Learning Objectives

 Provide a clear definition of the hydrologic cycle.

 Recognize and describe the key processes that are part of the hydrologic cycle.
 Develop an understanding of the significance of the hydrologic cycle.
 Explore the fundamental aspects of water and its connections to various scientific
disciplines.
 Distinguish various environmental challenges associated with the hydraulic cycle.

Overview

Water covers 71% of the Earth’s surface. Approximately 97% of the earth’s water resides
in the oceans while the remaining 3% is
freshwater 1. Of the freshwater, 68% is frozen in
glaciers and icebergs, while 30% is groundwater.
The remaining fraction of the freshwater is found
in rivers, lakes, and other sources. The same
water has been circulating around the earth for
the past 4 billion years! Yes, you read that right!
Possibilities are that the water that you drink
today or take a shower with might have been
recycled from the one that was consumed by the
Fig. 1 Water distribution on Earth
Tyrannosaurus of the Cretaceous period or Julius (image source: BYJU’S)
Caesar of the 100th BC. The gist is that earth’s
water has been moving on, above and below its surface in an ongoing cycle called the water
cycle.

Discussion

Hydrology
Hydrology is the study of water on and beneath the earth’s surface, with regards to its
occurrence, distribution, movement and properties as well as its relationship with the
environment within each phase of the water cycle.

What is Hydrologic Cycle?

The hydrologic cycle involves the continuous circulation of water in the Earth-Atmosphere
system. At its core, the water cycle is the motion of the water from the ground to the
atmosphere and back again. This gigantic system, powered by energy from the Sun, is a

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continuous exchange of moisture between the oceans, the atmosphere, and the land. The
hydrologic cycle is a complex and dynamic system that involves many interactions and
feedbacks among different components of the Earth system. It is influenced by various
factors, such as temperature, pressure, humidity, topography, vegetation, land use, and human
activities.

Fig. 2 Diagram of the hydraulic cycle

(image source: NASA.gov)

Components of the Hydrologic Cycle

The hydrologic cycle is composed of two phases, the first of which is the
atmospheric phase, which describes water movement as gas (water vapor) and liquid/solid
(rain and snow) in the atmosphere. The second phase is the terrestrial phase, which
describes water movement in, over, and through the Earth. The terrestrial phase is often
broken down into the surface water phase (runoff, streamflow) and the groundwater phase
(infiltration, percolation, aquifer recharge).

Hydrologic Cycle process

Evaporation: Evaporation is the process where water is changed from liquid form to water
vapor. It is the transfer of water from the surface of the Earth to the atmosphere. In the water
cycle, evaporation occurs when sunlight warms the surface of the water. The water
molecules move faster and faster, until they escape as water vapor. Studies have indicated
that seas, lakes, rivers, and oceans provide about 90 percent of the vapor in the atmosphere
through evaporation. The rest is provided by plant transpiration.
Transpiration: Transpiration involves the movement of moisture through green plants from
their roots to small openings underneath the leaves, where it is changed to vapor and then
released to the atmosphere. Transpiration is one of the major factors that contributes to the
water cycle by returning water vapor back into the atmosphere.
Evapotranspiration: is the combined components of
evaporation and transpiration, and is sometimes used to

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evaluate the movement of water in the atmosphere. Evapotranspiration (evaporation +
transpiration) Evapotranspiration is the combined effect of evaporation (movement of water
directly into the atmosphere as water vapor from a surface, such as the soil or a water body)
and transpiration (the process by which plants carry water from the soil into leaves, where it
is released to the atmosphere as water vapor). Due to the difficulty in separating the processes
of evaporation and transpiration, the two processes are generally considered together and
referred to as “ET.” This term includes the water that evaporates directly from soil, water, and
plant surfaces and the water that is pulled from the soil by plant transpiration.
Condensation: Condensation is the process where water vapor becomes liquid. The water
vapour evaporates from the water bodies like ocean, sea and river. These vapors after
reaching a height around 20km in the sky undergo condensation and forms clouds. These
later precipitate as rain, fog etc.
Condensation occurs when water vapor in the air turns back into liquid water. This can
happen in two ways:
 When the air is cooled to its dew point, which is the temperature at which
condensation happens.
 When the air becomes saturated with water vapor, which means it cannot hold any
more water.
Precipitation: Precipitation is water released from clouds in the form of rain, freezing rain,
sleet, snow, or hail. Precipitation is the main way atmospheric water returns to the surface of
the Earth, it forms in the clouds when water vapor condenses into bigger and bigger droplets
of water. When the drops are heavy enough, they fall to the earth. The amounts and types of
precipitation affect soil development, vegetation growth, and the generation of runoff, which
transports soils, nutrients, and pollutants. As rain falls from the atmosphere, some is caught
by vegetation (trees, grass, crops), and this is called “interception.” A portion of intercepted
rainfall is evaporated back to the atmosphere from the plant surfaces and never reaches the
ground. The rainfall that does reach the soil surface is referred to as net rainfall. Precipitation
that falls as liquid usually ends up as surface flow and stream flow. Surface runoff is
precipitation which travels over the soil surface to the nearest stream channel. Stream flow is
the movement of water in a natural channel, such as a river.

