Abstract
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 language | English (US) |
|---|---|
| Pages (from-to) | 90-98 |
| Number of pages | 9 |
| Journal | Soil Science |
| Volume | 168 |
| Issue number | 2 |
| DOIs | |
| State | Published - Feb 1 2003 |
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Keywords
- Electrical conductivity
- Small-scale TDR probe
- Solute transport
- Time Domain Reflectometry
ASJC Scopus subject areas
- Soil Science
- Earth-Surface Processes
Cite this
Time domain reflectometry developments in soil science : III. Small-scale probe for measuring bulk soil electrical conductivity. / Nissen, Henrik H.; Ferre, Paul A; Moldrup, Per.
In: Soil Science, Vol. 168, No. 2, 01.02.2003, p. 90-98.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Time domain reflectometry developments in soil science
T2 - III. Small-scale probe for measuring bulk soil electrical conductivity
AU - Nissen, Henrik H.
AU - Ferre, Paul A
AU - Moldrup, Per
PY - 2003/2/1
Y1 - 2003/2/1
N2 - 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.
AB - 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.
KW - Electrical conductivity
KW - Small-scale TDR probe
KW - Solute transport
KW - Time Domain Reflectometry
UR - http://www.scopus.com/inward/record.url?scp=0037329640&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0037329640&partnerID=8YFLogxK
U2 - 10.1097/00010694-200302000-00003
DO - 10.1097/00010694-200302000-00003
M3 - Article
AN - SCOPUS:0037329640
VL - 168
SP - 90
EP - 98
JO - Soil Science
JF - Soil Science
SN - 0038-075X
IS - 2
ER -