoring and/or where a relatively small number of leaves is available (as often occurs in laboratory experiments). The thermocouple psychrometer is not accurate at low water tension, is affected by the presence of solutes in the xylem water and is significantly sensitive to temperature. This paper explores a novel application of the high-capacity tensiometer (HCT), initially developed for pore-water pressure measurement in soils. The HCT was installed on the stem or branch of different trees and its measurement validated against pressure chamber measurements over a range of xylem water pressure down to −1300 kPa. In addition, its measurement was used to investigate the response of the soil–plant continuum. Results show that the HCT is a viable and convenient instrument to use for xylem water pressure measurement and can provide field-based data for the modelling of plant transpiration. Installing HCTs on stems and branches is quite straightforward and this will help achieve a step change in testing and modelling the effect of plant transpiration on the soil water regime in the vadose zone.