Fig 4. Different forms of precipitation

(image source: The Constructor.org)

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Types of Precipitation:
 Cyclonic Precipitation
A cyclone is a region in the atmosphere with large low pressure having circular wind motion.
The cyclonic precipitation is caused by the movement of moist air mass to this region due to
the difference in pressure.
 Convective Precipitation
The air above the land area gets heated up by some cause. The most warmer air rises up and
cools and precipitates. Convective precipitation is showery in nature. This type of
precipitation happens in varying intensities.
 Orographic Precipitation
Moving air masses have chances to strike barriers like mountains. Once they strike, they rise
up which causes condensation and precipitation. The precipitation is greater in the windward
side of the barrier compared to the leeward side of the barrier.
Infiltration: Infiltration is the downward entry
of water into the soil. It refers to the process
where precipitation or water infuses into
subsurface soils, is absorbed by the soil and
travels deeper through pore spaces and cracks
into rocks. Water is often soaked up by the soil
and can stay there for some time until it gets
evaporated. In an area with a lot of vegetation,
the infiltrated water can get absorbed by a plant
root and later transpired. Once precipitation has
reached the soil surface, some of it can infiltrate
the soil. The amount of water that infiltrates and Fig 5. Infiltration of precipitation in soil
how quickly it infiltrates varies widely from place (image credit: SlideShare)
to place and depends on soil properties such as
soil moisture content, texture, bulk density, organic matter content, permeability, porosity,
and the presence of any restrictive layers in the soil. Permeability is a measure of how fast
water flows through the soil.
Percolation and Groundwater Recharge: Aside from plant uptake, another path that water
can take after it enters the soil is percolation. Percolation is the downward movement of water
that has infiltrated out of the root zone under the pull of gravity. Generally, percolation is
beyond the reach of plant roots. Water that percolates downward through the soil, below the
plant root zone toward the underlying geologic formation, is responsible for recharging
aquifers.
Runoff: Runoff is the movement of water over the surface of land, or in some cases through
it. The water moves downhill due to gravity, through channels as small as a brook and as
large as a river. From here, the water will enter a pond, lake, or ocean. When the soil is
saturated and can no longer hold water, the excess water forms channels above ground that
follow a downhill gradient to reach a creek, stream, pond, or other waterbody.

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  Surface Runoff:
When the water after precipitation merges with the stream then it is known as surface runoff.
It occurs when all loss is satisfied, rainfall is continued and the rate of rainfall (intensity) is
greater than the infiltration rate.
 Subsurface Runoff:
When the part of precipitation after leaching into the soil moves laterally into any drain,
stream, or river is known as subsurface runoff. It is usually referred to as inter-flow.
 Base Runoff:
When the water after precipitation infiltrated into the soil and merges with the water table and
flows into the stream then it is known as base runoff. The movement of water in this is very
slow. Therefore, it is also referred to as delayed runoff.
Process of Hydrological Cycle
The Process of the hydrological cycle starts with oceans and other bodies of water,
even water from leaves of plats and soil moisture. Water from these sources, gets evaporated
due to heat energy provided by solar radiation and forms water vapor. This water vapor
moves upwards to higher altitudes forming clouds.
Most of the clouds condense and precipitate in any form like rain, hail, snow, sleet.
And a part of clouds is driven to land by winds. Even during the process of precipitation,
some parts of water molecules may evaporate back to atmosphere.
The Portion of water that reaches the ground, enters the earth’s surface infiltrating
various strata of soil. This process enhances the moisture content as well as the water table.
Vegetation sends a portion of water from the earth’s surface back to the atmosphere
through the process of transpiration. Once water percolates and infiltrates the earth’s surface,
runoff is formed over the land, flowing through the contours of land heading towards river
and lakes and finally joins into oceans after many years. Some amount of water is retained as
depression storage.
Further again the process of this hydrological cycle continues when the sun heats the
earth and by blowing of cool air over the ocean, carrying water molecules, forming into water
vapor then clouds getting condensed and precipitates as rainfall. Similarly, the water
percolates into the soil, thus increasing the water table and also the formation of runoff waters
heading towards water bodies. Thus, the cyclic process continues.

