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Encyclopedia > Water content
Soil composition
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. 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. Image File history File links Size of this preview: 800 × 485 pixelsFull resolution (1155 × 700 pixel, file size: 23 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Size of this preview: 800 × 485 pixelsFull resolution (1155 × 700 pixel, file size: 23 KB, MIME type: image/png) File historyClick on a date/time to view the file as it appeared at that time. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Loess field in Germany Surface-water-gley developed in glacial till, Northern Ireland Technically, soil forms the pedosphere: the interface between the lithosphere (rocky part of the planet) and the biosphere, atmosphere, and hydrosphere. ... “Rock” redirects here. ... Fixed Partial Denture, or Bridge The word ceramic is derived from the Greek word κεραμικός (keramikos). ... For other uses, see Wood (disambiguation). ... 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%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ...


Volumetric water content, θ, is defined mathematically as:

where Vw is the volume of water and Vb (or Vs for soil) is the bulk material volume. 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 mw is the mass of water and mb (or ms for soil) is the bulk material mass.


To convert gravimetric water content to volumetric water, multiply the gravimetric water content by the bulk density of the material.

Contents

Measurement

Direct methods

Volumetric water content can be directly measured using a known volume of the material, and a drying oven. Volumetric water content, θ, is calculated[1] using: Oven depicted in a painting by Millet An oven is an enclosed compartment for heating, baking or drying. ...

where

mwet and mdry are the masses of the sample before and after drying in the oven;
ρw is the density of water; and
Vb 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: This article or section is in need of attention from an expert on the subject. ... For other uses, see Density (disambiguation). ... For other uses, see Volume (disambiguation). ... For other uses, see Wood (disambiguation). ...

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. In the food industry the Dean-Stark method is also commonly used. Moisture analysis is the method of measuring moisture content for industrial applications. ... In medicine, titration is the process of gradually adjusting the dose of a medication until the desired effect is achieved. ... Karl Fischer titration is a classic method in analytical chemistry that uses coulometric titration to determine the moisture content of a sample. ... Freeze drying (also known as Lyophilization) is a dehydration process typically used to preserve a perishable material, or to make the material more convenient for transport. ... The Dean-Stark apparatus or Dean-Stark receiver or distilling trap is a piece of laboratory glassware used in synthetic chemistry to collect water (or occasionally other liquid) from a reactor. ...


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: ASTM International is an international voluntary standards organization that develops and produces technical standards for materials, products, systems and services. ...


p = 100(W-D)/D


where:


p = total evaporable moisture content of sample, percent, W = mass of original sample, g, and D = mass of dried sample, g.


Geophysical methods

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, electrical resistivity tomography, and others that are sensitive to the physical properties of water. Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications. ‹ The template below has been proposed for deletion. ... This article or section does not adequately cite its references or sources. ... 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 the surrounding medium. ... Electrical resistivity tomography (ERT) is a geophysical technique for imaging sub-surfaces structures from electrical measurements made at the surface, or by electrodes in bore holes. ... H2O and HOH redirect here. ...


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.


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 deals with soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils per se; and these properties in relation to the use and management of soils. ... Water covers 70% of the Earths surface. ... 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 of agriculture. ... Missing main definition------ someone add if you know it please. ... Recharge is the process where water moves downward from surface water to groundwater. ... Soil chemistry studies the chemical characteristics of soil. ...


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 θs 0 0.2–0.5 Fully saturated water, equivalent to effective porosity
Field capacity θfc −33 0.1–0.35 Soil moisture after 2–3 days after a rain or irrigation
Permanent wilting point θpwp or θwp −1500 0.01–0.25 minimum soil moisture at which a plant wilts
Residual water content θr −∞ 0.001–0.1 Remaining water at high tension

And lastly the available water content, θa, which is equivalent to: 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%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ... The term Field capacity originated from Israelson and West (1922) and Frank Veihmeyer and Arthur Hendrickson (1931). ... Permanent wilting point or permanent wilting percentage (PWP) is defined as the soil wetness at which a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours. ... Available water capacity (AWC) is defined as the range of available water that can be stored in soil and be available for growing crops (Richards and Wadleigh, 1952). ...

θa ≡ θfc − θpwp

which can range between 0.1 in gravel and 0.3 in peat. Gravel (largest fragment in this photo is about 4 cm) Gravel is rock that is of a certain particle size range. ... Peat in Lewis, Scotland 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. 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. Transpiration is the evaporation of excess water from aerial parts and of plants, especially leaves but also stems, flowers and fruits. ... Permanent wilting point (PWP) or wilting point (WP) is defined as the minimum soil moisture at which a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours. ... Fields outside Benambra, Victoria, Australia suffering from drought conditions A drought is an extended period of months or years when a region notes a deficiency in its water supply. ... Irrigation is the artificial application of water to the soil usually for assisting in growing crops. ... In general terms, the climate of a locale or region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life. ... Semi-arid generally describes regions that receive low annual rainfall (25 to 50 cm /10 to 20 in) and generally have scrub or grass vegetation. ...


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


Groundwater

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. Missing main definition------ someone add if you know it please. ... An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. ... 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%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ... 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%. The term porosity is used in multiple fields including manufacturing, earth sciences and construction. ... The capillary fringe , or tension-saturated zone, is the subsurface layer in which water molecules seep 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 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 vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe... The capillary fringe , or tension-saturated zone, is the subsurface layer in which water molecules seep up from a water table by capillary action to fill pores. ... Cross section showing the water table varying with surface topography as well as a perched water table The water table or phreatic surface is the surface where the 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) from which groundwater can be usefully extracted using a water well. ...


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 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. Water retention curve is the relationship between the volume fraction of water and the water potential of the soil, it is characteristic for different types of soil. ... This does not cite any references or sources. ... Hysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state: that is, systems whose states depend on their immediate history. ...


Normalized volumetric water content

The normalized water content, Θ, (also called effective saturation or Se) is a dimensionless value defined by van Genuchten[2] as:

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

where θ is the volumetric water content; θr is the residual water content, defined as the water content for which the gradient dθ / dh becomes zero; and, θs is the saturated water content.


See also

There are very few or no other articles that link to this one. ... Humidity is the amount of water vapor in air. ... Dew on a spider web Moldy bread Moisture generally refers to the presence of water, often in trace amounts. ... Moisture analysis is the method of measuring moisture content for industrial applications. ... Water retention curve is the relationship between the volume fraction of water and the water potential of the soil, it is characteristic for different types of soil. ...

References

  1. ^ Dingman, S.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. 
  2. ^ 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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