Campbell CS616, Instruction Manual

Page: 51 / 54

Appendix A. Discussion of Soil Water
Content
The water content reflectometer measures volumetric water content. Soil water
content is expressed on a gravimetric and a volumetric basis. To obtain the
independently determined volumetric water content, gravimetric water content
must first be measured. Gravimetric water content (
θ
g) is the mass of water
per mass of dry soil. It is measured by weighing a soil sample (m
wet
), drying
the sample to remove the water, then weighing the dried soil (m
dry
).
θ
g
water
soil
wet dry
dry
m
m
m m
m
= =
−
Volumetric water content (
θ
v
) is the volume of liquid water per volume of soil.
Volume is the ratio of mass to density (
ρ
b
) which gives:
θ
ρ
ρ
θ ρ
ρ
v
water
soil
water
water
soil
soil
g soil
water
volume
volume
m
m
= = =
*
The density of water is close to 1 and often ignored.
Soil bulk density (
ρ
bulk
) is used for
ρ
soil
and is the ratio of soil dry mass to
sample volume.
ρ
bulk
dry
sample
m
volume
=
Another useful property, soil porosity (
ε
), is related to soil bulk density as
shown by the following expression.
ε
ρ
ρ
= −
1
bulk
solid
The term
ρ
solid
is the density of the soil solid fraction and is approximately
2.65 g cm
-3
.
Notes:
A-1