A New Calibration Equation for Diviner2000 Capacitance Probe Accounting for Soil Shrinkage Characteristic Curve and Estimation of Parameters on the Basis of Soil Physical Properties

Real-time measurements of soil water status are quite often used for irrigation scheduling and particularly in precision irrigation, in order to identify the exact timing and amount to supply to crops according to their water requirements. Diviner2000 capacitance probes, measuring the apparent soil dielectric permittivity, have been extensively used in the last decade. Several studies have shown that, for different soils, there is a relationship between the volumetric soil water content (SWC) and the scaled frequency (SF) measured by the sensor. However, for swelling/shrinking clay soils, for which bulk density changes with soil water content, there is a lack of knowledge on how these variations influences the soil apparent dielectric permittivity and therefore the sensor calibration relationship, as a consequence of different contributions that soil solids, water and air, have on the SF. These contributions can be quantified on the basis of the so-called soil shrinkage characteristic curve, representing the relationship between the soil bulk density (BD) or specific volume and the gravimetric water content (U). The main objectives of this research are i) to propose a new calibration equation for Diviner2000 capacitance probe, taking into consideration the soil shrinkage characteristic curve as well as ii) to suggest an indirect procedure to estimate the parameters of the sensor calibration relationship by means of easily-measurable soil physical properties. Experiments were carried out at the University of Palermo, Italy, on nine different soils collected in agricultural area of SW Sicily for which, the textural analysis showed that the clay content ranged between 9% and 45%. Soil samples were sieved and compacted, using a standard method and a Proctor hammer, in containers (25 cm diameter and 25 cm height), to reach two different bulk densities. The soil shrinkage characteristic curVE and the U(SF) relationships were contextually determined during a drying process, in order to develop a new model, represented by an implicit U(SF,BD(U)) function. In order to validate the proposed model, the same procedure was used for undisturbed soil monoliths having identical dimensions of the sieved soil samples. Considering that the calibration equation parameters were recognized to depend only on the soil clay percentage, new empirical relationships for their indirect estimation were proposed and then validated on the basis of the independent measurements obtained on undisturbed soil samples, as well as by considering other experimental data collected in the published literature.