Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2004
Publication Date: 2/1/2005
Citation: Kelleners, T.J., Robinson, D.A., Shouse, P.J., Ayars, J.E., Skaggs, T.H., 2005. Frequency dependence of the complex permittivity and its impact on dielectric sensor calibration in soils. Soil Science Society of America Journal. 69:67-76. Interpretive Summary: Farmers can improve irrigation and water management by monitoring the water content of the soil during the growing season. Electromagnetic instruments that measure soil water content are increasingly used for monitoring because they are rapid, safe, non-destructive, and easily automated. In this work, we investigated the accuracy of two electromagnetic instruments and the impact that the instrument frequency has on performance. We found that the performance of commercial instruments would probably be improved if they were designed to operate at frequencies higher than those currently in use; the higher frequencies reduce interferences caused by various interactions between solid particles, water, and dissolved ions (particularly in clayey soils). This work contributes to a better understanding of electromagnetic techniques for measuring soil water content, and will aid scientists and industry in developing sensor and monitoring technologies that can be used to improve water management.
Technical Abstract: The capacitance method and time domain reflectometry (TDR) are two popular electromagnetic techniques used to estimate soil water content. However, the frequency dependence of the real and imaginary part of the permittivity complicates sensor calibration. The frequency dependence can be particularly significant in fine textured soils containing clay minerals. In this work, we applied both the capacitance method and TDR to a non-dispersive medium (fine sand) and a strongly dispersive medium (Bentonite). The measurements were conducted for a range of water contents. Results using a network analyzer showed that the frequency dependence of the real permittivity of the Bentonite was particularly strong below 500 MHz. Above this frequency, the real permittivity of the Bentonite was mainly a function of the water content. The capacitance sensor (frequency of 100-150 MHz) overestimated the real permittivity of the Bentonite at high water contents. An electric circuit model proved partially successful in correcting the capacitance data by taking the dielectric losses into account. The apparent permittivity as measured with TDR for Bentonite at low water contents did not match with the real and imaginary permittivity from the network analyzer. This was attributed to the dispersive nature of the Bentonite which resulted in the underestimation of the maximum frequencies (260-447 MHz) in the TDR signal using a rise time method. The TDR signal became attenuated at higher water contents. It seems worthwhile to raise the effective frequency of dielectric sensors above 500 MHz to benefit from the relatively stable permittivity region at this frequency.