# Water content

Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, 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.

Soil composition by Volume and Mass, by phase: air, water, void (pores filled with water or air), soil, and total.

## Definitions

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:

${\displaystyle u={\frac {m_{w}}{m}}}$

where ${\displaystyle m_{w}}$ is the mass of water and ${\displaystyle m}$ is the mass of the substance. Normally the latter is taken before drying:

${\displaystyle u'={\frac {m_{w}}{m_{\text{wet}}}}}$

except for woodworking, geotechnical and soil science applications where oven-dried material is used instead:

${\displaystyle u''={\frac {m_{w}}{m_{\text{dry}}}}}$

To convert gravimetric water content to volumetric water content, multiply the gravimetric water content by the bulk specific gravity ${\displaystyle SG}$ of the material:

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

### Derived quantities

In soil mechanics and petroleum engineering the water saturation or degree of saturation, ${\displaystyle S_{w}}$, is defined as

${\displaystyle S_{w}={\frac {V_{w}}{V_{v}}}={\frac {V_{w}}{V\phi }}={\frac {\theta }{\phi }}}$

where ${\displaystyle \phi =V_{v}/V}$ is the porosity, in terms of the volume of void or pore space ${\displaystyle V_{v}}$ and the total volume of the substance ${\displaystyle V}$. Values of Sw can range from 0 (dry) to 1 (saturated). In reality, Sw never reaches 0 or 1 - these are idealizations for engineering use.

The normalized water content, ${\displaystyle \Theta }$, (also called effective saturation or ${\displaystyle S_{e}}$) is a dimensionless value defined by van Genuchten[2] as:

${\displaystyle \Theta ={\frac {\theta -\theta _{r}}{\theta _{s}-\theta _{r}}}}$

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, ${\displaystyle \phi }$.

## Measurement

### Direct methods

Water content can be directly measured using a known volume of the material, and a drying oven. Volumetric water content, θ, is calculated[3] via the volume of water ${\displaystyle V_{w}}$ and the mass of water ${\displaystyle m_{w}}$:

${\displaystyle V_{w}={\frac {m_{w}}{\rho _{w}}}={\frac {m_{\text{wet}}-m_{\text{dry}}}{\rho _{w}}}}$

where

${\displaystyle m_{\text{wet}}}$ and ${\displaystyle m_{\text{dry}}}$ are the masses of the sample before and after drying in the oven;
${\displaystyle \rho _{w}}$ is the density of water; and

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:

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

However, geotechnics requires the moisture content to be expressed with respect to the sample's dry weight (often as a percentage, i.e. % moisture content = u×100%)

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

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

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.

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:

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

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 soil

### 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, amplitude domain reflectometry, electrical resistivity tomography, ground penetrating radar (GPR), and others that are sensitive to the physical properties of water .[4] 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 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.[5] 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.[6]

## 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 sciences, 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 .[7]

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)
Conditions
Saturated water content θs 0 0.2–0.5 Fully saturated soil, equivalent to effective porosity
Field capacity θfc −33 0.1–0.35 Soil moisture 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:

θa ≡ θfc − θpwp

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 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.

#### 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.

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.

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 (SSD) and damp (or wet).[8] 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.[8]
• 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.[8]
• Saturated surface dry (SSD) is defined as the condition of an 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[8]), but the inter-particle voids are saturated with water. In this condition aggregates will not affect the free water content of a composite material.[9][10]

The water adsorption by mass (Am) is defined in terms of the mass of saturated-surface-dry (Mssd) sample and the mass of oven dried test sample (Mdry) 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.[8]

