The Hydra Probes’ design features such as the geometry of the wave guide and the frequency at
50 MHz allows the Hydra Probe to simultaneously measure both real and imaginary dielectric
constants (Campbell, 1990). The algorithmic treatment method of the four raw voltages is
derived from the solution to a complex function.
5.2
Real Dielectric and Imaginary Permittivity (temperature
corrected)
Since both the real and imaginary dielectric permittivities will vary somewhat with temperature,
the user of the Hydra Probe has the option of selecting the temperature corrected values for the
real and imaginary dielectric permittivity. The uncorrected and the corrected dielectric values
may be of interest for some researchers. The soil moisture calibrations are based on the
temperature corrected values because the calibration curves were established at a constant
temperature. Similarly, the temperature corrected imaginary dielectric constant was used for the
electric conductivity measurement.
As the temperature increases, the molecular vibration increases (Levine, 1993). The molecular
vibration has a complex effect on both the orientation polarization and on the imaginary
dielectric constant. The temperature corrections are based on the small incremental changes of
Ei
and
Er
with temperature.
5.3
Soil Electrical Conductivity
.
The Hydra Probe measures the in situ electrical conductivity in units of Siemens per meter (S/m).
Soil electrical conductivity is indicative of dissolved salts, dissolved solids, and fertilizers
(McBride 1994). It may also be indicative of very high pH conditions. The soil electrical
conductivity is calculated from the imaginary dielectric permittivity [
E
i
]. by the theoretical
expression:
EC = 2
π
f
ε
0
E
i
[5.4]
Where EC is the electrical conductivity, f is the frequency (50 MHz for the Hydra Probe) and
ε
0
is the dielectric constant of a vacuum.
5.3.1 Electrical Conductivity Pathways in Soil
The electric conductivity of soil is complex. Figure [5.2] shows the three pathways the electrical
conductivity can propagate in soil. The bulk density, the porosity, the tortuosity, the water
content, and the dissolved ion concentration working in concert with the different pathways,
dramatically influences the electrical conductivity of a soil.
Pathway 1 is the electrical pathway that goes from water to the soil and back through the water
again. The electrical conductivity contribution of pathway 1 is a function of the conductivity of
the water and soil. As water increases, pathway 1 increases which may increase the electrical
conductivity of the soil as a whole.
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