Evaluation of instruments for monitoring the soil–plant continuum

The response of the shallow portion of the ground (vadose zone) 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 ground–atmosphere​ interaction. The soil and the plant form a continuous hydraulic system that needs to be characterised to model 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 water tension (negative water pressure) generated in the leaf stomata. To study the response of the soil–plant continuum, water tension needs to be measured not only in the soil but also in the plant (in addition to the water content in the soil). This paper first evaluates three instruments that can be used to measure xylem water tension, i.e. the High-Capacity Tensiometer (HCT) and the Thermocouple Psychrometer (TP) for continuous non-destructive measurement on the stem, and the Pressure Chamber (PC) for discontinuous destructive measurement on the leaves. Experimental procedures are presented and critically discussed, including data quality control and instrument calibration, accuracy, and precision. The performance of these three instruments is evaluated in terms of measurement precision and measurement accuracy via cross-validation. The paper then addresses the problem of monitoring soil suction (pore-water tension) and water content using a second generation profile probe (fully encapsulated) and the use of Electrical Resistivity Tomography (ERT) for coarse characterisation of water content spatial distribution to support the design of spatial configuration of suction and water content sensors.