Ground Water Hydrology: A Guide to the Water Cycle Below Ground

Ground Water Hydrology
Syllabus
• General Water Balance, Regional
groundwater balance, Distribution of
subsurface water, different type of
aquifers, occurrence of groundwater in
hydrological formations, structures and
type of wells, component of groundwater
studies.

• The ground water is considered a very
important natural resource, in arid , semi arid
and dry regions, this may be the only source of
water supply. Even in humid areas, groundwater
is considered a better resource for many
economic and hygienic reasons.

• Ground Water
• Has a suitable composition in most cases and is free
from turbidity, objectionable colors, and pathogenic
organisms and require not much treatment.
• Is relatively much safer from hazards of
chemical, radiogenic and biological pollution to which
surface water bodies are exposed
• Supplies are not quickly affected by drought and other
climatic changes and hence are more dependable.
• Being available locally in many cases may be tapped and
distributed at much lesser cost using very little network
of pipes

• Sources of groundwater
• Meteoric Water
• It is the water derived from precipitation (rain and snow)
although bulk of the rain water or melt water from snow
and ice reaches the sea through the surface flows or
runoffs a considerable part of precipitation gradually
infiltrates into ground water. This infiltrated water
continuous its downward journey till it reaches the zone
of saturation to become the ground water in the aquifer.
• Almost entire water obtained from ground water
supplies belongs to this category.

Connote Water
• This is the water present in the rocks right from the time
of their deposition in an aqueous environment. During
the process of formation of sedimentary rock in a lake or
sea or river, depositions is followed by compaction,
which leads to the squeezing out of most of the water
present between the sediments. Sometimes however,
incomplete compaction may cause retention of some
water by these rocks which is known as connote water.
And it may be found in rocks like limestone, sandstone
and gravels. It is saline in nature and is of no importance
as a source for exploitable groundwater.

• Juvenile Water
• It is also called magmatic water and is of only
theoretical importance as far as water supply
scheme is concerned. It is the water found in
the cracks or crevices or porous of rocks due to
condensation of steam emanating from hot
molten masses or magmas existing below the
surface of the earth. Some hot springs and
geysers are clearly derived from juvenile water.

• Distribution of Ground Water
• The water that goes below the surface of the
land may be found to exist in two main zones or
environments classified as Vadosa Water and
phreatic water or groundwater
• In the vadosa water zone itself, three different
types of environment are distinguished; soil
water, intermediate vadose water and capillary
water.

• The soil water forms a thin layer confined to the near surface depth
of the land. It may occur at depth between 1.0 to 9 m and is held up
by the root zone of vegetable cover of the globe It is lost to the
atmosphere by transpiration and evaporation.
• The intermediate vadosa zone occurs immediately below the zone of
soil water. It is in fact a zone of non saturation; water in this zone is
moving downward under the influence of gravity. It is generally of
smaller thickness and may be even absent in many cases. The above
zones are sometimes collectively referred as zone of aeration.
• The zone of capillary water, also called as capillary fringe. Is present
only in soil and rocks of fine particles size underlying the vadosa
zone. In the fine particle size zone, groundwater is drawn upward by
capillary action, sometimes to height of 2-3 m above saturated zone
lying underneath. Growth of vegetation in some desert is very often
dependent on presence of capillary fringe.

• The Phreatic Water Zone
• Also known as zone of saturation lies below the capillary
fringe and is the water held in this zone that is called
groundwater in the real sense. The upper surface of water in
the zone marks the water table in the area. In this zone the
layers or bodies of rocks which are porous and
permeable, have all their open spaces such as
pores, cavities, cracks etc. completely filled with water. All
these openings are interconnected, so that a well dug into this
openings are completely filled with water, there is no or very
little downward movement of groundwater. In all ground
water exploration programmes, the main objective is to locate
this zone and determine its extent, geometry and character.