What is the importance of Hydrologic cycle?

The hydrologic is a foundational process that sustains life, shapes the environment, and
influences the planet's overall health. Its interconnectedness with various Earth systems
underscores its significance in maintaining the delicate balance of ecosystems and the well-
being of all living organisms. It is important because it is how water reaches plants, animals
and us! it also moves things like nutrients, pathogens and sediment in and out of aquatic

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ecosystems and it enables the availability of water for all living organisms and regulates
weather patterns on our planet.
The role of Hydrological cycle is performed in two ways : (1) Physical role and (2)
Biological role.
(1) Physical Role:
(i) The maintenance of balance in exchange of waters between the ground and the sea.
Discharge and Recharge of waters: A good amount of waters through rivers and canals or
through the evaporation of waters on the ground surface and through carbon synthesis is
emitted. Most of waters return to the seas in the form of surface water and the tunnel water
and the rest gets evaporated and driven by air returns to the ocean. This process is known as
‘Discharge’. If this discharge of water had happened on one side only, The waters of canals
and rivers along with the ground water would have dried up. As a result, the plant world
would have been extinct and the land would have turned into a desert and the vast expanse of
the land would have been inundated with the waters of the sea. But it never happens in
reality. The waters of the seas evaporate and rise up and come towards the land driven by
wind. Then through condensation, fall by drops to the ground. Then, the canals, tunnels and
rivers become full of water and the ground water increases. This is called Recharge. If there
were no chance of evaporation, the land would have been flooded gradually. But, as the ratio
of discharge and the recharge is almost the same, there is hardly any possibility of inundation
or the aridity of the land. Thus, the discharge and the recharge through mutual exchange of
waters maintain the balance of the material world.
(ii) Transfer of water in between places in land: Through the amount of evaporation may be
less or more in some places on the surface of the land, the water vapour is transferred by the
wind. It may happen that where there is more evaporation, the precipitation is less and where
there is less evaporation, the precipitation is more. But as there is no hindrance caused by
mountains and hills and no arrival of water-vapours from the Seas, it does not rain. Again
because of the dry climate of the low river beds on account of their long journey, and want of
sufficient rainfall, the deficit of waters is compensated by rainfall, snow-fall and ground
water. Thus, through the transfer of water and water-vapour, the role of Hydrological Cycle is
efficiently performed.
(iii) Role of Hydrological Cycle in landscape evolution: The Hydrological cycle plays the
principal role in the changes that take place in the world. Rivers are responsible for the
change of the surface of the ground. The source of river water is the Hydrological Cycle. The
surface of the land is influenced by ground water and the role of Hydrological Cycle is very
important in this as well as in the change of the underground for if there were no recharge,
there would have been no ground water.
(2) Biological Role: The biological role in the Hydrological Cycle in preserving the material
world is very important. This may happen in many ways:
(i) Water-supply to the living world: The other name of water is life. Life first appeared in the
water (in the seas, 350 crore years ago). For want of water physical world is affected and so
life is not possible without water. The ground water (tubewells and wells), the Sweet water of
the rivers, and lakes are the source of drinking water.

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(ii) Growth of Plant Kingdom: No growth or birth of plants is possible without water. The
excessive amount of water creates wood-land in the moist areas. Even in dry climate, beside
the river-beds or where there is the existence of ground water in the neighbourhood oasis is
created. Plants are the refuge of the animal world.
Plants inhale the water-vapours through carbon synthesis and add water vapour to the
atmosphere and rain takes place.
(iii) Cultivation: By Hydrological process, the soil gets wet – and irrigation works flourish
and as a result crops grow in plenty.
(iv) Maintaining Aquatic Eco-system: It is possible to maintain aquatic ecosystem on the land
on account of the supply of water in the hydrological cycle. Fishes, Snakes, Frogs,
Crocodiles, and other aquatic animals and water – plants like marshes, grasses, water lotuses
etc. constitute aquatic eco-system.
(v) Growth of civilization and conservation: No animal or plant can live without water,
Because of the convenience offered by water in the river – beds, the civilization of mankind
centering round cultivation has become possible. In recent times, the system of cultivation
has been modernized, industries and technology have developed. As long as the advantage of
hydrological cycle is available, human civilization will be progressing leaps and bounds.

