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[[Image:soil-phase-diagram.svg|thumb|right|300px|[[Soil morphology|Soil composition]] by '''V'''olume and '''M'''ass, by phase: '''a'''ir, '''w'''ater, '''v'''oid (pores filled with water or air), '''s'''oil, and '''t'''otal.]]
'''Water content''' or '''moisture content''' is the quantity of [[water]] contained in a material, such as [[soil]] (called '''soil moisture'''), [[Rock (geology)|rock]], [[ceramic]]s, [[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.

==Definitions==

'''Volumetric water content''', θ, is defined mathematically as:
:<math>\theta = \frac{V_w}{V_\text{wet}}</math>
where <math>V_w</math> is the volume of water and <math>V_\text{wet} = V_s + V_w + V_a</math> 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) <math>V_s</math>, of water <math>V_w</math>, and of air <math>V_a</math>.

'''Gravimetric water content'''<ref>{{cite book |author1=T. William Lambe |author2=Robert V. Whitman |lastauthoramp=yes |year=1969 |title=Soil Mechanics |chapter=Chapter 3: Description of an Assemblage of Particles |edition=First |publisher=John Wiley & Sons, Inc. |isbn=978-0-471-51192-2 |page=553 |url-access=registration |url=https://archive.org/details/soilmechanics00lamb }}</ref> is expressed by mass (weight) as follows:
:<math>u = \frac{m_w}{m}</math>
where <math>m_w</math> is the mass of water and <math>m</math> is the mass of the substance. 

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): 
:<math>u' = \frac{m_{w}}{m_{\text{wet}}}</math>
However, [[woodworking]], [[geotechnics]] and [[soil science]] require the gravimetric moisture content to be expressed with respect to the sample's dry weight:
:<math>u'' = \frac{m_{w}}{m_{\text{dry}}}</math>
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]] <math>SG</math> of the material:
:<math>\theta = u \times SG</math>.

===Derived quantities===
In [[soil mechanics]] and [[petroleum engineering]] the '''water saturation''' or '''degree of saturation''', <math>S_w</math>, is defined as
:<math>S_w = \frac{V_w}{V_v} = \frac{V_w}{V \phi} = \frac{\theta}{\phi}</math>
where <math>\phi = V_v / V</math> is the [[porosity]], in terms of the volume of void or pore space <math>V_v</math> and the total volume of the substance <math>V</math>.{{clarify|reason=Dry or wet total volume?|date=March 2016}} Values of ''S<sub>w</sub>'' can range from 0 (dry) to 1 (saturated). In reality, ''S<sub>w</sub>'' never reaches 0 or 1 - these are idealizations for engineering use.

The '''normalized water content''', <math>\Theta</math>, (also called '''effective saturation''' or <math>S_e</math>) is a dimensionless value defined by van Genuchten<ref>{{cite journal |doi=10.2136/sssaj1980.03615995004400050002x |last1=van Genuchten |first1=M.Th. |title=A closed-form equation for predicting the hydraulic conductivity of unsaturated soils |journal=Soil Science Society of America Journal |year=1980 |volume=44 |issue=5 |pages=892–898|bibcode=1980SSASJ..44..892V |hdl=10338.dmlcz/141699 }}</ref> as:
:<math>\Theta = \frac{\theta - \theta_r}{\theta_s-\theta_r}</math>
where <math>\theta</math> is the volumetric water content; <math>\theta_r</math> is the residual water content, defined as the water content for which the gradient <math>d\theta/dh</math> becomes zero; and, <math>\theta_s</math> is the saturated water content, which is equivalent to porosity, <math>\phi</math>.

==Measurement==

===Direct methods===

Water content can be directly measured using a drying [[oven]]. 

Gravimetric water content, ''u'', is calculated<ref>[https://www.edaphic.com.au/soil-water-compendium/how-to-convert-gravimetric-soil-water-content-to-volumetric-soil-water-content/]</ref> via the mass of water <math>m_w</math>:
:<math>m_w = m_{\text{wet}}-m_{\text{dry}}</math>
where <math>m_{\text{wet}}</math> and <math>m_{\text{dry}}</math> are the [[mass]]es of the sample before and after drying in the oven.
This gives the numerator of ''u''; the denominator is either <math>m_{\text{wet}}</math> or <math>m_{\text{dry}}</math> (resulting in ''u''' or ''u"'', respectively), depending on the discipline.

