The document describes the hydrological cycle, also known as the water cycle. It involves the continuous movement of water on, above, and below the surface of the Earth. The sun evaporates water from oceans, lakes, and rivers, which rises into the atmosphere. Clouds carry water vapor and precipitation falls as rain, snow, or hail. Water runs over or seeps into the ground, with some evaporating again and some collecting in aquifers below ground. This water may eventually be discharged into streams, rivers, lakes, and oceans, completing the cycle.

1. What is the Hydrological Cycle?
•The total amount of water on the earth and in its
atmosphere does not change but the earth’s
water is always in movement.
•Oceans, rivers, clouds and rain, all of which
contain water, are in a frequent state of change
and the motion of rain and flowing rivers transfers
water in a never-ending cycle.
•This circulation and conservation of earth’s water
as it circulates from the land to the sky and back
again is called the ‘hydrological cycle’ or ‘water
cycle’.

3. How does the Hydrological Cycle work?
The stages of the cycle are:
• Evaporation
• Transport
• Condensation
• Precipitation
• Groundwater
• Run-off

4. Evaporation
• Water is transferred from the surface to the
atmosphere through evaporation, the process by
which water changes from a liquid to a gas.
• The sun’s heat provides energy to evaporate
water from the earth’s surface.
• Land, lakes, rivers and oceans send up a steady
stream of water vapour and plants also lose
water to the air (transpiration).

5. Transport
• The movement of water through the
atmosphere, specifically from over the oceans to
over land, is called transport.
• Clouds are propelled from one place to another
by either the jet stream, surface-based
circulations like land and sea breezes or other
mechanisms.
• Most water is transported in the form of water
vapour, which is actually the third most abundant
gas in the atmosphere.

6. Condensation
• The transported water vapour eventually
condenses, forming tiny droplets in clouds

7. Precipitation
• The primary mechanism for transporting water
from the atmosphere to the surface of the earth
is precipitation.
• When the clouds meet cool air over land,
precipitation, in the form of rain, sleet or snow, is
triggered and water returns to the land (or sea).
• A proportion of atmospheric precipitation
evaporates.

8. Groundwater
• Some of the precipitation soaks into the ground
and this is the main source of the formation of
the waters found on land - rivers, lakes,
groundwater and glaciers.
• Some of the underground water is trapped
between rock or clay layers - this is called
groundwater.
• Water that infiltrates the soil flows downward
until it encounters impermeable rock and then
travels laterally.

9. Groundwater cont.
• The locations where water moves laterally
are called ‘aquifers’.
• Groundwater returns to the surface
through these aquifers, which empty into
lakes, rivers and the oceans.

10. Run-off
• Most of the water which returns to land flows
downhill as run-off.
• Some of it penetrates and charges groundwater
while the rest, as river flow, returns to the
oceans where it evaporates.
• Different surfaces hold different amounts of
water and absorb water at different rates.
• As a surface becomes less permeable, an
increasing amount of water remains on the
surface, creating a greater potential for flooding.

11. Groundwater hydrology
• Groundwater is water that exists in the
pore spaces and fractures in rocks and
sediments beneath the earth’s surface.
• It originates as rainfall or snow, and then
moves through the soil and rock into the
groundwater system, where it eventually
makes its way back to the surface
streams, lakes, or oceans.

13. Occurrence of groundwater
• The occurrence and movement of groundwater
are related to physical forces acting in the
subsurface and the geologic environment in
which they occur.
• Groundwater occurs in the subsurface in two
broad zones:
1. the unsaturated zone and the saturated zone.

14. i. The unsaturated zone, also known as
the vadose zone, consists of soil pores
that are filled to a varying degree with air
and water.
ii. The zone of saturation consists of water-
filled pores that are assumed to be at
hydrostatic pressure.

