Specifications
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 (
T
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
).
T
g
water
soil
wet dry
dry
m
m
mm
m
Volumetric water content (
T
v
) is the volume of liquid water per volume of soil.
Volume is the ratio of mass to density (
U
) which gives
T
U
U
TU
U
v
water
soil
water
water
soil
soil
g soil
water
volume
volume
m
m
.
Soil bulk density (
U
bulk
) is used for
U
soil
and is the ratio of soil dry mass to
sample volume. The density of water,
U
water
,
is close to 1 and is often ignored.
Another useful property, soil porosity, (
H
) is related to soil bulk density as
shown by the following expression.
H
U
U
1
bulk
solid
The term
U
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 105qC for 24 hours.
m
wet
94 g
m
dry
78 g
sample volume 60 cm
3
T
g
water
soil
m
m
94 78
78
0 205
1
g g
g
g g.
U
bulk
dry
m
volume
78
60
13
3
3
g
cm
g cm.
A-1