The dependence of the dielectric constant, at frequencies between 1 MHz and 1 GHz, on the volumetric water content is determined empirically in the laboratory. The effect of varying the texture, bulk density, temperature, and soluble salt content on this relationship was also determined. Time‐domain reflectometry (TDR) was used to measure the dielectric constant of a wide range of granular specimens placed in a coaxial transmission line. The water or salt solution was cycled continuously to or from the specimen, with minimal disturbance, through porous disks placed along the sides of the coaxial tube. Four mineral soils with a range of texture from sandy loam to clay were tested. An empirical relationship between the apparent dielectric constant Ka and the volumetric water content θv, which is independent of soil type, soil density, soil temperature, and soluble salt content, can be used to determine θv, from air dry to water saturated, with an error of estimate of 0.013. Precision of θv to within ±0.01 from Ka can be obtained with a calibration for the particular granular material of interest. An organic soil, vermiculite, and two sizes of glass beads were also tested successfully. The empirical relationship determined here agrees very well with other experimenters' results, which use a wide range of electrical techniques over the frequency range of 20 MHz and 1 GHz and widely varying soil types. The results of applying the TDR technique on parallel transmission lines in the field to measure θv versus depth are encouraging.

For measurement of water content using TDR, parallel wire transmission lines varying in length from 0.125 to 1 m were installed vertically at planting time at three sites in a corn field. At one of the sites horizontal lines and additional vertical transmission lines with electrical impedance discontinuities were installed for comparison. Measurements of water content using a portable TDR cable tester were made periodically during the growing season. Comparisons of water contents by TDR with those from gravimetric samples showed that generally both were the same values. Standard deviations of differences between TDR and gravimetric values were ± 0.02 m3m−3 when measured locations were the same but increased to ± 0.06 m3m−3 when measured locations were different. Repeated measurements at the same location were highly correlated, one with another, over the season. Analysis of variance showed that all transmission line types were yielding equivalent values and that the horizontal transmission lines gave the minimum standard error of the mean. Data from transmission lines with impedance discontinuities gave water content profiles from a single measurement but the analyses of the TDR data curves were more complex than for the lines without impedance discontinuities. The variety of transmission line configurations for use in TDR measurement allows considerable flexibility of choice in relation to one's application.

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, ϵa, is a good approximation for the real component, ϵ′r, of the soil's complex relative permittivity with the magnitude of ϵ′r being determined primarily by water content. We examine this assumption and show that ϵa is influenced by both the real and imaginary components of the relative permittivity. Increases in ϵa 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 ϵa 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.

The water content-pressure head relationship for a small, well-confined, rectangular sample of fine sand was obtained under different water flow conditions. Water contents were measured by a gamma system and pressure heads were measured by a tensiometer-pressure transducer combination. During drying, more water was retained in the sand at a given pressure head in the unsteady flow case than in the static equilibrium and steadystate cases, which agreed with each other.

The time‐domain reflectometry (TDR) technique has been demonstrated to be a viable method for determining soil water content in uniformly wet soils. The TDR response in nonuniformly wet soils was examined herein both theroetically and experimentally. An extensive set of experiments was carried out in a controlled laboratory test facility which permitted synthesis of steep water content gradients and wetting fronts. The water content measured by the TDR technique has been found to be the same as the average water content to within 0.01 cm3cm−3 in extremely nonuniform conditions. This result is quite surprising at first but is readily explained by the nature of the dielectric constant and water content relationship. The TDR technique has also been found useful in detecting and monitoring the progression of wetting front advance through a soil.