• Water Balance
• In hydrology, a water balance equation can be used to describe the
flow of water in and out of a system.
• A system can be one of several hydrological domains, such as a
column of soil or a drainage basin.
• Water balance can also refer to the ways in which an organism
maintains water in dry or hot conditions. It is often discussed in
reference to plants or arthropods, which have a variety of water
retention mechanisms,
• general water balance equation is: P= Q+E+∆S
• Where,
• P= Precipitation
• Q= Runoff
• E= Evapotranspiration
• ∆S = Change in Storage

• This equation uses the principles of conservation of
mass in a closed system, whereby any water
entering a system (via precipitation), must be
transferred into either evaporation, surface runoff
(eventually reaching the channel and leaving in the
form of river discharge), or stored in the ground.
This equation requires the system to be closed, and
where it isn't (for example when surface runoff
contributes to a different basin), this must be
taken into account.
• Extensive water balances are discussed in
agricultural hydrology

• A water balance can be used to help manage water
supply and predict where there may be water
shortages. It is also used in irrigation, runoff
assessment (e.g. through the RainOff model ), flood
control and pollution control. Further it is used in
the design of subsurface drainage systems which
may be horizontal (i.e. using pipes, tile drains or
ditches) or vertical (drainage by wells).To estimate
the drainage requirement, the use of an
hydrogeological water balance and a groundwater
model may be instrumental.

Ground Water
• Study of sub surface flow is equally important
since about 30 % of the world’s fresh water
resources exist in the form of groundwater.
Further, the subsurface water forms a critical
input for the substance of life and vegetation in
arid zones. Due to its importance as a significant
source of water supply various aspects of ground
water dealing with exploration, development and
utilization have been extensively studied by
workers from different disciplines, such as
geology, geophysics, geochemistry, agricultural
engineering, fluid mechanics and civil Engineering.

• Forms of Subsurface Water
• Water in the soil mantle is called
subsurface water and is considered in two
zones
• Saturated Zone
• Aeration Zone.

Water table generally
below surface, so water
can seep in
Water can soak into
subsurface and become
groundwater
Where water table intersects
surface, water can flow out

Saturated Zone
• This Zone is also known as groundwater zone in
which all the pores of the soil are filled with water.
The water table forms the upper limit and marks a
free surface, i.e. a surface having atmospheric
pressure.

Zone of Aeration
• In this zone the soil pores are only partially
saturated with water. The spaces between the land
surface and the water table marks the extent of
this zone. The zone of aeration has three subzones.

Soil water zone
• This lies close to the ground surface in the major rrot
band of the vegetation from which the water is lost to
the atmosphere by evapotranspiration.
Capillary Fringe
• In this the water is held by the capillary action. This
zone extends from water table upwards to the limit of
the capillary rise.
Intermediate Zone
• This lies between the soil water zone and the capillary
fringe. The soil texture and moisture content and vary
from region to region. The soil moisture in the zone of
aeration is of importance in agricultural practices and
irrigation engineering.

• Saturated Formations
• All earth materials from soils to rocks have pore spaces.
Although these pores are completely saturated with
water table below, from the groundwater utilization
aspect only such material through which water moves
easily and hence can be extracted with ease are
significant. On this basis the saturated formation are
classified into four categories.
• Aquifer
• Aqitard
• Aquiclude
• Aquifuge

Aquifer
• An aquifer is a saturated formation of earth
material which not only stores water but
yields it in sufficient quantity. Thus an
aquifer transmits water relatively easily due
to high permeability. Unconsolidation
deposits off sand and gravel form good
aquifer.

Aquitard
• It is a formation through which only
seepage is possible and thus the yield is
insignificant compared to an aquifer. It is
partly permeable. A sandy clay unit is an
example of aquitard. Through an aquitard
appreciable quantities of water may leak to
an aquifer below it.

Aqiclude
It is a geological formation which is
essentially impermeable to the flow of
water. It may be considered as close to
water movement even though it may
contain large amount of water due to
its high porosity. Clay is an example
of an acquiclude.

Aquifuge
• It is a geological formation which
neither porous nor permeable. There
are no interconnected openings and
hence it cannot transmit water.
Massive compact rock without any
fracture is an acquifuge.

Aquifer
• Formation of ground which contain water
and may transmit water in usable quantity
are known as aquifer. Thus these are the
geological formations in which groundwater
occurs. (i.e. Sands, gravels).

