SPATIAL AVERAGING OF WATER-CONTENT BY TIME-DOMAIN REFLECTOMETRY - IMPLICATIONS FOR TWIN ROD PROBES WITH AND WITHOUT DIELECTRIC COATINGS

The averaging of apparent relative dielectric permittivities by time d omain reflectometry (TDR) is examined for properties varying along TDR waveguides and in the plane perpendicular to a TDR probe. A square ro ot averaging model with uniform weighting factors describes the measur ed apparent relative dielectric permittivity for axially varying mater ials; variations in the transverse plane are described by an inverse a veraging model with nonuniform spatial weighting. We define the sensit ivity of a TDR probe as the change in the measured travel time for a g iven change in the average soil water content for rods of a fixed leng th. For rods with dielectric coatings, this sensitivity increases with the dielectric permittivity of the coating and the rod radius and dec reases with the soil dielectric permittivity. Due to the dependence of the sensitivity on the soil water content, coated rods will not measu re the actual average water content for conditions of axially variable water contents.