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.
For materials that change in volume with water content, such as coal, the gravimetric water content, u, is expressed in terms of the mass of water per unit mass of the moist specimen (before drying): However, woodworking, geotechnics and soil science require the gravimetric moisture content to be expressed with respect to the sample's dry weight: And in food science, both
[clarification needed] Values of Sw can range from 0 (dry) to 1 (saturated).
The oven-dry method requires drying a sample (of soil, wood, etc.)
in a special oven or kiln and checking the sample weight at regular time intervals.
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 or until it stops losing weight).
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.
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
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 .
[6] Geophysical sensors are often used to monitor soil moisture continuously in agricultural and scientific applications.
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.
[7] 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.
At low relative humidities, moisture consists mainly of adsorbed water.
Many recent scientific research efforts have aimed toward a predictive-understanding of water content over space and time.
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.
Some agriculture professionals are beginning to use environmental measurements such as soil moisture to schedule irrigation.
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 flow of water through and unsaturated zone in soils often involves a process of fingering, resulting from Saffman–Taylor instability.
This results mostly through drainage processes and produces and unstable interface between saturated and unsaturated regions.
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.
Due to hysteresis, different wetting and drying curves may be distinguished.
They are Oven-dry (OD), Air-dry (AD), Saturated surface dry (SSD) and damp (or wet).
[11] 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.
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: Among these four moisture conditions of aggregates, saturated surface dry is the condition that has the most applications in laboratory experiments, research, and studies, especially those related to water absorption, composition ratio, or shrinkage tests in materials like concrete.
In saturated surface dry conditions, the aggregate's water content is in a relatively stable and static situation where its environment would not affect it.
