A new low-cost and low-power capacitive sensor for soil water content measurements: preliminary analysis for possible application in rooted soils

This short communication presents a new low-cost capacitive Soil Water Content (SWC) sensor, originally developed, whose application in situ in natural rooted soils could be of some interest for its impact in geotechnical engineering applications. It is very well known that in recent years, significant advancement has been made in laboratory and field testing for the understanding of the hydro-mechanical coupled behaviour of unsaturated soils. The complexity in characterizing such behaviour increases when the role of vegetation and the presence of organic matter is considered. The amount of literature on water content (SWC) measurements and related sensors is huge and involves several scientific fields. Among indirect methods to evaluate the SWC, time domain reflectometer (TDR), time domain transmissometer (TDT) and impedance sensors, such as resistive and capacitive, are the most common. Capacitive sensors are usually directly dependent on soil apparent dielectric constant K_a which increases with SWC. They have a little sensitivity compared to TDR/TDT, however, they find several applications due to their lower cost. Vegetation affects the hydrology and the effects of plant evapotranspiration may induce some changes in the water content and soil suction and therefore the soil water retention properties. The mutual interaction among roots and soils is very variable, depending on roots-type and soil type; the beneficial influence due to the reduction of water content/degree of saturation, due to the capacity of the plant system to absorb water from the surrounding soil and transfer it to the atmosphere through transpiration is also acknowledged in the literature. Therefore, quantifying root-induced modification in soil hydraulic properties, including SWRC, is vital to predict correctly the hydrology and, hence, for the analysis of slope stability of shallow soil covers. In this note, a new low-cost capacitive sensor, characterized by an interdigit layout and produced following a PCB process, is introduced (Figure 1).

The performance of this device are under evaluation with laboratory activities: several tests have been performed preparing samples of different-type granular materials at different SWC keeping constant the dry density: natural sandy soils, glass beads, and ground coffee mixtures were investigated. The electrical capacitance and conductance of the sensor were measured in the 10 – 100 kHz frequency range by using the HP 4275A LCR meter. Some results are shown in Figure 2. It is shown that the sensor response is affected by the measurement frequency. Moreover, a saturation behaviour is highlighted for both the capacitance and conductance at increasing SWC. The sensor impedance is affected also by the electrical conductivity of the medium surrounding the sensor, e.g. solid grains, water and organic materials, and for this reason the SWC estimation requires a correction to minimize the impact of water salinity. The experimental activity performed in the laboratory is a preliminary investigation aimed at identifying an analytical model of the electrical behaviour of the sensor. Once the model is defined, the sensor could be integrated with a portable system to be validated for in-situ applications.