#### 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.[11][12]

## References

1. ^ T. William Lambe & Robert V. Whitman (1969). "Chapter 3: Description of an Assemblage of Particles". Soil Mechanics (First ed.). John Wiley & Sons, Inc. p. 553. ISBN 978-0-471-51192-2.
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. Bibcode:1980SSASJ..44..892V. doi:10.2136/sssaj1980.03615995004400050002x.
3. ^ Dingman, S.L. (2002). "Chapter 6, Water in soils: infiltration and redistribution". Physical Hydrology (Second ed.). Upper Saddle River, New Jersey: Prentice-Hall, Inc. p. 646. ISBN 978-0-13-099695-4.
4. ^ 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" (PDF). Geophysical Research Abstracts. 7.
5. ^ Lakhankar, Tarendra; Ghedira, Hosni; Temimi, Marouane; Sengupta, Manajit; Khanbilvardi, Reza; Blake, Reginald (2009). "Non-parametric Methods for Soil Moisture Retrieval from Satellite Remote Sensing Data". Remote Sensing. 1: 3–21. doi:10.3390/rs1010003.
6. ^ "Archived copy". Archived from the original on 2007-09-29. Retrieved 2007-08-22.CS1 maint: Archived copy as title (link)
7. ^ Lawrence, J. E. & G. M. Hornberger (2007). "Soil moisture variability across climate zones". Geophys. Res. Lett. 34 (L20402): L20402. Bibcode:2007GeoRL..3420402L. doi:10.1029/2007GL031382.
8. "Water-to-Cement Ratio and Aggregate Moisture Corrections". precast.org. Retrieved 2018-11-18.
9. ^ "Aggregate Moisture in Concrete". Concrete Construction. Retrieved 2018-11-08.
10. ^ ftp://ftp.dot.state.tx.us/pub/txdot-info/cst/TMS/400-A_series/pdfs/cnn403.pdf
11. ^ Zaccardi, Y. A. Villagrán; Zega, C. J.; Carrizo, L. E.; Sosa, M. E. (2018-10-01). "Water absorption of fine recycled aggregates: effective determination by a method based on electrical conductivity". Materials and Structures. 51 (5): 127. doi:10.1617/s11527-018-1248-2. ISSN 1871-6873.
12. ^ Kawamura, Masashi; Kasai, Yoshio (2009-05-29). "Determination of saturated surface-dry condition of clay–sand mixed soils for soil–cement concrete construction". Materials and Structures. 43 (4): 571–582. doi:10.1617/s11527-009-9512-0. ISSN 1359-5997.

Atterberg limits

The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit.

Depending on its water content, a soil may appear in one of four states: solid, semi-solid, plastic and liquid. In each state, the consistency and behavior of a soil is different and consequently so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behavior. The Atterberg limits can be used to distinguish between silt and clay, and to distinguish between different types of silts and clays.The water content at which the soils changes from one state to the other are known as consistency limits or Atterberg's limit.

These limits were created by Albert Atterberg, a Swedish agriculturists in 1911. They were later refined by Arthur Casagrande.

Distinctions in soil are used in assessing the soils that are to have structures built on them. Soils when wet retain water, and some expand in volume. The amount of expansion is related to the ability of the soil to take in water and its structural make-up (the type of atoms present). These tests are mainly used on clayey or silty soils since these are the soils that expand and shrink due to moisture content. Clays and silts react with the water and thus change sizes and have varying shear strengths. Thus these tests are used widely in the preliminary stages of designing any structure to ensure that the soil will have the correct amount of shear strength and not too much change in volume as it expands and shrinks with different moisture contents.

Carbonic anhydrase

The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes.

The enzyme maintains acid-base balance and helps transport carbon dioxide.Carbonic anhydrase helps regulate pH and fluid balance. Depending on its location, the role of the enzyme changes slightly. For example, carbonic anhydrase produces acid in the stomach lining. In the kidney, the control of bicarbonate ions influences the water content of the cell. The control of bicarbonate ions also influences the water content in the eyes, and if the enzyme does not work properly, a buildup of fluid can lead to glaucoma.

Clay

Clay is a finely-grained natural rock or soil material that combines one or more clay minerals with possible traces of quartz (SiO2), metal oxides (Al2O3 , MgO etc.) and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to particle size and geometry as well as water content, and become hard, brittle and non–plastic upon drying or firing. Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.

Although many naturally occurring deposits include both silts and clay, clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts, which are fine-grained soils that do not include clay minerals, tend to have larger particle sizes than clays. There is, however, some overlap in particle size and other physical properties. The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 µm (clays being finer than silts), sedimentologists often use 4–5 μm, and colloid chemists use 1 μm. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg limits. ISO 14688 grades clay particles as being smaller than 2 μm and silt particles as being larger.

Mixtures of sand, silt and less than 40% clay are called loam. Loam makes good soil and is used as a building material.

