The phreatic zone, saturated zone, or zone of saturation, is the part of an

aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...

, below the

water table The water table is the upper surface of the phreatic zone or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater, which may be fresh, saline, or brackish, depending on the loc ...

, in which relatively all pores and fractures are saturated with water. The part above the water table is the

vadose zone The vadose zone (from the Latin word for "shallow"), also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at ...

(also called unsaturated zone). The phreatic zone size, color, and depth may fluctuate with changes of season, and during wet and dry periods. Depending on the characteristics of soil particles, their packing and

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

, the boundary of a saturated zone can be stable or instable, exhibiting fingering patterns known as

Saffman–Taylor instability The Saffman–Taylor instability, also known as viscous fingering, is the formation of patterns in a morphologically unstable interface between two fluids in a porous medium or in a Hele-Shaw cell, described mathematically by Philip Saffman and ...

. Predicting the onset of stable vs. unstable drainage fronts is of some importance in modelling phreatic zone boundaries. Dynamics of Drainage and Viscous Fingering
in ''Transport in Porous Media''
''Note that zones "behind" the drainage front are areas on the 'dry' (low-viscosity) (typically above / beyond the 'wet' zone).''

References

Aquifers Cave geology Hydrogeology Soil physics {{Hydrology-stub

See also

References