Campbell CS616 Instruction Manual download pdf (Page 35)

CS616 and CS625 Water Content Reflectometers

7.4 Operating Range

7.4.1 Soil Electrical Conductivity

The quality of soil water measurements which apply electromagnetic fields to

wave guides is affected by soil electrical conductivity. The propagation of

electromagnetic fields in the configuration of the CS616/CS625 is

predominantly affected by changing dielectric constant due to changing water

content, but it is also affected by electrical conductivity. Free ions in soil

solution provide electrical conduction paths which result in attenuation of the

signal applied to the waveguides. This attenuation both reduces the amplitude

of the high-frequency signal on the probe rods and reduces the bandwidth. The

attenuation reduces oscillation frequency at a given water content because it

takes a longer time to reach the oscillator trip threshold.

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)

σ σ

θ σ

bulk solution

v 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; and Τ,

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 CS616/CS625 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 same water content.

Discussion of the effects of soil electrical conductivity on CS616/CS625

performance will be on a soil solution or extract basis unless stated otherwise.

When soil solution electrical conductivity values exceed 2 dS m

-1

, the response

of the CS616/CS625 output begins to change. The slope decreases with

increasing electrical conductivity. The probe will still respond to water content

changes with good stability, but the calibration will have to be modified. (See

Section 8, Water Content Reflectometer User-Calibration.) At electrical

conductivity values greater than 5 dS m

-1

the probe output can become unstable.

7.4.2 Soil Organic Matter, Clay Content and Soil Bulk Density

The amount of organic matter and clay in a soil can alter the response of

dielectric-dependent methods to changes in water content. This is apparent

when mechanistic models are used to describe this measurement methodology.

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