Instruction manual

CS650 and CS655 Water Content Reflectometers
using an electrical conductivity measurement. A calibration equation converts
period and electrical conductivity to bulk dielectric permittivity. The Topp
equation is used to convert from permittivity to volumetric water content.
8.3.2 The Topp Equation
The relationship between dielectric permittivity and volumetric water content
in mineral soils has been described by Topp et al. (1980) in an empirical
fashion using a 3
rd
degree polynomial. With θ
v
the volumetric water content
and K
a
the bulk dielectric permittivity of the soil, the equation presented by
Topp et al. is
θ
v
= -5.3*10
-2
+ 2.92*10
-2
K
a
– 5.5*10
-4
K
a
2
+ 4.3*10
-6
K
a
3
It has been shown in numerous research efforts that this equation works well in
most mineral soils, so a soil specific calibration of the CS650 probe is usually
not necessary. If a soil specific calibration is desired, the user can generate an
equation relating K
a
to θ
v
following the methods described in Section 8.4,
Water Content Reflectometer User-Calibration
(p. 23).
8.3.3 Electrical Conductivity
8.3.3.1 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 CS650 is predominantly
affected by changing dielectric permittivity 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, σ
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)
σ σ θ σ
bulk solution
= +
v solid
Τ
with σ
bulk
being the electrical conductivity of the bulk soil; σ
solution
, the soil
solution; σ
solid
, the solid constituents; θ
v
, the volumetric water content; andΤ, 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 CS650 operation such as clay content
and compaction. The above equation is presented here to show the relationship
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