Phreatic zone in the context of "Groundwater flow"

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⭐ Core Definition: Phreatic zone

The phreatic zone, saturated zone, or zone of saturation, is the part of an aquifer, below the water table, in which relatively all pores and fractures are saturated with water. The part above the water table is the vadose zone (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, the boundary of a saturated zone can be stable or instable, exhibiting fingering patterns known as Saffman–Taylor instability. Predicting the onset of stable vs. unstable drainage fronts is of some importance in modelling phreatic zone boundaries.

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👉 Phreatic zone in the context of Groundwater flow

In hydrogeology, groundwater flow is defined as the "part of streamflow that has infiltrated the ground, entered the phreatic zone, and has been (or is at a particular time) discharged into a stream channel or springs; and seepage water." It is governed by the groundwater flow equation.Groundwater is water that is found underground in cracks and spaces in the soil, sand and rocks. Where water has filled these spaces is the phreatic (also called) saturated zone. Groundwater is stored in and moves slowly (compared to surface runoff in temperate conditions and watercourses) through layers or zones of soil, sand and rocks: aquifers. The rate of groundwater flow depends on the permeability (the size of the spaces in the soil or rocks and how well the spaces are connected) and the hydraulic head (water pressure).

In polar regions groundwater flow may be obstructed by permafrost.

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Phreatic zone in the context of 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 atmospheric pressure. Hence, the vadose zone extends from the top of the ground surface to the water table.

Water in the vadose zone has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion (funiculary groundwater), and capillary action (capillary groundwater). If the vadose zone envelops soil, the water contained therein is termed soil moisture. In fine grained soils, capillary action can cause the pores of the soil to be fully saturated above the water table at a pressure less than atmospheric. The vadose zone does not include the area that is still saturated above the water table, often referred to as the capillary fringe.

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Phreatic zone in the context of 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 locality. It can also be simply explained as the depth below which the ground is saturated. The portion above the water table is the vadose zone. It may be visualized as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity.

In coarse soils, the water table settles at the surface where the water pressure head is equal to the atmospheric pressure (where gauge pressure = 0). In soils where capillary action is strong, the water table is pulled upward, forming a capillary fringe.

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LINKED FROM (PARENTS)
← Vadose zone ← Water table ← Perched water table ← Groundwater flow
LINKS TO (CHILDREN)
Vadose zone → Capillary fringe → Water table → Phreatic → United States Geological Survey → Aquifer → Porosity → Saffman–Taylor instability →