The subsurface occurrence of groundwater may be divided into zones of aeration and saturation. The vertical distribution of groundwater is explained in this module.

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VERTICAL DISTRIBUTION OF
GROUNDWATER
By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore
Mysore

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Introduction:
Groundwater is water that exists in the pore
spaces and fractures in rocks and sediments
beneath the Earth’s surface.
Groundwater occurs everywhere beneath the
Earth’s surface, but is usually restricted to depth
less than 1000 meters.
Flow of groundwater , in soils and rocks, takes
place through void spaces, such as pores and
cracks. The hydraulic properties of soils and rocks
therefore depend on the sizes and shapes of the
void spaces.

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At many locations, the subsurface flow is
dominated by flow through complex networks of
macropores. The subsurface occurrence of
groundwater may be divided into zones of
aeration and saturation.
The zone of aeration consists of interstices
occupied partially by water and partially by air. In
the zone of saturation all interstices are filled with
water, under hydrostatic pressure.

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Unsaturated zone/Zone of Aeration:
It is also called as the Zones of aeration.
It is the zone existing just below the soil.
This is the zone between the land surface and the
water table. It is called as the Vadose zone. It is
partly filled with water and partly filled with air.
The water present in this zone is called as Vadose
water.
Vadose water is any water that occurs in the
unsaturated zone.
The vadose zone is also termed the
unsaturated zone.

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Water in the vadose zone has a pressure
head which is less than the atmospheric
pressure.The water is retained by a combination
of adhesion, 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.

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Vadose water is the water that occurs in the
unsaturated zone.
It includes the capillary fringe.
Characteristically this contains liquid water
under less than atmospheric pressure, and water
in the gas phase under atmospheric pressure, as
well as air and other gases.
This topmost layer is divided into 3 zones-
a. soil-moisture zone,
b. intermediate vadose zone,
c. capillary zone.

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Soil moisture zone:
This is the zone just below the ground surface.
It covers the top soil. It contains soil water and
pellicular water. Soil water is a vadose water .
It is the water that is near enough the surface to be
available to the roots of plants.
Pellicular water is the water held in the soil by
molecular attraction (adhesion) to the walls of
rock or soil particle s, in the form of a film or
skin, after gravity water has drained.

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The amount of soil moisture varies depending
upon the properties of the soil ( porosity &
permeability).
Soil moisture is expresses in percentage.
Tensiometer is the instrument used to measure
this percentage.
Soil moisture :
Water there is held to the soil particles by
capillary forces. The vadose zone may contain a
three-phase system:
• Solid. Mineral grains, and organic material.
• Liquid. Water with dissolved solutes.

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• Gaseous. Water vapor, and other gases.
Soil water can be sub-divided into three
categories:
1) hygroscopic water,
2) capillary water, and
3) gravitational water.
Hygroscopic water is found as a microscopic film
of water surrounding soil particles.

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This water is tightly bound to a soil particle by
molecular forces so powerful that it cannot be
removed by natural forces.
Hygroscopic water is bound to soil particles by
adhesive forces that exceed 31 bars and may be as
great as 10 000 bars (recall that sea level pressure
is equal to 1013.2 millibars which is just about 1
bar).
Capillary water is held by cohesive forces
between the films of hygroscopic water.

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The binding pressure for capillary water is much
less than hygroscopic water.
This water can be removed by air drying, or by
plant absorption, but cannot be removed by
gravity. Plants extract this water through their
roots until the soil capillary force (force holding
water to the particle) is equal to the extractive
force of the plant root.
At this point the plant cannot pull water from the
plant-rooting zone and it wilts, called the wilting
point.

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Infiltration process is affected by numerous
environmental factors that control the ability of
the soil matrix to transmit water and include:
• rate of water delivery to the surface via
precipitation or snowmelt
• depth of ponding on surface
• hydraulic conductivity of the soil
• soil porosity
• pre-existing moisture content in soil pores
• slope and roughness of the surface

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• chemical characteristics of the soil surface (i.e.,
hydrophobic or hydrophilic)
• physical & chemical properties of water (i.e.,
temp., acidity, dissolved constituents)
Soil water types
1. Hygroscopic water:
This is the thin (microscopic) layer of water
adhered to particles
It is ‘unavailable’ for any use.
It is always lost as vapour.

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2. Capillary water:
It is the water held in small pores & resistant to
gravitational drainage. It flows upward via
capillarity.
3. Gravitational water:
It is the water which is stored in largest pores.
It gets drained readily under it’s own weight.

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Pellicular water
The water held in the soil by molecular attraction
(adhesion) to the walls of rock or soil particle s, in
the form of a film or skin, after gravity water has
drained.
Intermediate vadose zone:
Transition zone
The part of unsaturated zone that lies between the
zone of soil water and the capillary fringe.

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Water that sinks into this zone is either drawn
down ward by gravity to the underlying
saturated zone, or is drawn by molecular
attraction into the capillary interstices in the
transition zone.
This zone lies between the soil moisture zone and
the capillary zone below. It separates them. It is
completely filled with air.
It is always under the dynamic influence of
capillary zone. It gets filled up when the water
table rises to the surface.

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Capillary zone
This is the lowermost zone of the zone of
aeration.
The zone immediately above the water table, into
which water may be drawn upward as a
consequence of capillary action. A typical height
for the capillary fringe in clay with a pore radius
of 0.0005 mm might be 3 m, compared with less
than 10 cm in a fine sand with a pore radius of
0.02 mm.

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It is originated due to the capillary fringes raising
from the zone of saturation through the tube like
interconnected interstitial openings.
Capillary water is the water held in capillary
openings of saturated or non-saturated materials
above a water surface - the water table.
Capillary fringe is the zone immediately above
the water table in which some or all of the
interstices are filled with water that is under
pressure less than atmospheric and that is
continuous with the water below the water
table.

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The zone is typically saturated to some distance
above its base, at the water table and its upper
limit is indistinct, varying with the grain size of
material above the water table and often is defined
arbitrarily.
Capillary potential is the amount of work
required to move a unit weight (or mass) of
water from a free-water surface to a specified
point in a soil at the level of the water surface.

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Capillary movement:
It is the movement of water by capillarity in a
porous medium.
Capillary potential
The amount of work required to move a unit
weight (or mass) of water from a free-water
surface to a specified point in a soil at the level
of the water surface.

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Zone of saturation:
The zone below the water table is the zone of
stauration. It is also called as phreatic zone.
The zone of saturation is referred to as an
aquifer.
The water table is defined as the level beneath the
Earth's surface below which all pore spaces are
filled with water, and above which the pore spaces
are filled with air.

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The zone of saturation is the area beneath the
water table where all pore spaces are completely
filled with water. Water that exists within this
zone is known as "ground water".
The thickness of the zone varies from a few feet
to several hundred feet, determined by local
geology, availability of pores in the formation,
and the movement of water from recharge to
points of discharge .

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Saturation can also be a transient (time-variant)
condition in the soil profile or vadose zone,
during times of high precipitation or infiltration .
Such saturation can vary in duration from a few
days or weeks to several months.

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Groundwater is regulated by the quantum and
speed of rains, extent of vaporization at the time
of rain, temperature, slope of land, dryness of air,
porosity and permeability of rocks, vegetative
cover and water absorbing capacity of the soil.
The main source of groundwater is rainfall. It
infiltrates through seepage slowly into the earth
and collects there. It is also called ‘plutonic
water’.

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Groundwater is an important part of the water
cycle, which also includes that part of surface and
atmospheric water which goes underground
through rainfall, rivers and lakes.