Campbell Scientific CS616 CS616 and CS625 Water Content Reflectometers - Page 47

Appendix A. Discussion of Soil Water, Content

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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 (θg) is the mass of water per mass of dry soil. It is measured by weighing a soil sample (mwet), drying the sample to remove the water, then weighing the dried soil (mdry). θg = m water msoil = mwet − mdry mdry Volumetric water content (θv) is the volume of liquid water per volume of soil. Volume is the ratio of mass to density (ρb) which gives: m water θv = volume water volumesoil = ρwater msoil = θg *ρsoil ρwater ρsoil The density of water is close to 1 and often ignored. Soil bulk density (ρbulk) is used for ρsoil and is the ratio of soil dry mass to sample volume. ρbulk = mdry volumesample Another useful property, soil porosity (ε), is related to soil bulk density as shown by the following expression. ε = 1 − ρbulk ρsolid The term ρsolid is the density of the soil solid fraction and is approximately 2.65 g cm-3. Notes: A-1

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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