Question

Determine the moisture content of the given soil sample

Answer 1

Volumetric water content, θ, is defined mathematically as:

{\displaystyle \theta ={\frac {V_{w}}{V_{\text{wet}}}}}

where {\displaystyle V_{w}} is the volume of water and {\displaystyle V_{\text{wet}}=V_{s}+V_{w}+V_{a}} 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) {\displaystyle V_{s}}, of water {\displaystyle V_{w}}, and of air {\displaystyle V_{a}}.

Gravimetric water content[1] is expressed by mass (weight) as follows:

{\displaystyle u={\frac {m_{w}}{m}}}

where {\displaystyle m_{w}} is the mass of water and {\displaystyle m} is the mass of the substance. Normally the latter is taken before drying:

{\displaystyle u'={\frac {m_{w}}{m_{\text{wet}}}}}

except for woodworking, geotechnical and soil science applications where oven-dried material is used instead:

{\displaystyle u''={\frac {m_{w}}{m_{\text{dry}}}}}

To convert gravimetric water content to volumetric water content, multiply the gravimetric water content by the bulk specific gravity {\displaystyle SG} of the material:

{\displaystyle \theta =u\times SG}.Derived quantities[edit]

In soil mechanics and petroleum engineering the water saturation or degree of saturation, {\displaystyle S_{w}}, is defined as

{\displaystyle S_{w}={\frac {V_{w}}{V_{v}}}={\frac {V_{w}}{V\phi }}={\frac {\theta }{\phi }}}

where {\displaystyle \phi =V_{v}/V} is the porosity, in terms of the volume of void or pore space {\displaystyle V_{v}} and the total volume of the substance {\displaystyle V}.[clarification needed] 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, {\displaystyle \Theta }, (also called effective saturation or {\displaystyle S_{e}}) is a dimensionless value defined by van Genuchten[2] as:

{\displaystyle \Theta ={\frac {\theta -\theta _{r}}{\theta _{s}-\theta _{r}}}}

where {\displaystyle \theta } is the volumetric water content; {\displaystyle \theta _{r}} is the residual water content, defined as the water content for which the gradient {\displaystyle d\theta /dh} becomes zero; and, {\displaystyle \theta _{s}} is the saturated water content, which is equivalent to porosity, {\displaystyle \phi }.

Measurement[edit]Direct methods[edit]

Water content can be directly measured using a known volume of the material, and a drying oven. Volumetric water content, θ, is calculated[3] via the volume of water {\displaystyle V_{w}} and the mass of water {\displaystyle m_{w}}:

{\displaystyle V_{w}={\frac {m_{w}}{\rho _{w}}}={\frac {m_{\text{wet}}-m_{\text{dry}}}{\rho _{w}}}}

where

{\displaystyle m_{\text{wet}}} and {\displaystyle m_{\text{dry}}} are the masses of the sample before and after drying in the oven;{\displaystyle \rho _{w}} is the density of water; and

For materials that change in volume with water content, such as coal, the water content, u, is expressed in terms of the mass of water per unit mass of the moist specimen:

{\displaystyle u'={\frac {m_{\text{wet}}-m_{\text{dry}}}{m_{\text{wet}}}}}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.