Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2002
Publication Date: 1/2/2004
Citation: Kelleners, T.J., Soppe, R.W., Robinson, D.A., Schaap, M.G., Ayars, J.E., Skaggs, T.H. 2004. Calibration of capacitance probe sensors using electric circuit theory. Soil Science Society of America Journal. 68:430-439. Interpretive Summary: Soil water content is an important parameter in many agricultural and environmental studies. Measuring soil water content by electromagnetic techniques has become very popular because it is rapid, safe, non-destructive, and easily automated. There is concern, however, about the influence of soil salinity on the instrument readings. We developed a model that describes the behaviour of a commercial electromagnetic sensor in conductive, saline media. Both the model and laboratory data showed that neglecting the conductivity effect on the instrument readings causes a significant overestimation of the soil water content. The model can be used to correct the instrument readings to a certain extent. This study contributes to a better understanding of the performance of electromagnetic sensors in saline soils. The findings will benefit the wide range of people who are interested in monitoring soil water content, including farmers, water managers, government agencies, and researchers.
Technical Abstract: Capacitance probe sensors are an attractive electromagnetic technique for estimating soil water content. There is concern, however, about the influence of soil salinity and soil temperature on the sensors. We present an electric circuit model that relates the sensor frequency to the permittivity of the medium and is able to correct for small values of ionic conductivity. The five parameters in the model are optimized using sensor readings in a range of non-conductive media with different permittivities. The effect of ionic conductivity on the sensor readings is assessed by mixing salts in three of the media. The influence is profound. The sensor frequency decreases with increasing conductivity. The effect is most pronounced for the medium with the lowest relative permittivity. The circuit model is able to correct for the conductivity effect on the sensors up to a conductivity level of ~0.25 S m-1, with the threshold being a function of the permittivity of the medium. Beyond this level, the model does not provide a solution. Calibration of the capacitance sensors can be simplified by fixing three of the constants and calculating the other two, using sensor readings in air and water.