Campbell CS616 User Manual - page 22
CS616 and CS625 Water Content Reflectometers
14
Table 8-2. Calibration coefficients for sandy clay loam with bulk
density 1.6 g cm
–3
and electrical conductivity at saturation 0.4 dS m
–1
for both linear and quadratic forms.
Linear
Quadratic
C0
C1
C0
C1
C2
–0.6200
0.0329
0.0950
–0.0211
0.0010
Table 8-3. Calibration coefficients for sandy clay loam with bulk
density 1.6 g cm
–3
and electrical conductivity at saturation 0.75 dS m
–1
for both linear and quadratic forms.
Linear
Quadratic
C0
C1
C0
C1
C2
–0.4470
0.0254
–0.0180
–0.0070
0.0006
8.1.3 Operating Range
8.1.3.1 Soil Electrical Conductivity
The quality of soil water measurements which apply electromagnetic fields
to wave guides is affected by soil electrical conductivity. The propagation
of electromagnetic fields in the configuration of the CS616/CS625 is
predominantly affected by changing dielectric constant due to changing
water content, but it is also affected by electrical conductivity. Free ions in
soil solution provide electrical conduction paths which result in attenuation
of the signal applied to the waveguides. This attenuation both reduces the
amplitude of the high-frequency signal on the probe rods and reduces the
bandwidth. The attenuation reduces oscillation frequency at a given water
content because it takes a longer time to reach the oscillator trip threshold.
It is important to distinguish between soil bulk electrical conductivity and
soil solution electrical conductivity. Soil solution electrical conductivity
refers to the conductivity of the solution phase of soil. Soil solution
electrical conductivity,
solution
can be determined in the laboratory using
extraction methods to separate the solution from the solid and then
measuring the electrical conductivity of the extracted solution.
The relationship between solution and bulk electrical conductivity can be
described by (Rhoades et al., 1976)
bulk
solution
v
solid
with
bulk
being the electrical conductivity of the bulk soil;
solution
, the
soil solution;
solid
, the solid constituents;
v
, the volumetric water
content; and
, a soil-specific transmission coefficient intended to account
for the tortuosity of the flow path as water content changes. See Rhoades
et al., 1989 for a form of this equation which accounts for mobile and
immobile water. This publication also discusses soil properties related to
CS616/CS625 operation such as clay content and compaction. The above
equation is presented here to show the relationship between soil solution
electrical conductivity and soil bulk electrical conductivity.
Most expressions of soil electrical conductivity are given in terms of
solution conductivity or electrical conductivity from extract since it is