Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 36)

CS650 and CS655 Water Content Reflectometers
7.5.1 Accurate Soil Temperature Measurement
The thermistor used for measuring soil temperature is located in the probe head
and is in contact with one of the stainless steel rods. In order to make an
accurate soil temperature measurement, the probe head should be buried in the
soil so that it is insulated from diurnal temperature fluctuations.
8. Water Content Reflectometer User-Calibration
8.1 Need for Soil Specific Calibration Equation
While the Topp equation has been determined to work well in a wide range of
mineral soils, there are soils for which a user-derived calibration will optimize
accuracy of the volumetric water content measurement. The Topp equation
underestimates the water content of some organic, volcanic, and fine textured
soils. Additionally, porous media with porosity greater than 0.5 or bulk density
greater than 1.55 g cm -3 may require a media-specific calibration equation.
In these cases, the user may develop a calibration equation to convert CS650
permittivity to volumetric water content over the range of water contents the
probe is expected to measure.
8.2 The User-Derived Calibration Equation
The relationship between soil permittivity and volumetric water content may be
described by a quadratic equation or a 3 rd order polynomial. In many
applications, a linear equation similar to Ledieu et al (1986) gives required
accuracy.
Quadratic form:
θ
v
(K
a
) = C
0
+ C
1
*K
a
+ C
2
*K
a
2
with θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the
soil, and C
n
, the calibration coefficient.
3 rd degree polynomial form:
θ
v
(K
a
) = C
0
+ C
1
*K
a
+ C
2
*K
a
2 + C
3
*K
a
3
with θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the
soil, and C
n
, the calibration coefficient.
Linear form:
θ
v
(K
a
) = C
0
+ C
1
*K
a
0.5
with θ
v
the volumetric water content, K
a
the bulk dielectric permittivity of the
soil, and C
n
, the calibration coefficient.
Two data points from careful measurements can be enough to derive a linear
calibration. A minimum of three data points are needed for a quadratic
calibration. With three evenly spaced water contents covering the expected
range, the middle water content data point will indicate whether a linear or
polynomial calibration equation is needed.
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