Environmental Problems
One of the most fundamental properties of water is that it is neither created nor destroyed.
That is, there is the same amount of water on Earth today as there was millions of years ago ;
water just changes phases — from liquid to solid to gas. However, while the mass of water is
conserved, water quality is not, and degradation of water quality effectively reduces
availability of accessible waters for domestic, industrial, or agricultural uses.

 Water scarcity- Water scarcity is the lack of

sufficient available water resources to meet
the demands of water usage within a region.
Water scarcity is a global problem that
threatens the health and development of
communities around the globe. Billions of
people around the world lack adequate access
to one of the essential elements of life: clean
water. This imbalance is caused by water
pollution, pressure from water-intensive
agriculture, population pressures, and climate Fig. 8 effect of Water Scarcity
(image credit: Telegdraf.id)
change effects on water sources. Water
scarcity affects the health of the population,
power generation, eco-systems, as well as other aspects of life.

 Water pollution- Water pollution is the release of substances into subsurface

groundwater or into lakes, streams, rivers, estuaries, and oceans to the point where the

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 substances interfere with beneficial use of the water or with the natural functioning of
ecosystems.
Two types of pollution impact water quality: (1) point source pollution, which is direct
discharge into the environment such as from wastewater treatment plants; and (2) nonpoint
source pollution, which is composed of diffuse inputs such as urban stormwater or
agricultural runoff.
Other causes of water pollution:
Domestic sewage
Domestic sewage are used water from houses and
apartments, mostly coming from the kitchen, bathroom,
and laundry sources. It is the primary source of pathogens
(disease-causing microorganisms) and putrescible organic
substances. Because pathogens are excreted in feces, all
sewage from cities and towns is likely to contain pathogens Fig. 9 Sewage
of some type, potentially presenting a direct threat to public (image source: Brittanica)
health.
Solid waste
The improper disposal of solid waste is a major source of
water pollution. Solid waste includes garbage,
rubbish, electronic waste, trash, and construction and
demolition waste, all of which are generated by individual,
residential, commercial, institutional, and industrial
activities.
Toxic waste
Waste is considered toxic if it is poisonous, radioactive, Fig. 10 Toxic waste
explosive, carcinogenic (causing cancer), mutagenic (causing (image source: Clark University)
damage to chromosomes), teratogenic (causing birth defects),
or bioaccumulative (that is, increasing in concentration at the higher ends of food chains).
Sources of toxic chemicals include improperly disposed wastewater from industrial plants
and chemical process facilities (lead, mercury, chromium) as well as surface runoff
containing pesticides used on agricultural areas and suburban lawns (chlordane, dieldrin,
heptachlor).
Effects of water pollution on groundwater and oceans aquifer:
Groundwater—water contained in underground geologic formations called aquifers—is a
source of drinking water for many people. Although groundwater may appear crystal clear
(due to the natural filtration that occurs as it flows slowly through layers of soil), it may still
be polluted by dissolved chemicals and by bacteria and viruses. In coastal areas, increasing
withdrawal of groundwater (due to urbanization and industrialization) can cause saltwater
intrusion: as the water table drops, seawater is drawn into wells.
Although estuaries and oceans contain vast volumes of water, their natural capacity to absorb
pollutants is limited. Contamination from sewage outfall pipes, from dumping of sludge or
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other wastes, and from oil spills can harm marine life, especially microscopic phytoplankton
that serve as food for larger aquatic organisms. Sometimes, unsightly and dangerous waste
materials can be washed back to shore, littering beaches with hazardous debris. By 2010, an
estimated 4.8 million and 12.7 million tonnes (between 5.3 million and 14 million tons) of
plastic debris had been dumped into the oceans annually, and floating plastic waste had
accumulated in Earth’s five subtropical gyres that cover 40 percent of the world’s oceans (see
also plastic pollution).
Another ocean pollution problem is the seasonal formation of “dead zones” (i.e., hypoxic
areas, where dissolved oxygen levels drop so low that most higher forms of aquatic life
vanish) in certain coastal areas. The cause is nutrient enrichment from dispersed agricultural
runoff and concomitant algal blooms. Dead zones occur worldwide; one of the largest of
these (sometimes as large as 22,730 square km [8,776 square miles]) forms annually in the
Gulf of Mexico, beginning at the Mississippi River delta.

