Campbell CS616, User Manual

Page: 33 / 44

A-1
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 (

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
).

g
water
soil
wet dry
dry
m
m
m m
m
 

Volumetric water content (

v
) is the volume of liquid water per volume of
soil. Volume is the ratio of mass to density (

b
) which gives:
water
soil g
soil
soil
water
water
soil
water
v
m
m
volume
volume

 




  
The density of water is close to 1 and often ignored.
Soil bulk density (

bulk
) is used for

soil
and is the ratio of soil dry mass to
sample volume.

bulk
dry
sample
m
volume

Another useful property, soil porosity (

), is related to soil bulk density as
shown by the following expression.



 
1
bulk
solid
The term

solid
is the density of the soil solid fraction and is approximately
2.65 g cm
–3
.