Confined aquifer
overlain by less
permeable
materials
Unconfined aquifer
open to Earth’s surface
and to infiltration
Perched aquifer underlain by
low-permeability unit
Artesian aquifer: water rises in
pipe (maybe to surface)

Aquifer are mainly of two types
Unconfined Aquifer
• An unconfined aquifer is the one in which water table
forms the upper surface of the zone of saturation. An
aquifer where the water table is the upper surface limit
and extends below till the impermeable rock strata is
called the unconfined aquifer.
Confined Aquifer
• When an aquifer is sandwiched between two
impermeable layers, it is known as a confined aquifer. It
is also known as a pressure aquifer, or an artesian
aquifer. Confined aquifers are completely filled with
water and they do not have a free water table and the
aquifer will be under pressure.

Leaky Aquifer
• An aquifer bound by one or two
aquitards is known as a leaky aquifer. It
is also known as semi-confined aquifer.
Perched Aquifer
Perched Aquifer is a special type of an
unconfined aquifer. An impermeable
saucer-shaped stratum of a small aerial
extent occurring in the zone of aeration
may retain and hold some amount of
water is called perched aquifer.

Water Table
A water table is the free water
surface in an unconfined aquifer
indicating the level of the water table
at that point. The water table is
constantly in motion adjusting its
surface to achieve a balance between
the recharge and outflow from the
surface storage.

Water Table
• Fluctuations in the water level in a dug well
during various seasons of the year, lowering
of the groundwater table in a region due to
heavy pumping of the wells and the rise in
the water table of an irrigated area with poor
drainage, are some common examples of the
fluctuation of the water table. In a general
sense, the water table follows the topographic
features of the surface. If the water table
intersects the land surface the ground water
comes out to the surface in the form of
springs or seepage.

Aquifer Functions
• As said earlier aquifer serves as a underground
reservoir and distribution system or conduct at
the same time.
• The storage capacity of rocks depends on the
porosity of the rock on the one hand and the
nature and inter-connections of the pores.

Aquifer Properties
• The importance properties of an aquifer are its
capacity to release the water held in its pores
and its ability to transmit the flow easily. These
properties essentially depends upon the
composition of the aquifer.

Aquifer Properties
Porosity
• The amount of pore space per unit volume of the
aquifer material is called porosity. It is expressed as
n = Vv
Vo
Where,
n= porosity,
Vv= Volume of voids
Vo= Volume of porous medium

Aquifer Properties
• In an unconsolidated material the size
distribution, packing and shape of particles
determine the porosity. In hard rocks the
porosity is dependent on the extent, spacing
and the pattern of fracturing or the nature of
solution channels.

Aquifer Properties
Specific Yield
• While the porosity give a measure of the water storage
capability of a formation, not all the water held in the
pores is available for extraction by pumping or drainage
by gravity. The pores hold back some water by
molecular attraction and surface tension. The actual
volume of water that can be extracted by the force of
gravity from a unit volume of aquifer material is known
as specific yield Sy, the fraction of water held back in the
aquifer is known as Specific retention Sr, thus porosity
• n= Sy + Sr

Aquifer Properties
Thus,
• Specific Yield: is the ratio of the volume of
water that, after saturation, can be drained by
gravity to its own volume. It is usually expressed
as percentage, Thus
• Sy= Wy x 100
V

Aquifer Properties
Specific Retention: Sr of a soil or rock is the ratio of the
volume of water it will retain after saturation against the force
of gravity to its own volume. It is also expressed as
percentage.
• Thus, Sr = W r x 100
V
Where, Wr is the volume of the retained water and V is the
bulk volume of the soil or rock
Since Wy and Wr constitutes the total volume of water in a
saturated material it is apparent that porosity will be equal to
sum of specific yield and the specific retention,
i.e. n= Sy + Sr

Flow in Aquifer
• Movement of water through the aquifer is in
general a function of three forms of energy
contained in groundwater; pressure, velocity
and elevation head. According to Bernoulli's
equation
• The sum of energy potential of these forms, H
is
• H= p + V2 + Z
r 2g

Flow in Aquifer
• Where,
• p= pressure,
• r= specific weight of water,
• V= is the velocity,
• g= acceleration due to gravity,
• Z= the elevation head
• The velocity factor in great many situations in
groundwater is almost negligible.

