Abstract
a USDA-ARS, George E. Brown, Jr. Salinity Lab., 450 W. Big Springs Rd, Riverside, CA 92507
b USDA-ARS, Water Management Research Lab., 9611 S. Riverbend Ave., Parlier, CA 93648
c Alterra-ILRI, P.O. Box 47, 6700 AA Wageningen, The Netherlands
d Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820
* Corresponding author ( tkelleners{at}ussl.ars.usda.gov ).
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 that is able to correct for dielectric losses due to ionic conductivity and relaxation. The circuit inductance L is optimized using sensor readings in a modified setup where ceramic capacitors replace the sensor's capacitance plates. The three other parameters in the model are optimized using sensor readings in a range of nonconductive media with different permittivities. The geometric factor for the plastic access tube g p is higher than the geometric factor for the medium g m , indicating that most of the electromagnetic field does not go beyond the access tube. 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 permittivity. The circuit model is able to correct for the conductivity effect on the sensors. However, as the dielectric losses increase, the frequency becomes relatively insensitive to permittivity and small inaccuracies in the measured frequency or in the sensor constants result in large errors in the calculated permittivity. Calibration of the capacitance sensors can be simplified by fixing two of the constants and calculating the other two using sensor readings in air and water.
Abbreviations: TDR, time domain reflectometry