Evaluation of a capacitance probe freque

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

Evaluation of a Capacitance Probe Frequency Response Model

Accounting for Bulk Electrical Conductivity: Comparison with

TDR and Network Analyzer Measurements

D. A. Robinson,* T. J. Kelleners, J. D. Cooper, C. M. K. Gardner,

P. Wilson, I. Lebron, and S. Logsdon

ABSTRACT cil, 2001). A great deal of effort has been expended in

trying to determine water content at a range of scales. Soils ranging in texture from sand to clay were used to compare

permittivity measurements made using a Surface Capacitance Inser- Many measurement methods have been reviewed by

tion Probe (SCIP) and time domain reflectometer (TDR). Measure- Gardner et al. (2001) and Topp and Ferre (2002), among

ments were made using the same electrodes embedded in each soil, others. Two of the more commonly used electromag￾making the measurements directly comparable. The objective of the netic methods for determining water content at the sam￾work was to test a model describing the frequency response of the ple scale are the TDR method (Topp et al., 1980; Jones

SCIP to both permittivity and electrode conductance, and to compare et al., 2002; Robinson et al., 2003) and the capacitance

results with TDR and network analyzer measurements. The model probe method (Wobschall, 1978; Dean et al., 1987; Bell

was tested using liquids of known permittivity and in saline, dielectric et al., 1987; Evett and Steiner, 1995; Paltineanu and solutions. Surface Capacitance Insertion Probe and TDR determined Starr, 1997; Kelleners et al., 2004). A surface capacitance permittivity values are similar for sandy soils but diverge for loam and insertion probe (SCIP) (Robinson and Dean, 1993; Dean, clay soils. Using Topp’s values as a reference, the SCIP-determined per￾mittivities for loams and clays lay close to the curve at water contents 1994; Robinson et al., 1998) is used in this work. Capaci-

0.25 m tance probes such as the EnviroSCAN (Sentek, Stepney, 3 m3

, then often rose above the curve with increasing water

content. Surface Capacitance Insertion Probe permittivity correction, Australia) have become popular for irrigation schedul￾using electrical conductivity (EC) measured at 1 kHz, corrected the ing (Evett and Steiner, 1995; Paltineanu and Starr, 1997;

results in sands reasonably well but not enough in loams and clays Kelleners et al. 2004). Hence, an understanding of the

for reliable calibration. We propose three possible reasons for the higher response of this instrument provides some insight into

than expected permittivity values observed using the SCIP: (i) higher the operation and performance of similar sensors. Both

than expected real permittivity created by dielectric dispersion, (ii) TDR and capacitance sensors attempt to measure the a large contribution of the imaginary permittivity due to relaxation permittivity of the soil medium. Because they may not processes assumed to be negligible, and (iii) poor model prediction do so perfectly, the measurement yielded by the instru- of permittivity due to excessive damping of the oscillator circuit with

high EC and dielectric losses. Results from network analyzer measure- ment is termed the apparent permittivity.

ments for one of the clay soils were used to aid data interpretation. The Many calibration equations to relate apparent permit￾TDR measurements were much more consistent, producing apparent tivity to water content have been presented in the litera￾relative permittivity values below those of the Topp curve for the ture (Topp et al., 1980; Roth et al., 1992; Jacobsen and

finer textured soils. Schjonning, 1993; Malicki et al., 1996). Many of these

equations are used interchangeably among different

types of sensors. Water content determination is a two￾T

he life-sustaining reservoir for plant and micro- step process—from sensor response to permittivity (Jones

bial communities is soil water, a key component of et al., 2005; Blonquist et al., 2005), and from permittivity

the hydrological cycle. As such, knowledge of soil water to water content. Errors or invalid assumptions in the

content is required for using global circulation models first step will lead to difficulty in making interpretations

to estimate heat and vapor fluxes in what has been re- at the next step. In this study we compared measure￾ferred to as the “critical zone” (Committee on Basic Re- ments made using a surface capacitance insertion probe

search Opportunities in the Earth Sciences, Board on and TDR in 12 soils. The initial objective of this work

Earth Sciences and Resources, National Research Coun- was to evaluate a calibration model developed for the

SCIP. This uses well-defined dielectric solutions and di￾electric solutions with ionic conductivity to determine

D.A. Robinson and I. Lebron, Dep. of Plants, Soils and Biometeorol- whether accounting for EC measured at 1 kHz improves ogy, Utah State University, Logan, UT, USA; T.J. Kelleners, George permittivity measurement. Solution electrical conduc- E. Brown Jr. Salinity Lab USDA-ARS, Riverside CA, USA; J.D.

tivity changes by about 2% C1 Cooper, Instrument Section, Centre for Ecology and Hydrology, Wall- and can have a strong im￾ingford, Oxon, UK; C.M.K. Gardner, IAHS Press, Centre for Ecology pact on the apparent permittivity measurement. Elimi￾and Hydrology, Wallingford, Oxon, UK; P. Wilson, School of Environ- nating it from the apparent permittivity measurement

mental Sciences, University of Ulster, Coleraine, Co. Londonderry, can considerably improve water content determination. N. Ireland, UK; S. Logsdon, National Soil Tilth Lab USDA-ARS,

Ames/Ankeny, IA, USA. Received 14 Sept. 2004. *Corresponding The second objective was therefore to evaluate the model

author ([email protected]). permittivity predictions with and without accounting for

bulk soil EC to determine whether accounting for EC Published in Vadose Zone Journal 4:992–1003 (2005). could improve the permittivity and water content cali- Special Section: Soil Water Sensing

doi:10.2136/vzj2004.0131

 Soil Science Society of America Abbreviations: EC, electrical conductivity; SCIP, Surface Capacitance

677 S. Segoe Rd., Madison, WI 53711 USA Insertion Probe; TDR, time domain reflectometer.

992

Published online November 16, 2005