Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: Experimental evidence and hypothesis development

Reports on temperature (T) effects on time domain reflectometry (TDR) measurements of soil water content (theta) are contradictory and often exhibit conflicting trends. We imposed step T changes on sealed columns of four sails having variable theta, while monitoring bulk apparent dielectric constant (or permittivity epsilon(b)) and bulk electrical conductivity (sigma(b)) using TDR. Measured epsilon(b) increased substantially with increasing temperature for one silt loam soil, for all theta. For another silt loam soil and for an Oxisol, measured epsilon(b) increased with increasing T at relatively low theta but decreased with increasing T at higher theta. For a sandy loam soil, measured epsilon(b) decreased with increasing T for all theta. The experimental results led to the hypothesis that TDR-measured epsilon(b) is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric constant of bulk water with increased T; and (2) the increase in TDR-measured epsilon(b) with increased T due to release of bound water. TDR-measured epsilon(b) is thus dependent on solid surface area and wetness. Our results have implications for routine use of TDR in fine-textured and organic soils and potentially for microwave remote sensing of soil water status.