The fundamental principle for CS650 water content measurement is that the velocity of
electromagnetic wave propagation along the sensor rods is dependent on the dielectric
permittivity of the material surrounding the rods. As water content increases, the propagation
velocity decreases because of increasing dielectric permittivity. Therefore, the two-way travel
time of the rod signal is dependent upon water content, hence the name water content
reflectometer. Digital circuitry scales the high-speed oscillator output to an appropriate
frequency for measurement by an onboard microprocessor. Increases in oscillation period
resulting from signal attenuation are corrected 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 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 third-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
Research has shown that this equation works well in most mineral soils, so a soil specific
calibration of the CS650 sensor is usually not necessary. For a soil specific calibration, you can
generate an equation relating K
a
to θ
v
following the methods described in
reflectometer user-calibration
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
sensor 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. In the laboratory, extraction methods can determine soil solution electrical conductivity,
CS650 and CS655 Water Content Reflectometers
23
