Instruction manual

CS650 and CS655 Water Content Reflectometers
The effect of temperature on probe electronics is minimal with period average
readings varying by less than 0.5% of the 20 °C reading over the range of 10 to
30 °C and less than 2% of the 20 °C reading over the range of –10 to 70 °C.
The larger error is caused by the change in dielectric permittivity of soil with
temperature. This is mostly due to the high temperature dependence of the
permittivity of water, which varies from a value of 88 at 0 °C to 64 at 70 °C.
Since water is the major contributor to bulk dielectric permittivity of soil,
temperature related changes to the permittivity of water will lead to
overestimation of volumetric water content at temperatures below 20 °C and
underestimation of volumetric water content at temperatures above 20 °C.
The Topp equation does not account for soil temperature. The effect of
temperature on the soil permittivity is related to soil specific properties such as
porosity and the permittivity of the soil solid phase with temperature.
Consequently, a general equation that corrects volumetric water content for
temperature for all soils is not available.
A temperature correction equation that works well in quartz sand is given by:
θ
Corr
= θ - 0.0044*Tθ
3
+ 0.0014*Tθ
2
+ 0.0029*Tθ – 0.0002*T + 2.4*θ
3
–
1.6*θ
2
+ 0.32*θ – 0.046
where θ
Corr
is the temperature corrected volumetric water content, T is soil
temperature in °C, and θ is the volumetric water content value at soil
temperature T.
8.3.5.1 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.
8.4 Water Content Reflectometer User-Calibration
8.4.1 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.
8.4.2 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.
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