8.3.5.1 Accurate soil temperature measurement
The thermistor used for measuring soil temperature is located in the sensor head and is in
contact with one of the stainless steel rods. To make an accurate soil temperature measurement,
the sensor head should be buried in the soil so that it is insulated from diurnal temperature
fluctuations.
8.4 Water content reflectometer user-
calibration
8.4.1 Need for soil specific calibration equation
While the Topp equation works well in a wide range of mineral soils, there are soils for which a
user-derived calibration optimizes 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 sensor is expected to measure.
8.4.2 User-derived calibration equation
A quadratic equation or third order polynomial can describe the relationship between soil
permittivity and volumetric water content. 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 Cn, the
calibration coefficient.
Third-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 Cn, the
calibration coefficient.
Linear form:
θ
v
(K
a
) = C
0
+ C
1
•K
a
0.5
CS650 and CS655 Water Content Reflectometers
26