On the other hand, volumetric water content, ''θ'', is calculated<ref>{{cite book |author=Dingman, S.L. |year=2002 |title=Physical Hydrology |chapter=Chapter 6, Water in soils: infiltration and redistribution |edition=Second |publisher=Prentice-Hall, Inc. |location=Upper Saddle River, New Jersey |isbn=978-0-13-099695-4 |page=646}}</ref> via the volume of water <math>V_w</math>:
:<math>V_w = \frac{m_w}{\rho_w}</math>
where <math>\rho_w</math> is the [[density of water]].
This gives the numerator of ''θ''; the denominator, <math>V_\text{wet}</math>, 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.

===Laboratory methods===
{{main|Moisture analysis}}

Other methods that determine water content of a sample include chemical [[titration]]s (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 apparatus|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: 
:<math>p = \frac{W-D}{W}</math>
where <math>p</math> is the fraction of total evaporable moisture content of sample, <math>W</math> is the mass of the original sample, and <math>D</math> is mass of dried sample.

===Soil moisture measurement{{anchor|Soil}}{{anchor|Soil moisture}}===
{{further|Soil moisture}}
In addition to the direct and laboratory methods above, the following options are available.

====Geophysical methods====
{{further|Soil moisture sensors}}

There are several [[Geophysics|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 (ADR)|amplitude domain reflectometry]], [[electrical resistivity tomography]], [[ground penetrating radar]] (GPR), and others that are sensitive to the [[Water (molecule)|physical properties of water]] .<ref>{{cite journal |author1=F. Ozcep |author2=M. Asci |author3=O. Tezel |author4=T. Yas |author5=N. Alpaslan |author6=D. Gundogdu |title=Relationships Between Electrical Properties (in Situ) and Water Content (in the Laboratory) of Some Soils in Turkey|journal=Geophysical Research Abstracts |year=2005 |volume=7 |url=http://www.cosis.net/abstracts/EGU05/08470/EGU05-J-08470.pdf }}</ref> Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications.

====Satellite remote sensing method====
{{further|Remote sensing}}

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.<ref>{{Cite journal | doi=10.3390/rs1010003| title=Non-parametric Methods for Soil Moisture Retrieval from Satellite Remote Sensing Data| journal=Remote Sensing| volume=1| pages=3–21| year=2009| last1=Lakhankar| first1=Tarendra| last2=Ghedira| first2=Hosni| last3=Temimi| first3=Marouane| last4=Sengupta| first4=Manajit| last5=Khanbilvardi| first5=Reza| last6=Blake| first6=Reginald| bibcode=2009RemS....1....3L}}</ref> 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.<ref>{{cite web |url=http://hydrolab.arsusda.gov/rsbarc/RSofSM.htm |title=Archived copy |accessdate=2007-08-22 |url-status=dead |archiveurl=https://web.archive.org/web/20070929131729/http://hydrolab.arsusda.gov/rsbarc/RSofSM.htm |archivedate=2007-09-29 }}</ref>

==Classification and uses==

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, the [[water 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.

===Earth and agricultural sciences===

In [[soil science]], [[hydrology]] and [[agricultural science]]s, 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 .<ref>{{cite journal |author1=Lawrence, J. E. |author2=G. M. Hornberger  |lastauthoramp=yes |title=Soil moisture variability across climate zones |journal=Geophys. Res. Lett. |year=2007 |volume=34 | issue=L20402 |doi=10.1029/2007GL031382 |pages=L20402 |bibcode = 2007GeoRL..3420402L }}</ref>

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

{|class="wikitable"
! Name
! Notation
! Suction pressure<br>(J/kg or kPa)
! Typical water content<br>(vol/vol)
! Conditions
|-
| Saturated water content
|align="center"| θ<sub>s</sub>
|align="right"| 0
|align="center"| 0.2–0.5
| Fully saturated soil, equivalent to [[effective porosity]]
|-
| [[Field capacity]]
|align="center"| θ<sub>fc</sub>
|align="right"| −33
|align="center"| 0.1–0.35
| Soil moisture 2–3 days after a rain or irrigation
|-
| [[Permanent wilting point]]
|align="center"| θ<sub>pwp</sub> or θ<sub>wp</sub> 
|align="right"| −1500
|align="center"| 0.01–0.25
| Minimum soil moisture at which a plant wilts
|-
| Residual water content
|align="center"|θ<sub>r</sub>
|align="right"| −∞
|align="center"| 0.001–0.1
| Remaining water at high tension
|}

And lastly the [[Available water capacity|available water content]], θ<sub>a</sub>, which is equivalent to:
:θ<sub>a</sub> ≡ θ<sub>fc</sub> − θ<sub>pwp</sub>
which can range between 0.1 in [[gravel]] and 0.3 in [[peat]].

