Calibration of time domain reflectometry for water content measurement using a composite dielectric approach

Time domain reflectometry (TDR) has been developed to an operational level for the measurement of soil water content during the past decade. Because it is able to provide fast, precise and nondestructive in situ measurements, it has become an alternative to the neutron scattering method, in particular for monitoring water content under field conditions. One of the major disadvantages of the neutron scattering technique is that, due to the relatively high sensitivity of the signal to factors other than water content, site-specific calibration is usually required. In this paper a calibration curve for the TDR method is presented which is not restricted to specific soil conditions. The calibration is based on the dielectric mixing model of Dobson et al. (1985). Measurements of volumetric water content and dielectric number at eleven different field sites representing a wide range of soil types were used to determine the parameter of the model by weighted nonlinear regression. The uncertainty (root mean square error) of water content values calculated with the optimized calibration curve was estimated not to exceed 0.013 cm3/cm3. This value is comparable to the precision of the thermogravimetric method. From a sensitivity analysis it was determined that the temperature dependence of the TDR signal may have to be corrected to obtain optimum accuracy.

[1]  R. L. Smith-Rose,et al.  The electrical properties of soil at frequencies up to 100 megacycles per second; with a note on the resistivity of ground in the United Kingdom , 1935 .

[2]  W. A. G. Voss,et al.  Generalized approach to multiphase dielectric mixture theory , 1973 .

[3]  Allan J. Delaney,et al.  Dielectric properties of soils at UHF and microwave frequencies 36R. J. Geophys. Res. V79, N11, Apr. 1974, P1699–1708 , 1974 .

[4]  C. G. Gardner,et al.  High dielectric constant microwave probes for sensing soil moisture , 1974 .

[5]  J L Davis.  Relative permittivity measurements of a sand and clay soil, in situ , 1975 .

[6]  M. Brereton.  Classical Electrodynamics (2nd edn) , 1976 .

[7]  D. Wobschall,et al.  A Theory of the Complex Dielectric Permittivity of Soil Containing Water: The Semidisperse Model , 1977, IEEE Transactions on Geoscience Electronics.

[8]  A. P. Annan,et al.  Electromagnetic determination of soil water content: Measurements in coaxial transmission lines , 1980 .

[9]  T. Schmugge,et al.  An Empirical Model for the Complex Dielectric Permittivity of Soils as a Function of Water Content , 1980, IEEE Transactions on Geoscience and Remote Sensing.

[10]  V. Nesterov,et al.  Dependence of dielectric permittivity of moist disperse materials on the temperature , 1982 .

[11]  F. Ulaby,et al.  Microwave Dielectric Behavior of Wet Soil-Part II: Dielectric Mixing Models , 1985, IEEE Transactions on Geoscience and Remote Sensing.

[12]  F. Ulaby,et al.  Microwave Dielectric Behavior of Wet Soil-Part 1: Empirical Models and Experimental Observations , 1985, IEEE Transactions on Geoscience and Remote Sensing.

[13]  S. Dasberg,et al.  Time domain reflectometry field measurements of soil water content and electrical conductivity , 1985 .

[14]  F. N. Dalton,et al.  THE TIME-DOMAIN REFLECTOMETRY METHOD FOR MEASURING SOIL WATER CONTENT AND SALINITY , 1986 .

[15]  A. Alharthi,et al.  Soil water saturation: Dielectric determination , 1987 .

[16]  W. Press,et al.  Numerical Recipes: The Art of Scientific Computing , 1987 .