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| [[Image:soil-phase-diagram.svg|thumb|right|300px|[[Soil composition]] by phase: s-soil (dry), v-void (pores filled with water or air), w-water, a-air. V is volume, M is mass.]]
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| '''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, fruit, 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.
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| ==Definitions==
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| '''Volumetric water content''', θ, is defined mathematically as:
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| :<math>\theta = \frac{V_w}{V_T}</math>
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| where <math>V_w</math> is the volume of water and <math>V_T = V_s + V_v = V_s + V_w + V_a</math> is the total volume (that is soil volume + water volume + air space).
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| '''Gravimetric water content'''<ref>{{cite book |author=T. William Lambe & Robert V. Whitman |year=1969 |title=Soil Mechanics |chapter=Chapter 3: Description of an Assemblage of Particles |edition=First |publisher=John Wiley & Sons, Inc. |isbn= 0-471-51192-7 |page=553 }}</ref> is expressed by mass (weight) as follows:
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| :<math>u = \frac{m_w}{m_t}</math>
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| where <math>m_w</math> is the mass of water and <math>m_t</math> is the bulk mass. The bulk mass is taken as the total mass, except for [[geotechnical]] and soil science applications where oven-dried soil (<math>m_s</math>, see the diagram) is conventionally used instead of <math>m_t</math>. | |
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| To convert gravimetric water content to volumetric water, multiply the gravimetric water content by the bulk [[specific gravity]] of the material.
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| In [[soil mechanics]] and [[petroleum engineering]], the term '''water saturation''' or '''degree of saturation''', <math>S_w</math> is used, defined as
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| :<math>S_w = \frac{V_w}{V_v} = \frac{V_w}{V_T\phi} = \frac{\theta}{\phi}</math>
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| where <math>\phi = V_v / V_T</math> is the [[porosity]] and <math>V_v</math> is the volume of void or pore space. 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.
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| 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 |author=van Genuchten, 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 |url=http://hydro.nevada.edu/courses/gey719/vg.pdf }}</ref> as:
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| :<math>\Theta = \frac{\theta - \theta_r}{\theta_s-\theta_r}</math>
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| 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>.
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| ==Measurement==
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| ===Direct methods===
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| Water content can be directly measured using a known volume of the material, and a drying [[oven]]. 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=0-13-099695-5 |page=646}}</ref> using:
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| :<math>\theta = \frac{m_{\text{wet}}-m_{\text{dry}}}{\rho_w \cdot V_b}</math>
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| where
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| :<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;
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| :<math>\rho_w</math> is the [[density]] of water; and
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| :<math>V_b</math> is the [[volume]] of the sample before drying the sample.
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| 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:
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| :<math>u = \frac{m_{\text{wet}} - m_{\text{dry}}}{m_{\text{wet}}}</math>
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| However, geotechnics requires the moisture content to be expressed as a percentage of the sample's dry weight
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| i.e. % moisture content = <math>u*100%</math>
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| :Where
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| :<math>u = \frac{m_{\text{wet}} - m_{\text{dry}}}{m_{\text{dry}}}</math>
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| 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.
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| ===Laboratory methods===
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| {{main|Moisture analysis}}
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| 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.
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| 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:
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| :<math>p = \frac{W-D}{D}</math>
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| 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.
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| ===Geophysical methods===
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| {{see also|Soil moisture sensors}}
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| 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 |author=F. Ozcep, M. Asci, O. Tezel, T. Yas, N. Alpaslan, 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.
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| ===Satellite remote sensing method===
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| 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 [http://www.mdpi.com/2072-4292/1/1/3/]. The data from microwave remote sensing satellite such as: WindSat, AMSR-E, RADARSAT, ERS-1-2, Metop/ASCAT are used to estimate surface soil moisture [http://hydrolab.arsusda.gov/rsbarc/RSofSM.htm].
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| ==Classification and uses==
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| 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 on the pore size. In wood-based materials, however, almost all water is adsorbed at humidities below 98% RH.
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| 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.
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| 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.
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| ===Earth and agricultural sciences===
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| 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 |author=Lawrence, J. E., and G. M. Hornberger |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>
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| There are four standard water contents that are routinely measured and used, which are described in the following table:
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| {|class="wikitable"
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| ! Name
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| ! Notation
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| ! Suction pressure<br>(J/kg or kPa)
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| ! Typical water content<br>(vol/vol)
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| ! Conditions
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| |-
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| | Saturated water content
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| |align="center"| θ<sub>s</sub>
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| |align="right"| 0
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| |align="center"| 0.2–0.5
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| | Fully saturated soil, equivalent to [[effective porosity]]
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| |-
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| | [[Field capacity]]
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| |align="center"| θ<sub>fc</sub>
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| |align="right"| −33
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| |align="center"| 0.1–0.35
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| | Soil moisture 2–3 days after a rain or irrigation
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| |-
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| | [[Permanent wilting point]]
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| |align="center"| θ<sub>pwp</sub> or θ<sub>wp</sub>
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| |align="right"| −1500
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| |align="center"| 0.01–0.25
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| | Minimum soil moisture at which a plant wilts
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| |-
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| | Residual water content
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| |align="center"|θ<sub>r</sub>
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| |align="right"| −∞
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| |align="center"| 0.001–0.1
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| | Remaining water at high tension
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| |}
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| And lastly the [[Available water capacity|available water content]], θ<sub>a</sub>, which is equivalent to:
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| :θ<sub>a</sub> ≡ θ<sub>fc</sub> − θ<sub>pwp</sub>
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| which can range between 0.1 in [[gravel]] and 0.3 in [[peat]].
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| ====Agriculture====
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| 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.
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| 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}}
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| ====Groundwater====
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| 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.
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| 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.
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| 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.
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| 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.
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| ==See also==
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| *[[Humidity]], "water content" in air
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| *[[Moisture]]
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| *[[Moisture analysis]]
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| *[[Soil moisture sensors]]
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| *[[Water activity]]
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| *[[Water retention curve]]
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| ==References==
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| {{Reflist}}
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| ==Further reading==
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| *{{Citation |title=Field Estimation of Soil Water Content: A Practical Guide to Methods, Instrumentation and Sensor Technology |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= }}
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| {{Aquiferproperties}}
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| {{Geotechnical engineering|state=collapsed}}
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| {{DEFAULTSORT:Water Content}}
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| [[Category:Analytical chemistry]]
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| [[Category:Hydrology]]
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| [[Category:Physical chemistry]]
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| [[Category:Soil mechanics]]
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| [[Category:Soil physics]]
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| [[Category:Water]]
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| [[Category:Woodworking]]
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| [[et:Veesisaldus]]
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| [[fr:Teneur en eau (Milieux poreux)]]
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