Water content

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Water content is a ratio used in hydrogeology, soil science and soil mechanics to indicate the amount of water a porous medium contains. 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. When the porous medium in question is soil, water content is synonymous with soil moisture.

Water content can either be the volumetric (by volume) or gravimetric (by weight) fraction of the total rock which is filled with liquid water. Water content is a fraction ranging between 0 and 1 or 0% to 100%, indicating the percentage of porosity filled with water. Saturated conditions occur when the porosity and the water content are equal. Unsaturated conditions are everything other than this case, 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.

Basically every point in the Earth's subsurface has a non-zero water content. Even in the driest places (e.g., deserts) have measurable amounts of soil moisture. The amount may be so low that it is physically impossible to be removed.

In certain groundwater analyses, however, the vadose zone is often approximated as being fully unsaturated. This simplifies calculations of groundwater dynamics and is generally accurate because the effects of a moist vadose zone are sufficiently small.

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.