Campbell Hydrosense, Instruction Manual

Page: 19 / 27

A-1
Appendix A. Definition of Water
Content Terms
A.1 Definition of Water Content Terms
Soil water content is expressed on a gravimetric or volumetric basis.
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
) which gives
θ
ρ
ρ
θ ρ
ρ
v
water
soil
water
water
soil
soil
g soil
water
volume
volume
m
m
= = =
∗
.
Soil bulk density (
ρ
bulk
) is used for
ρ
soil
and is the ratio of soil dry mass to
sample volume. The density of water,
ρ
water
, is close to 1 and is often ignored.
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 well approximated
by the value 2.6 g cm
-3
.
A simple data set is given here as an example. A sample of known volume was
weighed before and after oven drying at 105
°
C for 24 hours.
m
wet
94 g
m
dry
78 g
sample volume 60 cm
3
θ
g
water
soil
m
m
= =
−
=
−
94 78
78
0 205
1
g g
g
g g .
ρ
bulk
dry
m
volume
= = =
−
78
60
13
3
3
g
cm
g cm .