Bulk densityFrom Wikipedia, the free encyclopediaJump to: navigation, search

Bulk density is a property of powders, granules and other "divided" solids, especially used in reference to mineral components (soil, gravel), chemical substances, (pharmaceutical) ingredients, foddstuff or any other masses of corpuscular or particulate matter. It is defined as the mass of many particles of the material divided by the total volume they occupy. The total volume includes particle volume, inter-particle void volume and internal pore volume.[1]

Bulk density is not an intrinsic property of a material; it can change depending on how the material is handled. For example, a powder poured in to a cylinder will have a particular bulk density; if the cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported both as "freely settled" (or "poured" density) and "tapped" density (where the tapped density refers to the bulk density of the powder after a specified compaction process, usually involving vibration of the container.)

Contents

[hide]

• 1 Soil • 2 See also • 3 Notes

• 4 External links

[edit] Soil

The bulk density of soil depends greatly on the mineral make up of soil and the degree of compaction. The density of quartz is around 2.65g/cm³ but the bulk density of a mineral soil is normally about half that density, between 1.0 and 1.6g/cm³. Soils high in organics and some friable clay may have a bulk density well below 1g/cm³

Bulk density of soil is usually determined on Core samples which are taken by driving a metal corer into the soil at the desired depth and horizon. The samples are then oven dried and weighed.

Bulk density = mass of oven dry soil/core volume

The bulk density of soil is inversely related to the porosity of the same soil: the more pore space in a soil the lower the value for bulk density. Bulk density of a region in the interior of the earth is also related to the seismic velocity of waves travelling through it: for P-waves, this has been quantified with Gardner's relation. The higher the density, the faster the velocity.

Water contentFrom Wikipedia, the free encyclopediaJump to: navigation, search

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.

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

Volumetric water content, θ, is defined mathematically as:

where Vw is the volume of water and VT = Vs + Vv = Vs + Vw + Va is the total volume (that is soil volume + water volume + air space).

Gravimetric water content[1] is expressed by mass (weight) as follows:

where mw is the mass of water and mb is the bulk mass. The bulk mass is taken as the total mass, except for geotechnical and soil science applications where oven-dried soil (ms, see the diagram) is conventionally used as mb.

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

Contents

[hide]

• 1 Other definitions o 1.1 Degree of saturation o 1.2 Normalized volumetric water content

• 2 Measurement o 2.1 Direct methods o 2.2 Laboratory methods o 2.3 Geophysical methods o 2.4 Satellite remote sensing method

• 3 Classification and uses o 3.1 Earth and agricultural sciences

3.1.1 Agriculture 3.1.2 Groundwater

• 4 See also

• 5 References

[edit] Other definitions

[edit] Degree of saturation

In soil mechanics and petroleum engineering, the term water saturation or degree of saturation, Sw is used, defined as

where φ = Vv / VT is the porosity and Vv is the volume of void or pore space.

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.

[edit] 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:

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, which is equivalent to porosity, φ.

[edit] Measurement

[edit] 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] using:

where

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

ρw is the density of water; andVb is the volume of the sample before drying the sample.

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:

However, geotechnics requires the moisture content to be expressed as a percentage of the sample's dry weight i.e. % moisture content = u * 100

Where

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.

[edit] Laboratory methodsMain 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.

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:

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.

[edit] 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, 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.

[edit] 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 [1]. 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 [2].

[edit] 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 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.

[edit] 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 temperature regions .[5]

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 0.2–0.5Fully saturated water, equivalent to effective porosity

Field capacity θfc −33 0.1–0.35Soil moisture 2–3 days after a rain or irrigation

Permanent wilting point

θpwp or θwp −1500 0.01–0.25Minimum soil moisture at which a plant wilts

Residual water content

θr −∞ 0.001–0.1Remaining 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.

[edit] 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.

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]

[edit] 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

What is Soil Moisture?Soil moisture is difficult to define because it means different things in different disciplines. For example, a farmer's concept of soil moisture is different from that of a water resource manager or a weather forecaster. Generally, however, soil moisture is the

water that is held in the spaces between soil particles. Surface soil moisture is the water that is in the upper 10 cm of soil, whereas root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 200 cm of soil.

Why is Measuring Soil Moisture Important?Compared to other components of the hydrologic cycle, the volume of soil moisture is small; nonetheless, it of fundamental importance to many hydrological, biological and biogeochemical processes. Soil moisture information is valuable to a wide range of government agencies and private companies concerned with weather and climate, runoff potential and flood control, soil erosion and slope failure, reservoir management, geotechnical engineering, and water quality. Soil moisture is a key variable in controlling the exchange of water and heat energy between the land surface and the atmosphere through evaporation and plant transpiration. As a result, soil moisture plays an important role in the development of weather patterns and the production of precipitation. Simulations with numerical weather prediction models have shown that improved characterization of surface soil moisture, vegetation, and temperature can lead to significant forecast improvements. Soil moisture also strongly affects the amount of precipitation that runs off into nearby streams and rivers. Large-scale dry or wet surface regions have been observed to impart positive feedback on subsequent precipitation patterns, such as in the extreme conditions over the central U.S. during the 1988 drought and the 1993 floods. Soil moisture information can be used for reservoir management, early warning of droughts, irrigation scheduling, and crop yield forecasting.

Remote Sensing of Soil MoistureDespite the importance of soil moisture information, widespread and/or continuous measurement of soil moisture is all but nonexistent. "The lack of a convincing approach of global measurement of soil moisture is a serious problem" (National Research Council, 1992). Clearly, a need exists for continuous measurements of surface soil moisture with global coverage. Remote sensing of soil moisture from the vantage point of space is advantageous because of its spatial coverage and temporal continuity, but this capability does not yet exist. Research in soil moisture remote sensing began in the mid 1970's shortly after the surge in satellite development. Subsequent research has occurred along many diverse paths. Quantitative measurements of soil moisture in the surface layer of soil have been most successful using passive remote sensing in the microwave region. The potential exists today to retrieve soil moisture estimates from space-based instruments at frequencies of about 6 GHz (C band). However, observations at frequencies between 1 and 3 GHz (L band) are best suited for detection of soil moisture because energy is emitted from a deeper soil layer and less energy is absorbed or reflected by vegetation.

Soil moisture remote sensing is fraught with challenges. Only the moisture in the top few centimeters of soil can be detected. Algorithm development is complicated by the need for surface roughness and vegetation corrections, which are based on empirical relationships of limited breadth. Extending ground-based techniques to space-based systems requires innovative antenna technology. In spite of these challenges, recent advances in aperture synthesis and thinned array technology applied at L band have shown great promise for soil moisture mapping. Scientists that the Global Hydrology and Climate Center have been involved in experiments to address some the challenges we face in developing this technology.