Dairy salt

Dairy salt is a salt product used in the preparation of butter and cheese products that serves to add flavor and act as a food preservative. Dairy salt can vary in terms of quality and purity, with purer varieties being the most desirable for use in foods. Dairy salt has been used since at least the 1890s in England and the United States. In butter preparation, it serves to retain moisture, while in cheeses, it tends to reduce water content and slow the ripening process.

Dried lime

Dried lime (also known as: black lime; noomi basra (Iraq); limoo amani (Iran); loomi (Oman)) is a lime that has lost its water content, usually after having spent a majority of their drying time in the sun. They are used whole, sliced or ground, as a spice in Middle Eastern dishes. Originating in Oman -- hence the name limoo amani and Iraqi name noomi basra (lemon from Basra) -- dried limes are popular in cookery across the Middle East.

Fit for Life

Fit for Life (FFL) is a diet and lifestyle book series stemming from the principles of orthopathy. It is promoted mainly by the American writers Harvey and Marilyn Diamond. The Fit for Life book series recommends dietary principles including eating only fruit in the morning, eating predominantly "live" and "high-water-content" food, and if eating animal protein to avoid combining it with complex carbohydrates.

While the diet has been praised for encouraging the consumption of raw fruits and vegetables, several other aspects of the diet have been disputed by dietitians and nutritionists, and the American Dietetic Association and the American Academy of Family Physicians list it as a fad diet.

Gas dwarf

A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having as result a total radius between 1.7 and 3.9 Earth radii (1.7–3.9 R⊕). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 R⊕ and planets greater than 3.9 R⊕, namely ice giants and gas giants.Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out.The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density (2.1(+2.2/-1.2) grams per cubic centimetre) that indicates either a substantial water content or possibly a thick gas envelope.A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.

Honey

Honey is a sweet, viscous food substance produced by bees and some related insects. Bees produce honey from the sugary secretions of plants (floral nectar) or from secretions of other insects (such as honeydew), by regurgitation, enzymatic activity, and water evaporation. Bees store honey in wax structures called a honeycomb. The variety of honey produced by honey bees (the genus Apis) is the best-known, due to its worldwide commercial production and human consumption. Honey is collected from wild bee colonies, or from hives of domesticated bees, a practice known as beekeeping or apiculture.

Honey gets its sweetness from the monosaccharides fructose and glucose, and has about the same relative sweetness as sucrose (granulated sugar). It has attractive chemical properties for baking and a distinctive flavor when used as a sweetener. Most microorganisms do not grow in honey, so sealed honey does not spoil, even after thousands of years.Honey provides 46 calories in a serving of one tablespoon (15 ml). Honey is regarded as safe when not taken in excessive amounts.Honey use and production have a long and varied history as an ancient activity. Several cave paintings in Cuevas de la Araña, Spain, depict humans foraging for honey at least 8,000 years ago.

Hydraulic debarker

A hydraulic debarker is a machine removing bark from wooden logs by the use of water under a pressure of 100 psia (6.8 atmospheres) or greater. Hydraulic debarking can reduce soil and rock content of bark, but may increase the water content. Debarking water may be recycled after effective settling, but suspended solids may increase wear on high-pressure pumps. Hydraulic debarking has declined where water quality problems have arisen.

Infiltration (hydrology)

Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meters per day but can also be measured in other units of distance over time if necessary. The infiltration capacity decreases as the soil moisture content of soils surface layers increases. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier.

Infiltrometers , permeameters and rainfall simulators are all devices that can be used to measure infiltration rates. Infiltration is caused by multiple factors including; gravity, capillary forces, adsorption and osmosis. Many soil characteristics can also play a role in determining the rate at which infiltration occurs.

Lotion

A lotion is a low-viscosity topical preparation intended for application to the skin. By contrast, creams and gels have higher viscosity, typically due to lower water content. Lotions are applied to external skin with bare hands, a brush, a clean cloth, or cotton wool.

While a lotion may be used as a medicine delivery system, many lotions, especially hand lotions and body lotions are meant instead to simply smooth, moisturize, soften and perhaps perfume the skin.Some skincare products, such as sunscreen and moisturizer, may be available in multiple formats, such as lotions, gels, creams, or sprays.

Moisture

Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in various commercial products. Moisture also refers to the amount of water vapour present in the air.

Perlite

Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its low density after processing.