15. Unsaturated zone
• The unsaturated zone commonly consists
of three sub-zones: the root zone, an
intermediate zone, and the capillary fringe.
• The root zone varies in thickness
depending upon growing season and type
of vegetation.
• The water content in the root zone is
usually less than that of saturation.

16. • In the zone of aeration (unsaturated
zone), vadose water occurs.
• This general zone may be further
subdivided into the soil water zone, the
intermediate vadose zone (sub-soil zone),
and capillary zone.

18. Saturated zone
• In the zone of saturation, all communicating voids
are filled with water under hydrostatic pressure.
• Water in the saturated zone is known as
groundwater or phreatic water.
• The saturated zone extends from the upper
surface of saturation down to underlying
impermeable rock.

19. • In the absence of overlying impermeable strata,
the water table, or phreatic surface, forms the
upper surface of the zone of saturation.
• This is defined as the surface of atmospheric
pressure and appears as the level at which
water stands in a well penetrating the aquifer.

20. Forces Acting on groundwater
External forces which act on water in the
subsurface include:
(i) gravity,
(ii) pressure from the atmosphere and overlying
water, and
(iii)molecular attraction between solids and water.

21. In the subsurface, water can occur in the following
as:
(i) water vapour which moves from regions of
higher pressure to lower pressure,
(ii) condensed water which is absorbed by dry soil
particles,
(iii) water which is retained on particles under the
molecular force of adhesion, and
(iv) water which is not subject to attractive forces
towards the surface of solid particles and is
under the influence of gravitational forces.

22. In the saturated zone, groundwater flows through
interconnected voids in response to the difference
in fluid pressure and elevation.
•The driving force is measured in terms of
hydraulic head.
•Hydraulic head (or potentiometric head) (h) is
defined by Bernoulli's equation:

23. Where,
h = hydraulic head
z = elevation above datum
p = fluid pressure with constant density D
g = acceleration due to gravity
v = fluid velocity

24. • Pressure head (or fluid pressure) hp is defined
as:
• In the unsaturated zone, water is held in tension
and pressure head is less than atmospheric
pressure (hp< atmo).
• Below the water table, in the saturated zone,
pressure head is greater than atmospheric
pressure (hp> atmo).
• Because groundwater velocities are usually very
low, the velocity component of hydraulic head
can be neglected.

25. Thus, hydraulic head can usually be
expressed as:
The following figure depicts this equation
within a well.

27. Water Table and Piezometric Surface
Water table
• Water table is the surface of water level in an
unconfined aquifer at which the pressure is
atmospheric.
• It is the level at which the water will stand in a
well drilled in an unconfined aquifer.
• The water table fluctuates whenever there is a
recharge or an outflow from the aquifer.
• In fact, the water table is constantly in motion
adjusting its surface to achieve a balance
between the recharge and the out flow.

28. Perched water table
• Perched water table is formed when a small
water body is separated from the main
groundwater body by a relatively small
impermeable stratum.
• Wells drilled below the perched water table up to
the small impervious stratum yield very small
quantity of water and soon go dry.

29. Piezometric surface
• The water in a confined aquifer is under
pressure. When a well is drilled in a confined
aquifer, the water level in it will rise above the
top of aquifer.
• The piezometric surface is an imaginary surface
to which the water level would rise if a
piezometer was inserted in the aquifer.
• Thus, it indicates the pressure of the water in the
aquifer.
• Hence, a piezometric surface is the water table
equivalent of the unconfined aquifer

31. Aquifer
• An aquifer is a ground-water reservoir composed
of geologic units that are saturated with water
and sufficiently permeable to yield water in a
usable quantity to wells and springs.
• Sand and gravel deposits, sandstone, limestone,
and fractured, crystalline rocks are examples of
geological units that form aquifers.

