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Concept
Water content
Summary
Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 (completely dry) to the value of the materials' porosity at saturation. It can be given on a volumetric or mass (gravimetric) basis.
Volumetric water content, θ, is defined mathematically as:
where is the volume of water and is equal to the total volume of the wet material, i.e. of the sum of the volume of solid host material (e.g., soil particles, vegetation tissue) , of water , and of air .
Gravimetric water content is expressed by mass (weight) as follows:
where is the mass of water and is the mass of the solids.
For materials that change in volume with water content, such as coal, the gravimetric water content, u, is expressed in terms of the mass of water per unit mass of the moist specimen (before drying):
However, woodworking, geotechnics and soil science require the gravimetric moisture content to be expressed with respect to the sample's dry weight:
And in food science, both and are used and called respectively moisture content wet basis (MC_wb) and moisture content dry basis (MC_db).
Values are often expressed as a percentage, i.e. u×100%.
To convert gravimetric water content to volumetric water content, multiply the gravimetric water content by the bulk specific gravity of the material:
In soil mechanics and petroleum engineering the water saturation or degree of saturation, , is defined as
where is the porosity, in terms of the volume of void or pore space and the total volume of the substance . Values of Sw can range from 0 (dry) to 1 (saturated). In reality, Sw never reaches 0 or 1 - these are idealizations for engineering use.
The normalized water content, , (also called effective saturation or ) is a dimensionless value defined by van Genuchten as:
where is the volumetric water content; is the residual water content, defined as the water content for which the gradient becomes zero; and, is the saturated water content, which is equivalent to porosity, .
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