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 (θg) 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
).
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:
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.
Another useful property, soil porosity (ε), is related to soil bulk density as shown by the following
expression.
The term ρ
solid
is the density of the soil solid fraction and is approximately 2.65 g cm
–3
.
CS650 and CS655 Water Content Reflectometers
40
