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).
Highlights
The response of the shallow portion of the ground and of earth structures is affected by the interaction with the atmosphere
Very frequently the ground surface is covered by vegetation and, as a result, transpiration plays a major role in the mechanisms of water removal by the atmosphere
This paper presents the use of High-Capacity Tensiometer (HCT) to monitor the negative xylem water pressure in plants
Summary
The response of the shallow portion of the ground (vadose zone) and of earth structures is affected by the interaction with the atmosphere. The psychrometer is widely used in the field for continuous monitoring of xylem/leaf water pressure It measures total and not ‘matric’ water pressure, i.e. its measurement is affected by the presence of solutes in the xylem water. This pressure probe has never registered xylem water pressures below -0.65 MPa [11] and for no more than a few hours due to cavitation occurring in the probe [10] This was probably due to the absence of a high-air-entry porous interface, which is incorporated into high-capacity tensiometer used to measure pore-water tension in soil [4]. The measurement of the HCT was validated against the measurement of leaf water potential via the Pressure Chamber and the Transistor Psychrometer over a relatively wide range of xylem water pressures via field and laboratory experiments [12]. This paper only focuses on one single experiment carried out on a cherry tree in the laboratory
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