Campbell CS650, User Manual

Page: 49 / 56

C-1
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 (

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:



 
 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 (

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
.