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

Abstract

Reports on temperature (T) effects on time domain reflectometry (TDR) measurements of soil water content (θ) are contradictory and often exhibit conflicting trends. We imposed step T changes on sealed columns of four soils having variable θ, while monitoring bulk apparent dielectric constant (or permittivity ϵb) and bulk electrical conductivity (σb) using TDR. Measured ϵb increased substantially with increasing temperature for one silt loam soil, for all θ. For another silt loam soil and for an Oxisol, measured ϵb increased with increasing T at relatively low θ but decreased with increasingT at higher θ. For a sandy loam soil, measured ϵb decreased with increasing T for all θ. The experimental results led to the hypothesis that TDR‐measured ϵb 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 ϵb with increased T due to release of bound water. TDRmeasured ϵb 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.