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.