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a Crop Production, Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, 960 Carling Ave., Ottawa, Canada, K1A 0C6
b Land and Water Commonwealth Scientific and Industrial Research Organization, G.P.O. Box 1666, Canberra, ACT 2601, Australia
c Water Research Foundation of Australia, Centre for Resources and Environmental Studies, Australian National Univ., Canberra, ACT 2601, Australia
toppc{at}em.agr.ca
Time domain reflectometry (TDR) is widely used for routine field monitoring of water content and salts in soils. Most estimates of water content assume the TDR-measured apparent relative permittivity, 
a, is a good approximation for the real component, 'r, of the soil's complex relative permittivity with the magnitude of 'r being determined primarily by water content. We examine this assumption and show that a is influenced by both the real and imaginary components of the relative permittivity. Increases in a resulted from the dc conductivity and dielectric loss arising from the presence of ions in solution and clay content. At water contents above 0.15 m3 m-3 in soils with high clay content and/or salt, specific calibrations are needed for precise determinations of water content from TDR. We use the wave propagation equations to separate the real and imaginary component contributions to a. The Giese and Tiemann interpretation for dc conductivity was again shown to be within 10% of that from a conductance meter and this fact was used to propose a method using only TDR data to separate real and imaginary components of the relative permittivity. It was found that the dielectric losses and conductive losses did not differ according to the source of conductivity, whether from clay content in the soil matrix or electrolyte in the soil solution.
Abbreviations: EC, electrical conductivity, TDR, time domain reflectrometry
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