Purée

A purée (or mash) is cooked food, usually vegetables, fruits or legumes, that has been ground, pressed, blended or sieved to the consistency of a creamy paste or liquid. Purées of specific foods are often known by specific names, e.g., applesauce or hummus. The term is of French origin, where it meant in Old French (13th century) purified or refined.

Purées overlap with other dishes with similar consistency, such as thick soups, creams (crèmes) and gravies—although these terms often imply more complex recipes and cooking processes. Coulis (French for "strained") is a similar but broader term, more commonly used for fruit purées. The term is not commonly used for paste-like foods prepared from cereal flours, such as gruel or muesli; nor with oily nut pastes, such as peanut butter. The term "paste" is often used for purées intended to be used as an ingredient, rather than eaten.

Purées can be made in a blender, or with special implements such as a potato masher, or by forcing the food through a strainer, or simply by crushing the food in a pot. Purées generally must be cooked, either before or after grinding, in order to improve flavour and texture, remove toxic substances, and/or reduce their water content.

It is common to purée entire meals (without use of salt or other additives) to be served to toddlers, babies, and those unable to chew as sufficient, nutritious meals.

Rác Thermal Bath

The Rác Thermal Bath, located in Budapest, Hungary, is an 8000-square metre bath and is renowned for its Turkish bath dating back to the 16th century, and its imperial pools and shower corridor built in the age of the Austro-Hungarian Monarchy. The bath is listed as a UNESCO World Heritage site and is now part of the newly built complex of the Rac Hotel & Thermal Spa. Its name derives from the ancient Hungarian name of Serbs, one of the former cohabiting populations in Tabán.

Scoville scale

The Scoville scale is a measurement of the pungency (spiciness or "heat") of chili peppers and other spicy foods, as recorded in Scoville Heat Units (SHU) based on the concentration of capsaicinoids, among which capsaicin is the predominant component. The scale is named after its creator, American pharmacist Wilbur Scoville, whose 1912 method is known as the Scoville organoleptic test. In the 21st century, high-performance liquid chromatography (HPLC) is used to quantify the capsaicinoid content as an indicator of pungency. The most expedient method for estimating SHU is a subjective assessment derived from the capsaicinoid sensitivity by people experienced with eating hot chilis.

Soil moisture sensor

Soil moisture sensors measure the volumetric water content in soil. Since the direct gravimetric measurement of free soil moisture requires removing, drying, and weighting of a sample, soil moisture sensors measure the volumetric water content indirectly by using some other property of the soil, such as electrical resistance, dielectric constant, or interaction with neutrons, as a proxy for the moisture content.

The relation between the measured property and soil moisture must be calibrated and may vary depending on environmental factors such as soil type, temperature, or electric conductivity. Reflected microwave radiation is affected by the soil moisture and is used for remote sensing in hydrology and agriculture. Portable probe instruments can be used by farmers or gardeners.

Soil moisture sensors typically refer to sensors that estimate volumetric water content. Another class of sensors measure another property of moisture in soils called water potential; these sensors are usually referred to as soil water potential sensors and include tensiometers and gypsum blocks.

Spring (hydrology)

A spring is a point at which water flows from an aquifer to the Earth's surface. It is a component of the hydrosphere.

Water-tube boiler

A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam.

High Pressure Water Tube Boiler:

The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a superheater to become superheated. Superheated steam is defined as steam that is heated above the boiling point at a given pressure. Superheated steam is a dry gas and therefore used to drive turbines, since water droplets can severely damage turbine blades.

Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore 'downcomer tubes', where it pre-heats the feedwater supply. (In large utility boilers, the feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the waterwalls). To increase economy of the boiler, exhaust gases are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such watertube boilers in thermal power stations are also called steam generating units.

The older fire-tube boiler design, in which the water surrounds the heat source and gases from combustion pass through tubes within the water space, is a much weaker structure and is rarely used for pressures above 2.4 MPa (350 psi). A significant advantage of the watertube boiler is that there is less chance of a catastrophic failure: there is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

A water tube boiler was patented by Blakey of England in 1766 and was made by Dallery of France in 1780.

Physical aquifer properties used in hydrogeology
Soil
Foundations
Retaining walls
Stability
Earthquakes
Geosynthetics
Numerical analysis

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.