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) measu rements of soil water content (theta) are contradictory and often exhibit c onflicting trends. We imposed step T changes on sealed columns of four sail s having variable theta, while monitoring bulk apparent dielectric constant (or permittivity epsilon(b)) and bulk electrical conductivity (sigma(b)) u sing TDR. Measured epsilon(b) increased substantially with increasing tempe rature for one silt loam soil, for all theta. For another silt loam soil an d for an Oxisol, measured epsilon(b) increased with increasing T at relativ ely 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. T he experimental results led to the hypothesis that TDR-measured epsilon(b) is determined by an interplay between two competing phenomena: (1) the redu ction in the dielectric constant of bulk water with increased T; and (2) th e increase in TDR-measured epsilon(b) with increased T due to release of bo und 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-t extured and organic soils and potentially for microwave remote sensing of s oil water status.