Calculation of soil water content using dielectric permittivity measurements; benefits of soil-specific calibration

Abstract. Soil Water Content (SWC) probes are widely used around the Earth for scientific and agricultural usage. Most of them are based on soil dielectric permittivity measurement as the dry soil relative dielectric permittivity ε, typically from 3 to 5, is much smaller than the water relative dielectric permittivity: about 80. The measure of dielectric permittivity in wet soils allows deducing the soil volumetric water content. Capacitance, Time Domain Reflectometry (TDR), Frequency Domain Reflectometry (FDR) and even remote sensing techniques such as Ground-Penetrating Radar (GPR) and microwave-based techniques are concerned. This study presents SWC measurements with commercial FDR probes on clayey soil highlighting the benefit of a meticulous soil-specific calibration although constructors indicate probes calibrated with generic constants as convenient. Locally, the use of the manufacturer’s transfer equation can lead to a strong overestimation or underestimation of the actual soil water content. A simple protocol for clayey soil calibration is proposed. Without soil-specific calibration on clayey soil, we observe errors as important as 115 % with a factory-calibrated probe based on the real part of the dielectric permittivity and up to 245 % with the factory-calibrated probes based on the modulus of the dielectric permittivity.