ARS | Publication request: Soil water sensor response to bulk electrical conductivity

Submitted to: American Society of Agronomy Meetings
Publication Type: Abstract Only
Publication Acceptance Date: July 27, 2012
Publication Date: October 22, 2012
Citation: Schwartz, R.C., Casanova, J.J., Evett, S.R., Bell, J.M., Pelletier, M.G., Baumhardt, R.L. 2012. Soil water sensor response to bulk electrical conductivity. {abstract}. American Society of Agronomy Meetings. 2012 CDROM. Paper No. 59-10.

Technical Abstract: Soil water monitoring using electromagnetic (EM) sensors can facilitate observations of water content at high temporal and spatial resolutions. These sensors measure soil dielectric permittivity (Ka) which is largely a function of volumetric water content. However, bulk electrical conductivity BEC can dominate the low frequency loss spectrum in soils, masking changes in the real permittivity and thus water content. All EM sensors exhibit some degree of sensitivity to bulk electrical conductivity. The objectives for this talk are to review the origin of the Ka dependence on BEC for broadband and capacitance technologies, present study results evaluating sensitivities of selected sensors in two contrasting soils, and discuss implications for sensor calibrations for use in soil water management. Study results demonstrated that time-domain reflectometry (TDR) measures Ka at an effective frequency of 160 MHz for a clay loam and 520 MHz for a sand. Accordingly, the slope of the permittivity – bulk electrical conductivity relationship (delta-Ka/delta-BEC)) for TDR averaged 3.5 m dS**1 in clay loam, similar to a capacitance sensor running at 70 MHz. However, in sand, TDR sensitivity to sa was negligible. Permittivity measured using a digital time-domain transmission probe exhibited little or no sensitivity to sa (< 0.32 m dS**1) for both clay loam and sand. Including sa in the calibration equation can improve water content estimation. However, such calibrations can be impractical for sensors that exhibit nonlinear Ka responses to sa or measure permittivity and bulk electrical conductivity in differing or disproportionate sampling volumes.