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United States Department of Agriculture

Agricultural Research Service

Title: Time Domain Reflectometry Calibration in Terms of Travel Time, Bulk Electrical Conductivity, and Effective Frequency

item Evett, Steven
item Howell, Terry
item Tolk, Judy

Submitted to: ASA-CSSA-SSSA Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: November 6, 2005
Publication Date: November 6, 2005
Citation: Evett, S.R., Howell, T.A., Tolk, J.A. 2005. Time domain reflectometry calibration in terms of travel time, bulk electrical conductivity, and effective frequency [abstract]. Agronomy Abstracts, ASA-CSSA-SSSA Annual Meeting, Salt Lake City, Utah. 2005 CDROM.

Technical Abstract: Many calibrations of conventional time domain reflectometry (TDR) have been done with the apparent electrical permittivity as the independent variable and water content as the dependent variable (or vice versa). Since the apparent permittivity is calculated as a function of the TDR pulse two-way travel time (tt) down along a waveguide of length, L, this is practically the same as calibrating in terms of [(tt)/2L]. Recent studies have confirmed that the travel time increases not only with water content, but also with increasing bulk electrical conductivity (BEC). Also, the apparent permittivity (and thus travel time) increases as the effective frequency, Fe, of the signal decreases. We conducted calibrations for TDR derived water content in three soils (17%, 30%, and 48% by mass of smectitic and montmorillonitic clays) in terms of travel time, BEC, and Fe. Slopes on the travel time term varied significantly when only tt was considered, and calibrations in the two more clayey soils did not account for temperature induced variation in reported water content. Slopes varied even more when both tt and BEC were included. When all three properties were considered, slope differences on the travel time term were minimized and temperature effects on reported water contents were practically eliminated. All three properties may be measured from TDR waveforms, allowing correction for temperature effects without separate measurements of temperature.

Last Modified: 5/5/2015
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