Time domain reflectometry developments in soil science: III. Small-scale probe for measuring bulk soil electrical conductivity

Henrik H. Nissen, Paul A. Ferré, Per Moldrup

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


It is commonly believed that Time Domain Reflectometry (TDR) measures bulk soil electrical conductivity (EC) and volumetric water content within the same, well-defined sample volume. However, recent studies have shown that the sample volume is a function of the distribution of EC and dielectric permittivity near the probe. One result of this spatially distributed sensitivity is measurement-induced dispersion. That is, when TDR is used to measure a sharp advancing solute front, the measured EC is some average across the sharp front, leading to incorrect smoothing of the breakthrough curve. A reduction of the probe dimensions is the only solution to this artificial smoothing problem. In this study, a small scale TDR probe is presented and tested. The small probe dimensions produce a near point measurement of EC but make water content measurements unreliable. The small scale EC TDR (SEC-TDR) probe is simple, inexpensive, and made with readily available components. A solute transport experiment was carried out under saturated conditions in a plastic pipe packed with coarse silica sand. Five SEC-TDR probes were inserted, monitoring the EC at various positions along the column, and a coaxial flow cell was used to monitor the effluent EC. Step solute breakthrough and displacement breakthrough responses were created using tap water and a KCl solution. Highly detailed measurements of EC were obtained from which the dispersivity (λ) was inferred. The λ measured by the SEC-TDR probes was significantly lower than λ measured in the effluent by the coaxial flow cell, suggesting that the SEC-TDR probe can reduce the problem of TDR-induced dispersion under even the most challenging conditions.

Original languageEnglish (US)
Pages (from-to)90-98
Number of pages9
JournalSoil Science
Issue number2
StatePublished - Feb 1 2003


  • Electrical conductivity
  • Small-scale TDR probe
  • Solute transport
  • Time Domain Reflectometry

ASJC Scopus subject areas

  • Soil Science


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