Water content - Citizendia

Soil composition

Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, or wood on a volumetric or gravimetric basis. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere The word ceramic is derived from the Greek word κεραμικός ( keramikos) Wood is hard fibrous lignified structural tissue produced as secondary Xylem in the stems of Woody plants notably trees but also shrubs The property 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. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100%

Volumetric water content, θ, is defined mathematically as:

$\theta = \frac{V_w}{V_T}$

where $V w$ is the volume of water and $V T$ is the total volume (that is Soil Volume + Water Volume +Void Space). Water content may also be based on its mass or weight, thus the gravimetric water content is defined as:

$u = \frac{m_w}{m_b}$

where $m w$ is the mass of water and $m b$ (or $m s$ for soil) is the bulk material mass. To convert gravimetric water content to volumetric water, multiply the gravimetric water content by the bulk specific gravity of the material. Specific gravity is defined as the ratio of the Density of a given solid or liquid substance to the density of water at a specific temperature and pressure typically

In petroleum engineering, the term water saturation, $S w$ is used, defined as

$S_w = \frac{V_w}{V_b\phi} = \frac{\theta}{\phi}$

where $φ$ is the porosity. Petroleum engineering refers to the subsurface engineering activities related to the production of Hydrocarbons, which can be either Crude oil or Gas. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100%

1 Measurement
2 Classification and uses
- 2.1 Earth and agricultural sciences
3 See also
4 References

Measurement

Direct methods

Volumetric water content can be directly measured using a known volume of the material, and a drying oven. An oven is an enclosed compartment for Heating, Baking or Drying. Volumetric water content, θ, is calculated^[1] using:

$\theta = \frac{m_{\text{wet}}-m_{\text{dry}}}{\rho_w \cdot V_b}$

where

m wet

and

m dry

are the masses of the sample before and after drying in the oven;

ρ w

is the density of water; and

V b

is the volume of the sample before drying the sample

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

$u = \frac{m_{\text{wet}} - m_{\text{dry}}}{m_{\text{wet}}}$

However, geotechnics requires the moisture content to be expressed as a percentage of the sample's dry weight i. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The volume of any solid plasma vacuum or theoretical object is how much three- Dimensional space it occupies often quantified numerically Wood is hard fibrous lignified structural tissue produced as secondary Xylem in the stems of Woody plants notably trees but also shrubs e. % moisture content = $u * 100$

Where

$u = \frac{m_{\text{wet}} - m_{\text{dry}}}{m_{\text{dry}}}$

Laboratory methods

Main article: Moisture analysis

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. Moisture analysis covers a variety of methods for measuring Moisture content in both high level and trace amounts in solids liquids or gases Titration is a common laboratory method of Quantitative chemical analysis that is used to determine the unknown Concentration of a known Reactant Karl Fischer titration is a classic Titration method in Analytical chemistry that uses coulometric or Volumetric titration to determine trace Freeze drying (also known as lyophilization or cryodesiccation) is a Dehydration process typically used to preserve a perishable material In the food industry the Dean-Stark method is also commonly used. The Dean-Stark apparatus or Dean-Stark receiver or distilling trap is a piece of Laboratory glassware used in Synthetic chemistry to collect

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:

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

where $p$ is the fraction of total evaporable moisture content of sample, $W$ is the mass of the original sample, and $D$ is mass of dried sample. ASTM International ( ASTM) originally known as the American Society for Testing and Materials is an international Standards organization that develops and publishes

Geophysical methods

There are several geophysical methods available that can approximate in situ soil water content. Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic These methods include: time-domain reflectometry (TDR), neutron probe, frequency domain sensor, capacitance probe, electrical resistivity tomography, and others that are sensitive to the physical properties of water ^[2]. Time-domain reflectometry or TDR is a measurement technique used to determine the characteristics of electrical lines by observing reflected A neutron probe is a device used to measure the quantity of Water present in Soil. Frequency domain ( FD) sensor is an instrument developed for measuring Soil moisture content Capacitance sensors use Capacitance to measure the Dielectric Permittivity of a surrounding medium Electrical resistivity tomography (ERT or electrical resistivity imaging (ERI is a geophysical technique for imaging sub-surfaces structures from electrical Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications.

Satellite remote sensing method

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 data from microwave remote sensing satellite such as: WindSat, AMSR-E, RADARSAT, ERS-1-2 are used to estimate surface soil moisture [1].

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 on the pore size. 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 activity or aw is a measurement of the energy status of the water in a system

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 sciences, water content has an important role for groundwater recharge, agriculture, and soil chemistry. Soil science is the study of Soil as a Natural resource on the surface of the Earth including soil formation, classification and mapping Hydrology (from Greek Yδωρ hudōr, "water" and λόγος logos, "study" is the study of the movement distribution and quality of Agricultural science is a broad multidisciplinary field that encompasses the parts of exact natural economic and Social sciences that are used in the practice and understanding Groundwater recharge or deep drainage or deep percolation is a hydrologic Process where Water moves downward from Surface water Agriculture refers to the production of goods through the growing of plants and fungi and the raising of domesticated Animals The study of agriculture Soil chemistry studies the chemical characteristics of soil Soil chemistry is affected by mineral composition organic matter and environmental factors 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 temperature regions ^[3] .

