Time Domain Reflectometry: Temperature-dependent Measurements of Soil Dielectric Permittivity

Abstract

The purpose of this study is to determine the temperature influence on the soil bulk dielectric permittivity, e b, calculated from the measurement of the electromagnetic wave velocity of propagation along the parallel waveguide in a TDR probe, i.e. a probe working in Time Domain Reflectometry technique. The experimental evidence shows that the existing models do not completely describe the temperature effect. However, it has been confirmed that the observed temperature effect is the result of two competing phenomena; e b increases with temperature following the release of bound water from soil solid particles and e b decreases with temperature increase following the temperature effect of free water molecules. It has been found that there is a soil type characteristic moisture value, θ eq, named the equilibrium water content, having the specific temperature property. The temperature effect for this moisture is not present, which means that for soils with the moisture value equal to θ eq the both competing phenomena mentioned earlier compensate each other. The equilibrium water content, θ eq, decrease is correlated with the soil specific surface area. The temperature correction formula adjusting the soil moisture determined by TDR, θ TDR, at various temperatures to the corresponding value at 25°C, based on knowledge of θ eq, decreases the standard deviation of the absolute measurement error of soil moisture θ TDR by the factor of two as compared to the uncorrected values. The majority of measurements of physical, chemical and biological properties of porous materials including soil should be accompanied with the measurement of soil water content and temperature. These parameters determine almost all processes in natural environment. It seems obvious to accompany the sensors for the measurement of soil salinity, oxygenation, content of nutrients, soil water potential, and others with the temperature and moisture sensors having the same measurement volume and performing measurements at the same time. Temperature sensors of various accuracy and size are easily available and together with the necessary electronics they can fit into the desired sensor enclosure. More problems are encountered with moisture sensors of porous materials because they do not measure water content directly, but use other parameters of the measured matter that indirectly and selectively determine its moisture. Such a property of porous materials accompanied with moisture is dielectric permittivity.