Water content ormoisture content is the quantity ofwater contained in a material, such assoil (calledsoil moisture),rock,ceramics,crops, orwood. 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:

${\displaystyle \theta =\frac{V_w}{V_{\text{wet}}}}$

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:

${\displaystyle u=\frac{m_w}{m_s}}$

where${\displaystyle m_w}$ is the mass of water and${\displaystyle m_s}$ is the mass of the solids.

For materials that change in volume with water content, such ascoal, the gravimetric water content,u, is expressed in terms of the mass of water per unit mass of the moist specimen (before drying):

${\displaystyle u^{\prime }=\frac{m_w}{m_{\text{wet}}}}$

However,woodworking,geotechnics andsoil science require the gravimetric moisture content to be expressed with respect to the sample's dry weight:

${\displaystyle u^{″}=\frac{m_w}{m_{\text{dry}}}}$

And infood science, both${\displaystyle u^{\prime }}$ and${\displaystyle u^{″}}$ are used and called respectivelymoisture content wet basis (MC_wb) andmoisture contentdry basis (MC_db).^[2]

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 bulkspecific gravity${\displaystyle SG}$ of the material:

${\displaystyle \theta =u\times SG}$.

Insoil mechanics andpetroleum engineering thewater saturation ordegree of saturation,${\displaystyle S_w}$, is defined as

${\displaystyle S_w=\frac{V_w}{V_v}=\frac{V_w}{V\varphi }=\frac{\theta }{\varphi }}$

where${\displaystyle \varphi =V_v/V}$ is theporosity, 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 ofS_w can range from 0 (dry) to 1 (saturated). In reality,S_w never reaches 0 or 1 - these are idealizations for engineering use.

Thenormalized water content,${\displaystyle \mathrm{\Theta }}$, (also calledeffective saturation or${\displaystyle S_e}$) is a dimensionless value defined by van Genuchten^[3] as:

${\displaystyle \mathrm{\Theta }=\frac{\theta -{\theta }_r}{{\theta }_s-{\theta }_r}}$

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,${\displaystyle \varphi }$.

Water content can be directly measured using a dryingoven.Theoven-dry method requires drying a sample (of soil, wood, etc.) in a special oven or kiln and checking the sample weight at regular time intervals. When the drying process is complete, the sample's weight is compared to its weight before drying, and the difference is used to calculate the sample's original moisture content.

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 themasses of the sample before and after drying in the oven.This gives the numerator ofu; the denominator is either${\displaystyle m_{\text{wet}}}$ or${\displaystyle m_{\text{dry}}}$ (resulting inu' oru", 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 thedensity of water.This gives the numerator ofθ; 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., atin can) when taking a sample.

Forwood, 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 or until it stops losing weight). Inwood drying, this is an important concept.

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

From the Annual Book ofASTM (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 severalgeophysical methods available that can approximatein 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 thephysical 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 addition to the primary methods above, another method exists to measure the moisture content of wood: an electronicmoisture meter.Pin and pinless meters are the two main types of moisture meters.

Pin meters require driving two pins into the surface of the wood while making sure that the pins are aligned with the grain and not perpendicular to it. Pin meters provide moisture content readings by measuring the resistance in the electrical current between the two pins. The drier the wood, the more resistance to the electrical current, when measuring below the fiber saturation point of wood. Pin meters are generally preferred when there is no flat surface of the wood available to measure

Pinless meters emit an electromagnetic signal into the wood to provide readings of the wood's moisture content and are generally preferred when damage to the wood's surface is unacceptable or when a high volume of readings or greater ease of use is required.

Moisture may be present as adsorbed moisture at internal surfaces and as capillary condensed water in small pores. At low relative humidities, moisture consists mainly of adsorbed water. At higher relative humidities, liquid water becomes more and more important, depending or not depending on the pore size can also be an influence of volume. In wood-based materials, however, almost all water is adsorbed at humidities below 98% RH.

In biological applications there can also be a distinction between physisorbed water and "free" water — the physisorbed water being that closely associated with and relatively difficult to remove from a biological material. The method used to determine water content may affect whether water present in this form is accounted for. For a better indication of "free" and "bound" water, thewater activity of a material should be considered.

Water molecules may also be present in materials closely associated with individual molecules, as "water of crystallization", or as water molecules which are static components of protein structure.

Insoil science,hydrology andagricultural sciences, water content has an important role forgroundwater recharge,agriculture, andsoil 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 theavailable water content, θ_a, which is equivalent to:

θ_a ≡ θ_fc − θ_pwp

which can range between 0.1 ingravel and 0.3 inpeat.

When a soil becomes too dry, planttranspiration drops because the water is increasingly bound to the soil particles by suction. Below thewilting 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 asagriculturaldrought, and is a particular focus ofirrigation management. Such conditions are common inarid andsemi-arid environments.

Some agriculture professionals are beginning to use environmental measurements such as soil moisture to scheduleirrigation. This method is referred to assmart irrigation orsoil cultivation.^[10]

In saturatedgroundwateraquifers, all availablepore spaces are filled with water (volumetric water content =porosity). Above acapillary 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 ofvadose zone hydrogeology. Thecapillary fringe of thewater table is the dividing line betweensaturated and unsaturated conditions. Water content in the capillary fringe decreases with increasing distance above thephreatic surface. The flow of water through and unsaturated zone in soils often involves a process of fingering, resulting fromSaffman–Taylor instability. This results mostly throughdrainage 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.

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

Generally, an aggregate has four different moisture conditions. They are Oven-dry (OD), Air-dry (AD),Saturated surface dry (SSD) and damp (or wet).^[11] Oven-dry and Saturated surface dry can be achieved by experiments in laboratories, while Air-dry and damp (or wet) are aggregates' common conditions in nature.

• 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.^[11]
• 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.^[11]
• Saturated surface dry (SSD) is defined as the condition of anaggregate 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^[11]), but theinter-particle voids are saturated with water. In this condition aggregates will not affect the free water content of acomposite material.^[12][13]

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.^[11]

Among these four moisture conditions of aggregates, saturated surface dry is the condition that has the most applications in laboratory experiments, research, and studies, especially those related to water absorption, composition ratio, or shrinkage tests in materials like concrete. For many related experiments, a saturated surface dry condition is a premise that must be realized before the experiment. In saturated surface dry conditions, the aggregate's water content is in a relatively stable and static situation where its environment would not affect it. Therefore, in experiments and tests where aggregates are in saturated surface dry condition, there would be fewer disrupting factors than in the other three conditions.^[14][15]

• Humidity, "water content" in air
• Moisture
• Viscous fingering
• Moisture analysis
• Soil moisture sensors
• Water activity
• Water retention curve

• Robinson, David A. (2008),"Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation and Sensor Technology"(PDF),Soil Science Society of America Journal,73 (4), Vienna, Austria: International Atomic Energy Agency: 131,Bibcode:2009SSASJ..73.1437R,doi:10.2136/sssaj2008.0016br,ISSN 1018-5518, IAEA-TCS-30
• Wessel-Bothe, Weihermüller (2020): Field Measurement Methods in Soil Science. New practical guide to soil measurements explains the principles of operation of different moisture sensor types (independent of manufacturer), their accuracy, fields of application and how such sensors are installed, as well as subtleties of the data so obtained. Also deals with other crop-related soil parameters.

• Analytical chemistry
• Hydrology
• Physical chemistry
• Soil mechanics
• Soil physics
• Water
• Woodworking

• Articles with short description
• Short description matches Wikidata
• Wikipedia articles needing clarification from March 2016