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

Name	Notation	Suction pressure (J/kg or kPa)	Typical water content (vol/vol)	Description
Saturated water content	θ_s	0	0. 2–0. 5	Fully saturated water, equivalent to effective porosity
Field capacity	θ_fc	−33	0. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Field capacity is the amount of Soil moisture or Water content held in Soil after excess water has drained away and the rate of downward movement has materially 1–0. 35	Soil moisture after 2–3 days after a rain or irrigation
Permanent wilting point	θ_pwp or θ_wp	−1500	0. Permanent wilting point ( PWP) or wilting point ( WP) is defined as the minimal point of Soil moisture the plant requires not to wilt 01–0. 25	Minimum soil moisture at which a plant wilts
Residual water content	θ_r	−∞	0. 001–0. 1	Remaining water at high tension

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

θ_a ≡ θ_fc − θ_pwp

which can range between 0. Available water capacity or available water content ( AWC) is the range of available water that can be stored in soil and be available for growing crops 1 in gravel and 0. Gravel is rock that is of a specific Particle size range In Geology, gravel is any loose rock that is larger than two millimeters (2mm 3 in peat. Peat is an accumulation of partially Decayed Vegetation matter.

Agriculture

When a soil gets too dry, plant transpiration drops because the water is becoming increasingly bound to the soil particles by suction. Transpiration is the Evaporation of water from the aerial parts of Plants especially leaves but also stems Flowers and Roots Below the wilting point plants are no longer able to extract water. Permanent wilting point ( PWP) or wilting point ( WP) is defined as the minimal point of Soil moisture the plant requires not to wilt 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. Agriculture refers to the production of goods through the growing of plants and fungi and the raising of domesticated Animals The study of agriculture A drought is an extended period of months or years when a region notes a deficiency in its water supply Irrigation is an artificial application of water to the soil usually for assisting in growing crops Such conditions are common in arid and semi-arid environments. In general terms the Climate of a local or region is said to be arid when it is characterized by a severe lack of available Water, to the extent of hindering A Semi-arid climate or steppe climate generally describes climatic regions that receive low annual Rainfall (250-500 mm or 10-20 in

Some agriculture professionals are beginning to use environmental measurements such as soil moisture to schedule irrigation. Irrigation is an artificial application of water to the soil usually for assisting in growing crops This method is referred to as "Smart Irrigation. "

Groundwater

In saturated groundwater aquifers, all available pore spaces are filled with water (volumetric water content = porosity). Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations An aquifer is an underground layer of Water -bearing Permeable rock or unconsolidated materials ( Gravel, Sand, Silt, or Clay Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% Above a capillary fringe, pore spaces have air in them too. The capillary fringe is the subsurface layer in which Groundwater seeps up from a Water table by Capillary action to fill pores

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 vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the phreatic zone or zone of saturation ("vadose" The capillary fringe of the water table is the dividing line between saturated and unsaturated conditions. The capillary fringe is the subsurface layer in which Groundwater seeps up from a Water table by Capillary action to fill pores The water table is the level at which the ground water pressure is equal to Atmospheric pressure. An aquifer is an underground layer of Water -bearing Permeable rock or unconsolidated materials ( Gravel, Sand, Silt, or Clay Water content in the capillary fringe decreases with increasing distance above the phreatic surface.

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. Water retention curve is the relationship between the Water content, θ and the soil water potential ψ Water potential is the Potential energy of Water relative to pure water (e It is characteristic for different types of porous medium. Due to hysteresis, different wetting and drying curves may be distinguished. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system

Normalized volumetric water content

The normalized water content, $Θ$ , (also called effective saturation or $S e$ ) is a dimensionless value defined by van Genuchten^[4] as:

$\Theta = \frac{\theta - \theta_r}{\theta_s-\theta_r}$

where $θ$ is the volumetric water content; $θ r$ is the residual water content, defined as the water content for which the gradient $d θ / d h$ becomes zero; and, $θ s$ is the saturated water content.

References

^ Dingman, S. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Moisture analysis covers a variety of methods for measuring Moisture content in both high level and trace amounts in solids liquids or gases Water retention curve is the relationship between the Water content, θ and the soil water potential ψ Water activity or aw is a measurement of the energy status of the water in a system L. (2002). "Chapter 6, Water in soils: infiltration and redistribution", Physical Hydrology, Second Edition, Upper Saddle River, New Jersey: Prentice-Hall, Inc. , 646 pp. ISBN 0-13-099695-5.
^ F. Ozcep, M. Asci, O. Tezel, T. Yas, N. Alpaslan, D. Gundogdu (2005). "Relationships Between Electrical Properties (in Situ) and Water Content (in the Laboratory) of Some Soils in Turkey.". Geophysical Research Abstracts 7.
^ Lawrence, J. E. , and G. M. Hornberger (2007). "Soil moisture variability across climate zones". Geophys. Res. Lett. 34 (L20402): L20402. doi:10.1029/2007GL031382. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
^ van Genuchten, M. Th. (1980). "A closed-form equation for predicting the hydraulic conductivity of unsaturated soils". Soil Science Society of America Journal 44 (5): 892–898.

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