32. Aquifers provide two important functions:
i. they transmit ground water from areas of
recharge to areas of discharge, and
ii. they provide a storage medium for useable
quantities of ground water.
• The amount of water a material can hold
depends upon its porosity.
• The size and degree of interconnection of
those openings (permeability) determine the
materials’ ability to transmit fluid.
• Aquifers may be classed as unconfined or
confined, depending on the presence or
absence of a water table, while a leaky aquifer
represents a combination of the two types.

33. Unconfined Aquifer
• An unconfined aquifer is one in which a water
table varies in undulating form and in slope,
depending on areas of recharge and discharge,
pumpage from wells, and permeability.
• Rises and falls in the water table correspond to
changes in the volume of water in storage within
an aquifer.
• Contour maps and profiles of the water table can
be prepared from elevations of water in wells
that tap the aquifer to determine the quantities of
water available and their distribution and
movement

34. • A special case of an unconfined aquifer involves
perched water bodies.
• This occurs wherever a groundwater body is
separated from the main groundwater by a
relatively impermeable stratum of small areal
extent and by the zone of aeration above the
main body of groundwater.
• Clay lenses in sedimentary deposits often have
shallow perched water bodies overlying them.
• Wells tapping these sources yield only
temporary or small quantities of water.

35. Confined Aquifers
• Confined aquifers, also known as artesian or
pressure aquifers, occur where groundwater is
confined under pressure greater than
atmospheric by overlying relatively impermeable
strata.
• In a well penetrating such an aquifer, the water
level will rise above the bottom of the confining
bed.
• Water enters a confined aquifer in an area
where the confining bed rises to the surface;
where the confining bed ends underground, the
aquifer becomes unconfined.
• A region supplying water to a confined area is
known as a recharge area

36. Aquitard
• An aquitard is a partly permeable geologic
formation.
• It transmits water at such a slow rate that the
yield is insufficient.
• Pumping by wells is not possible.
For example, sand lenses in a clay formation will
form an aquitard.

37. Aquiclude
• An aquiclude is composed of rock or
sediment that acts as a barrier to
groundwater flow.
• Aquicludes are made up of low porosity
and low permeability rock/sediment such
as shale or clay.
• Aquicludes have normally good storage
capacity but low transmitting capacity.

38. Aquifuge
• An aquifuge is a geologic formation which
doesn’t have interconnected pores.
• It is neither porous nor permeable. Thus, it
can neither store water nor transmit it.
Examples of aquifuge are rocks like basalt,
granite, etc. without fissures.

39. Aquifer characteristics
The following properties of the aquifer are required
for study of groundwater hydrology:
i. Porosity
ii. Specific Yield
iii. Specific Retention
iv. Coefficient of permeability
v. Transmissibility
vi. Specific Storage
vii. Storage Coefficient

40. Porosity (n)
Porosity (n) is the percentage of rock or soil
that is void of material.
It is defined mathematically by the equation:

41. Porosity Cont.
Where,
n is the porosity (percentage)
Vv is the volume of void space in a unit volume of
earth materials (L3
, cm3
or m3
)
V is the unit volume of earth material, including
both voids and solids (L3
, cm3
or m3
)

42. Specific Yield (Sy)
Specific yield (Sy) is the ratio of the volume of
water that drains from a saturated rock owing to
the attraction of gravity (or by pumping from
wells) to the total volume of the saturated
aquifer.
It is defined mathematically by the equation:

43. Where,
Vw = volume of water in a unit volume of earth
materials (L3
, cm3
or m3
)
V = volume of earth material, including both
voids and solids (L3
, cm3
or m3
)
All the water stored in a water bearing stratum
cannot be drained out by gravity or by pumping,
because a portion of the water is rigidly held in
the voids of the aquifer by molecular and surface
tension forces

44. Specific Retention (Sr)
Specific retention (Sr) is the ratio of the volume of
water that cannot be drained out to the total
volume of the saturated aquifer.
Since the specific yield represents the volume of
water that a rock will yield by gravity drainage,
hence the specific retention is the remainder.
The sum of the two equals porosity.