C-1
Appendix C. 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
=
m
water
m
soil
=
m
wet
−
m
dry
m
dry
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
v
=
volume
water
volume
soil
=
m
water
ρ
water
m
soil
ρ
soil
=
θ
g
•
ρ
soil
ρ
water
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
=
m
dry
volume
sample
Another useful property, soil porosity (
ε
), is related to soil bulk density as
shown by the following expression.
ε
= 1
−
ρ
bulk
ρ
solid
The term
ρ
solid
is the density of the soil solid fraction and is approximately
2.65 g cm
–3
.
