Measurement of xylem water pressure using High-Capacity Tensiometer and benchmarking against Pressure Chamber and Thermocouple Psychrometer

Abstract

The response of the shallow portion of the ground (vadose zone) and of earth structures is affected by the interaction with the atmosphere. Rainwater infiltration and evapotranspiration affect the stability of man-made and natural slopes and cause shallow foundations and embankments to settle and heave. Very frequently, the ground surface is covered by vegetation and, as a result, transpiration plays a major role in ground-atmosphere interaction. The soil, the plant, and the atmosphere form a continuous hydraulic system, which is referred to as Soil-Plant-Atmosphere Continuum (SPAC). The SPAC actually represents the ‘boundary condition’ of the geotechnical water flow problem. Water flow in soil and plant takes place because of gradients in hydraulic head triggered by the negative water pressure (water tension) generated in the leaf stomata. To study the response of the SPAC, (negative) water pressure needs to be measured not only in the soil but also in the plant. The paper presents a novel technique to measure the xylem water pressure based on the use of the High-Capacity Tensiometer (HCT), which is benchmarked against conventional techniques for xylem water pressure measurements, i.e. the Pressure Chamber (PC) and the Thermocouple Psychrometer (TP).

Evaluation of instruments for monitoring the soil–plant continuum

“…The High-Capacity Tensiometer (HCT) is composed of an integral strain gauge, a diaphragm 0.4 mm thick and a ceramic filter with nominal air-entry value of 1.5 MPa (Tarantino & Mongiovi, 2002). The working principle and the experimental procedures adopted i) to saturate the porous ceramic filter and i) to check its saturation prior to and after the measurement are discussed in Tarantino (2004) Pressure chamber Sapflow meter 7 installation on the stem are provided in Dainese et al (2020a). The measurement of xylem water potential using the HCT has been validated by and Dainese et al (2020b) by comparison with Pressure Chamber and Thermocouple…”