Water content

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 ${\displaystyle V}$. Values of S_w can range from 0 (dry) to 1 (saturated). In reality, S_w 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[3] 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, ${\displaystyle \phi }$.

Water content can be directly measured using a drying oven.

Gravimetric water content, u, is calculated[4] via the mass of water ${\displaystyle m_{w}}$:

${\displaystyle m_{w}=m_{\text{wet}}-m_{\text{dry}}}$

where ${\displaystyle m_{\text{wet}}}$ and ${\displaystyle m_{\text{dry}}}$ are the masses of the sample before and after drying in the oven. This gives the numerator of u; the denominator is either ${\displaystyle m_{\text{wet}}}$ or ${\displaystyle m_{\text{dry}}}$ (resulting in u' or u", respectively), depending on the discipline.

On the other hand, volumetric water content, θ, is calculated[5] via the volume of water ${\displaystyle V_{w}}$:

${\displaystyle V_{w}={\frac {m_{w}}{\rho _{w}}}}$

where ${\displaystyle \rho _{w}}$ is the density of water. This gives the numerator of θ; the denominator, ${\displaystyle V_{\text{wet}}}$, is the total volume of the wet material, which is fixed by simply filling up a container of known volume (e.g., a tin can) when taking a sample.

For wood, the convention is to report moisture content on oven-dry basis (i.e. generally drying sample in an oven set at 105 deg Celsius for 24 hours). In wood drying, this is an important concept.

Other methods that determine water content of a sample include chemical titrations (for example the Karl Fischer titration), determining mass loss on heating (perhaps in the presence of an inert gas), or after freeze drying. In the food industry the Dean-Stark method is also commonly used.

From the Annual Book of ASTM (American Society for Testing and Materials) Standards, the total evaporable moisture content in Aggregate (C 566) can be calculated with the formula:

${\displaystyle p={\frac {W-D}{W}}}$

where ${\displaystyle p}$ is the fraction of total evaporable moisture content of sample, ${\displaystyle W}$ is the mass of the original sample, and ${\displaystyle D}$ is mass of dried sample.

In addition to the direct and laboratory methods above, the following options are available.

There are several geophysical methods available that can approximate in situ soil water content. These methods include: time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, amplitude domain reflectometry, electrical resistivity tomography, ground penetrating radar (GPR), and others that are sensitive to the physical properties of water .[6] Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications.

Satellite microwave remote sensing is used to estimate soil moisture based on the large contrast between the dielectric properties of wet and dry soil. The microwave radiation is not sensitive to atmospheric variables, and can penetrate through clouds. Also, microwave signal can penetrate, to a certain extent, the vegetation canopy and retrieve information from ground surface.[7] The data from microwave remote sensing satellites such as WindSat, AMSR-E, RADARSAT, ERS-1-2, Metop/ASCAT, and SMAP are used to estimate surface soil moisture.[8]

In soil science, hydrology and agricultural sciences, water content has an important role for groundwater recharge, agriculture, and soil chemistry. Many recent scientific research efforts have aimed toward a predictive-understanding of water content over space and time. Observations have revealed generally that spatial variance in water content tends to increase as overall wetness increases in semiarid regions, to decrease as overall wetness increases in humid regions, and to peak under intermediate wetness conditions in temperate regions .[9]

There are four standard water contents that are routinely measured and used, which are described in the following table:

And lastly the available water content, θ_a, which is equivalent to:

θ_a ≡ θ_fc − θ_pwp

which can range between 0.1 in gravel and 0.3 in peat.

When a soil becomes too dry, plant transpiration drops because the water is increasingly bound to the soil particles by suction. Below the wilting point plants are no longer able to extract water. At this point they wilt and cease transpiring altogether. Conditions where soil is too dry to maintain reliable plant growth is referred to as agricultural drought, and is a particular focus of irrigation management. Such conditions are common in arid and semi-arid environments.

Some agriculture professionals are beginning to use environmental measurements such as soil moisture to schedule irrigation. This method is referred to as smart irrigation or soil cultivation.

In saturated groundwater aquifers, all available pore spaces are filled with water (volumetric water content = porosity). Above a capillary fringe, pore spaces have air in them too.

Most soils have a water content less than porosity, which is the definition of unsaturated conditions, and they make up the subject of vadose zone hydrogeology. The capillary fringe of the water table is the dividing line between saturated and unsaturated conditions. Water content in the capillary fringe decreases with increasing distance above the phreatic surface. The flow of water through and unsaturated zone in soils often involves a process of fingering, resulting from Saffman–Taylor instability. This results mostly through drainage processes and produces and unstable interface between saturated and unsaturated regions.

One of the main complications which arises in studying the vadose zone, is the fact that the unsaturated hydraulic conductivity is a function of the water content of the material. As a material dries out, the connected wet pathways through the media become smaller, the hydraulic conductivity decreasing with lower water content in a very non-linear fashion.

A water retention curve is the relationship between volumetric water content and the water potential of the porous medium. It is characteristic for different types of porous medium. Due to hysteresis, different wetting and drying curves may be distinguished.

• Oven-dry (OD) is defined as the condition of an aggregate where there is no moisture within any part of the aggregate. This condition can be achieved in a laboratory by heating the aggregate to 220 °F (105 °C) for a period of time.[10]
• Air-dry (AD) is defined as the condition of an aggregate in which there are some water or moisture in the pores of the aggregate, while the outer surfaces of it is dry. This is a natural condition of aggregates in summer or in dry regions. In this condition, an aggregate will absorb water from other materials added to the surface of it, which would possibly have some impact on some characters of the aggregate.[10]
• Saturated surface dry (SSD) is defined as the condition of an aggregate in which the surfaces of the particles are "dry" (i.e., they will neither absorb any of the mixing water added; nor will they contribute any of their contained water to the mix[10]), but the inter-particle voids are saturated with water. In this condition aggregates will not affect the free water content of a composite material.[11][12]

The water adsorption by mass (A_m) is defined in terms of the mass of saturated-surface-dry (M_ssd) sample and the mass of oven dried test sample (M_dry) by the formula:

${\displaystyle A={\frac {M_{ssd}-M_{dry}}{M_{dry}}}}$

• Damp (or wet) is defined as the condition of an aggregate in which water is fully permeated the aggregate through the pores in it, and there is free water in excess of the SSD condition on its surfaces which will become part of the mixing water.[10]

Among these four moisture condition of aggregates, saturated surface dry is the condition that has the most applications in laboratory experiments, researches and studies, especially these related to water absorption, composition ratio or shrinkage test in materials like concrete. For many related experiments, a saturated surface dry condition is a premise that must be realize before the experiment. In saturated surface dry condition, the aggregate's water content is in a relatively stable and static situation where it would not be affected by its environment. Therefore, in experiments and tests where aggregates are in saturated surface dry condition, there would be fewer disrupting factors than in other three conditions.[13][14]