 Floods- A flood happens when water overflows or

soaks land that is normally dry. A flood can develop
in a many ways. The most common is
when rivers or streams overflow their banks. These
floods are called riverine floods. Heavy rain, a
broken dam or levee, rapid ice melt in the
mountains, or even a beaver dam in
a vulnerable spot can overwhelm a river and send it
spreading over nearby land. The land surrounding
Fig. 11 Aereal shot of the aftermath of typhoon
a river is called a flood plain. Coastal flooding, Ursula in Balasan, Iloilo
also called estuarine flooding, happens when a (image credit: projectlupad.com)
large storm or tsunami causes the sea to rush
inland.
Artificial Causes of Floods:
Floods can also have artificial sources. Many man-made floods are intentional and controlled.
 Rice farmers, for instance, rely on flooded fields. Rice is a semi-aquatic crop—it grows in
water. After rice seedlings are planted, farmers flood their fields, called rice paddies.

 Sometimes, engineers flood an area to restore an ecosystem.

Example: In 2008, the U.S.'s Grand Canyon was deliberately flooded. Water was released
from dams on the Colorado River, which runs through the Grand Canyon. In 20 minutes,
enough water was released from a dam at Lake Powell, Utah, to fill up the Empire State
Building. Hydrologists, engineers, and environmentalists hoped that flooding the canyon
would help redistribute sediment—which had been blocked up by dams—and create
sandbars. Sandbars provide a wildlife habitat, often serving as a shallow bridge for animals
such as beavers and bighorn sheep to cross from one side of the river to the other.

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 Dams control the natural flood plains of lakes and rivers. Hydrologists may intentionally
flood areas to prevent damage to the dam or increase the water supply for agriculture,
industry, or consumer use.
 Engineers may also intentionally flood areas to prevent the possibility of worse flooding.
Example: When heavy rains caused the Souris River to flood in 2011, for example, the water
level nearly reached the top of the Alameda Reservoir in Oxbow, Saskatchewan, Canada.
Faced with the prospect of catastrophic flooding if the entire dam broke, engineers chose to
release huge amounts of water. The reservoir remained intact, but the release contributed to
massive floods in both Saskatchewan and the U.S. city of Minot, North Dakota.
Effects of Floods:
When floodwaters recede, affected areas are often blanketed in silt and mud.
This sediment can be full of nutrients, benefiting farmers and agribusinesses in the area.
However, floods have enormous destructive power. When a river overflows its banks or the
sea moves inland, many structures are unable to withstand the force of the water. Bridges,
houses, trees, and cars can be picked up and carried off. Floods erode soil, taking it from
under a building's foundation, causing the building to crack and tumble.
Floods can cause even more damage when their waters recede. The water and landscape can
be contaminated with hazardous materials, such as sharp debris, pesticides, fuel, and
untreated sewage.
As flood water spreads, it carries disease. Flood victims can be left for weeks without clean
water for drinking or hygiene. This can lead to outbreaks of deadly diseases
like typhoid, malaria, hepatitis A, and cholera.

Aral Sea Disaster

The Aral Sea is located between Kazakhstan and Uzbekistan and was once the fourth largest
lake in the world
but since the 1960s,
it has been

catastrophically shrinking.

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 Fig. 11 The gradual shrinking of Aral Sea through the years
(image credit: envrionego.com)

 Reason: In the 1940s, the

European USSR was going through a widespread drought and famine, and as a result,
Stalin launched what is known as the Great Plan for the Transformation of Nature. Its
purpose was to improve the overall agriculture of the country. The Soviet Union turned
lands of the Uzbek SSR into cotton plantations—which operated on a system of forced
labor—and ordered the construction of irrigation canals to provide water to the crops in
the middle of the plateau of the region. These hand-dug, irrigation canals moved water
from the Anu Darya and Syr Darya rivers, the same rivers that fed the freshwater into the
Aral Sea.
 Effect: Prior to the evaporation of the lake, the
Aral Sea produced about 20,000 to 40,000 tons
of fish a year. This was reduced to a low of 1,000
tons of fish a year at the height of the crisis. And
today, instead of supplying food to the region,
the shores have become ship graveyards, a
curiosity for occasional travelers. Besides the sad
fact that the Aral Sea has been disappearing, its
huge, dried-up lakebed is also a source of
disease-causing dust that blows throughout the
region. The dried remnants of the lake contain
not only salt and minerals but also pesticides Fig. 12 The Aral Sea today
(image source: Sogda-tour.com)
like DDT that were once used in huge quantities
by the Soviet Union (ironically, to make up for the lack of water). Additionally, the USSR
once had a biological-weapons testing facility on one of the lakes within the Aral Sea.
Although now closed, the chemicals used at the facility help to make the destruction of
the Aral Sea one of the great environmental catastrophes of human history.

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