Flow in Aquifer
• Permeability, hydraulic conductivity and
transmissibility are important terms related to
various aspects of movements of water in
aquifer.

Flow in Aquifer
• Permeability:
• When used in general sense, permeability is the
capacity of a rock to transmit fluids through it.
It is often expressed as Intrinsic Permeability, of
which darcy d is the unit
• Permebility is essentially related to the quantity
of pores and other interices in a rock.

Flow in Aquifer
• Transmissibility
• The terms was introduced by theiss in 1935 to
express the capacity of an entire aquifer to
transmit water. The coefficient of
transmissibility is defined as:
• Rate of flow in gallons/ minute through a
vertical section of an aquifer 1 foot wide
extending to the full saturated length of the
aquifer under a unit hydraulic head.

Flow in Aquifer
• Hydraulic Conductivity
• In groundwater geology or hydrology, the
quantitative measurement of flow or water is
generally expressed by the terms of hydraulic
Conductivity rather than permeability.
• The hydraulic conductivity K, may be defined
as the flow velocity per unit hydraulic gradient.
It is expressed as meter / day or meter / sec

Flow in Aquifer
• Darcy’s Law
As stated earlier groundwater is not static but it
slowly moving through aquifers. The rate of
flow of groundwater through the aquifers most
of which are natural porous media, can be
expressed by Darcy’s Law

Darcy’s Law
• Henry Darcy a French hydraulic engineer investigated the
flow of water through the horizontal beds of sand to be used
for water filtration. On the basis of his experimentation
Darcy in 1856 indicated that for laminar flow conditions the
velocity of flow of water through saturated porous media is
proportional to the hydraulic gradient. It is universally known
as Darcy’s law may be expressed as
V= Ki
Where,
V= Velocity of flow
K= coefficient of permeability
i= Hydraulic gradient=
= Δh / L; Δh = head loss in a length L of flow path

Darcy’s Law
• Further since
• V= Q/A
• Q= KiA
• Where,
• Q= rate of flow (or Discharge)
• A= total c/s area of the porous medium
perpendicular to the direction of flow.

Wells
• Wells as defined as opening or hole dug or drilled
into an aquifer with the view of withdrawing water
for drinking, agriculture, or other uses. Mostly
these are vertical holes drilled or dug into the
ground upto the aquifer. Water may flow through
these wells either due to natural hydrostatic
pressure or may have to be pumped out. These
may be quite shallow or deep depending on the
depth at which the water bearing strata are
encountered.

Types of Wells
Gravity Well
• Gravity well, it is also called a water table well and is a
vertical or nearly vertical hole penetrating the zone of
saturation below the ground. The essential character of
such a well which is at atmospheric pressure, and
represents, when at rest, the water table of the area
around the well. Water will not normally flow out of
such well on its own; it has to be pumped out or taken
out. Most wells driven in the aquifer for withdrawal of
water are actually gravity well. When water has to be
pumped out of such wells it forms the location of the
Tube wells.

Types of Wells
Galleries
• These are horizontal tunnels or open ditches
that are dug out through a water bearing layer
formation to intercept water. These are dug
generally perpendicular to the direction of flow
of water in the aquifer.

Types of Wells
• Artesian Wells
• These are the holes drilled through the
confined or artesian aquifer. In such wells water
generally flows out at the ground surface, and
even may gush out to some height.

References
• “Irrigation Water Resource and Water Power Engineering”
By Prof:- P.N. Modi Standard Book House
• “Hydrology and Water Resource Engineering” By
Prof:- R.B.Khasia
• Publications of C. P. Kumar
• http://www.angelfire.com/nh/cpkumar/publication/
• MIT Open-CourseWare
• http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-72-groundwater-
hydrology-fall-2005/lecture-notes/

Ground Water Hydrology: A Guide to the Water Cycle Below Ground

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