Campbell Scientific CS650 and CS655 Water Content Reflectometers User Manual

Page 34

CS650 and CS655 Water Content Reflectometers

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. Soil solution electrical

conductivity, σ

solution

can be determined in the laboratory using extraction

methods to separate the solution from the solid and then measuring the

electrical conductivity of the extracted solution.

The relationship between solution and bulk electrical conductivity can be

described by (Rhoades et al., 1976)

bulk

solution

solid

with σ

bulk

being the electrical conductivity of the bulk soil; σ

solution

, the soil

solution; σ

solid

, the solid constituents; θ

, the volumetric water content; andΤ, a

soil-specific transmission coefficient intended to account for the tortuosity of

the flow path as water content changes. See Rhoades et al., 1989 for a form of

this equation which accounts for mobile and immobile water. This publication

also discusses soil properties related to CS650 operation such as clay content

and compaction. The above equation is presented here to show the relationship

between soil solution electrical conductivity and soil bulk electrical

conductivity.

Most expressions of soil electrical conductivity are given in terms of solution

conductivity or electrical conductivity from extract since it is constant for a

soil. Bulk electrical conductivity increases with water content so comparison

of the electrical conductivity of different soils must be at the same water

content.

The calibration equation in the CS650 firmware corrects the oscillation

frequency for the effects of σ

solution

up to 3 dS m

-1

for the CS650 and up to

10 dS m

-1

for the CS655. This is equivalent to σ

bulk

values of approximately

0.8 dS m

-1

and 2.7 dS m

-1

respectively. If σ

bulk

exceeds these limits, the

CS650 probe will return 99999 for dielectric permittivity and volumetric water

content. The measured period average and voltage ratio values will continue to

be reported even if the bulk EC is outside the operational range of the probe.

7.3.2 Temperature Correction of Soil Electrical Conductivity

The EC value reported by the CS650 is bulk electrical conductivity. This value

is temperature dependent, changing by 2% per degree Celsius. To compensate

for the effect of temperature, EC readings may be converted to a standard

temperature, such as 25 °C using the following equation:

= EC

/ (1 + 0.02*(T

soil

-25)

where EC

is the σ

bulk

value at 25 °C and EC

is the σ

bulk

value at soil

temperature T

soil

(°C).