====Agriculture====

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 [[agriculture|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''.{{citation needed|date=September 2009}}

====Groundwater====

In saturated [[groundwater]] [[aquifer]]s, all available [[Porosity|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 [[Aquifer#Saturated versus unsaturated|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.

== In aggregates ==
Generally, an aggregate has four different moisture conditions. They are Oven-dry (OD), Air-dry (AD), [[Saturated-surface-dry|Saturated surface dry (SSD)]] and damp (or wet).<ref name=":0">{{Cite web|url=https://precast.org/2010/05/water-to-cement-ratio-and-aggregate-moisture-corrections/|title=Water-to-Cement Ratio and Aggregate Moisture Corrections|website=precast.org|language=en-US|access-date=2018-11-18}}</ref> 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.

==== Four Conditions ====

* '''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.<ref name=":0" />

* '''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.<ref name=":0" />

* '''Saturated surface dry''' (SSD) is defined as the condition of an [[Aggregate (composite)|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<ref name=":0" />), but the [[Porosity|inter-particle voids]] are saturated with water. In this condition aggregates will not affect the free [[water content]] of a [[composite material]].<ref>{{cite web|url=https://www.concreteconstruction.net/producers/aggregate-moisture-in-concrete_o|title=Aggregate Moisture in Concrete|publisher=Concrete Construction|accessdate=2018-11-08}}</ref><ref>ftp://ftp.dot.state.tx.us/pub/txdot-info/cst/TMS/400-A_series/pdfs/cnn403.pdf</ref>

The water adsorption by mass (A<sub>m</sub>) is defined in terms of the mass of saturated-surface-dry (M<sub>ssd</sub>) sample and the mass of oven dried test sample (M<sub>dry</sub>) by the formula:
:<math>A = \frac{M_{ssd}-M_{dry}}{M_{dry}}</math>

* '''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.<ref name=":0" />

==== Application ====
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.<ref>{{Cite journal|last=Zaccardi|first=Y. A. Villagrán|last2=Zega|first2=C. J.|last3=Carrizo|first3=L. E.|last4=Sosa|first4=M. E.|date=2018-10-01|title=Water absorption of fine recycled aggregates: effective determination by a method based on electrical conductivity|journal=Materials and Structures|language=en|volume=51|issue=5|pages=127|doi=10.1617/s11527-018-1248-2|issn=1871-6873}}</ref><ref>{{Cite journal|last=Kawamura|first=Masashi|last2=Kasai|first2=Yoshio|date=2009-05-29|title=Determination of saturated surface-dry condition of clay–sand mixed soils for soil–cement concrete construction|journal=Materials and Structures|volume=43|issue=4|pages=571–582|doi=10.1617/s11527-009-9512-0|issn=1359-5997}}</ref>

==See also==
*[[Humidity]], "water content" in air
*[[Moisture]]
*[[Viscous fingering]]
*[[Moisture analysis]]
*[[Soil moisture sensors]]
*[[Water activity]]
*[[Water retention curve]]

==References==
{{Reflist}}

==Further reading==
*{{Citation |title=Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation and Sensor Technology |journal=Soil Science Society of America Journal |volume=73 |issue=4 |publisher=International Atomic Energy Agency |location=Vienna, Austria |id=IAEA-TCS-30 |year=2008 |pages=131 |issn=1018-5518 |url=http://www-pub.iaea.org/mtcd/publications/pdf/tcs-30_web.pdf |doi= 10.2136/sssaj2008.0016br|bibcode=2009SSASJ..73.1437R |last1=Robinson |first1=David A. }}

{{Aquiferproperties}}
{{Geotechnical engineering|state=collapsed}}

{{DEFAULTSORT:Water Content}}
[[Category:Analytical chemistry]]
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[[Category:Physical chemistry]]
[[Category:Soil mechanics]]
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