Campbell Scientific HydroSense® Soil Water Content System (CS620, CD620) - Manual (Page 21)

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 .
A-1