CS650 and CS655 Water Content Reflectometers
22
voids around the rods indicate lower water content than actual. In some
applications, the CS650G insertion and pilot tool helps maintain the proper
spacing and parallel orientation of the rods during sensor insertion.
8.3.5 Temperature Dependence and Correction
The two temperature dependent sources of error in CS650 water content
measurements are the effect of temperature on the operation of the sensor
electronics and the effect of temperature on the dielectric permittivity of the
soil.
The effect of temperature on sensor 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 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•T
θ
3
+ 0.0014•T
θ
2
+ 0.0029•T
θ
– 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 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.