The water cycle describes how water is exchanged (cycled) through Earth's land,

ocean, and atmosphere. Water always exists in all three places, and in many forms—
as lakes and rivers, glaciers and ice sheets, oceans and seas, underground aquifers,
and vapor in the air and clouds.

Evaporation, Condensation, and Precipitation

The water cycle consists of three major processes: evaporation, condensation,

and precipitation.

Evaporation
Evaporation is the process of a liquid's surface changing to a gas. In the water cycle,
liquid water (in the ocean, lakes, or rivers) evaporates and becomes water vapor.

Water vapor surrounds us, as an important part of the air we breathe. Water vapor is
also an important greenhouse gas. Greenhouse gases such as water vapor and carbon
dioxide insulate the Earth and keep the planet warm enough to maintain life as we know
it.

The water cycle's evaporation process is driven by the sun. As the sun interacts with
liquid water on the surface of the ocean, the water becomes an invisible gas (water
vapor). Evaporation is also influenced by wind, temperature, and the density of the body
of water.

Condensation
Condensation is the process of a gas changing to a liquid. In the water cycle, water
vapor in the atmosphere condenses and becomes liquid.

Condensation can happen high in the atmosphere or at ground level. Clouds form as
water vapor condenses, or becomes more concentrated (dense). Water vapor
condenses around tiny particles called cloud condensation nuclei (CCN). CCN can be
specks of dust, salt, or pollutants. Clouds at ground level are called fog or mist.

Like evaporation, condensation is also influenced by the sun. As water vapor cools, it
reaches its saturation limit, or dew point. Air pressure is also an important influence on
the dew point of an area.

Precipitation
Unlike evaporation and condensation, precipitation is not a process. Precipitation
describes any liquid or solid water that falls to Earth as a result of condensation in the
atmosphere. Precipitation includes rain, snow, and hail.

Fog is not precipitation. The water in fog does not actually precipitate, or liquify and fall
to Earth. Fog and mist are a part of the water cycle called suspensions: They are liquid
water suspended in the atmosphere.
Precipitation is one of many ways water is cycled from the atmosphere to the Earth or
ocean.

Other Processes
Evaporation, condensation, and precipitation are important parts of the water cycle.
However, they are not the only ones.

Runoff, for instance, describes a variety of ways liquid water moves across
land. Snowmelt, for example, is an important type of runoff produced as snow or
glaciers melt and form streams or pools.

Transpiration is another important part of the water cycle. Transpiration is the process of
water vapor being released from plants and soil. Plants release water vapor
through microscopic pores called stomata. The opening of stomata is strongly
influenced by light, and so is often associated with the sun and the process of
evaporation. Evapotranspiration is the combined components of evaporation and
transpiration, and is sometimes used to evaluate the movement of water in the
atmosphere.

States of Water

Through the water cycle, water continually circulates through three states: solid, liquid,
and vapor.

Ice is solid water. Most of Earth's freshwater is ice, locked in massive glaciers, ice
sheets, and ice caps.

As ice melts, it turns to liquid. The ocean, lakes, rivers, and underground aquifers all
hold liquid water.

Water vapor is an invisible gas. Water vapor is not evenly distributed across the
atmosphere. Above the ocean, water vapor is much more abundant, making up as
much as 4% of the air. Above isolated deserts, it can be less than 1%.

The Water Cycle and Climate

The water cycle has a dramatic influence on Earth's climate and ecosystems.

Climate is all the weather conditions of an area, evaluated over a period of time. Two
weather conditions that contribute to climate include humidity and temperature. These
weather conditions are influenced by the water cycle.

Humidity is simply the amount of water vapor in the air. As water vapor is not evenly
distributed by the water cycle, some regions experience higher humidity than others.
This contributes to radically different climates. Islands or coastal regions, where water
vapor makes up more of the atmosphere, are usually much more humid than inland
regions, where water vapor is scarcer.

A region's temperature also relies on the water cycle. Through the water cycle, heat is
exchanged and temperatures fluctuate. As water evaporates, for example, it absorbs
energy and cools the local environment. As water condenses, it releases energy and
warms the local environment.

The Water Cycle and the Landscape

The water cycle also influences the physical geography of the Earth. Glacial melt
and erosion caused by water are two of the ways the water cycle helps create Earth's
physical features.

As glaciers slowly expand across a landscape, they can carve away entire valleys,
create mountain peaks, and leave behind rubble as big as boulders. Yosemite Valley,
part of Yosemite National Park in the U.S. state of California, is a glacial valley. The
famous Matterhorn, a peak on the Alps between Switzerland and Italy, was carved as
glaciers collided and squeezed up the earth between them. Canada's "Big Rock" is one
of the world's largest "glacial erratics," boulders left behind as a glacier advances or
retreats.

Glacial melt can also create landforms. The Great Lakes, for example, are part of the
landscape of the Midwest of the United States and Canada. The Great Lakes were
created as an enormous ice sheet melted and retreated, leaving liquid pools.

The process of erosion and the movement of runoff also create varied landscapes
across the Earth's surface. Erosion is the process by which earth is worn away by liquid
water, wind, or ice.

Erosion can include the movement of runoff. The flow of water can help carve
enormous canyons, for example. These canyons can be carved by rivers on
high plateaus (such as the Grand Canyon, on the Colorado Plateau in the U.S. state of
Arizona). They can also be carved by currents deep in the ocean (such as the Monterey
Canyon, in the Pacific Ocean off the coast of the U.S. state of California).

Reservoirs and Residence Time

Reservoirs are simply where water exists at any point in the water cycle. An
underground aquifer can store liquid water, for example. The ocean is a reservoir. Ice
sheets are reservoirs. The atmosphere itself is a reservoir of water vapor.

Residence time is the amount of time a water molecule spends in one reservoir. For
instance, the residence time of "fossil water," ancient groundwater reservoirs, can be
thousands of years. Some fossil water reservoirs beneath the Sahara Desert have
existed for 75,000 years.

Residence time for water in the Antarctic ice sheet is about 20,000 years. That means
that a molecule of water will stay as ice for about that amount of time.

The residence time for water in the ocean is much shorter—about 3,200 years.

The residence time of water in the atmosphere is the shortest of all—about nine days.

Calculating residence time can be an important tool for developers and engineers.
Engineers may consult a reservoir's residence time when evaluating how quickly a
pollutant will spread through the reservoir, for instance. Residence time may also
influence how communities use an aquifer.

Rain is part of the water cycle.

Photograph by Issa AlKindy, My Shot
Breaking the Cycle
The water cycle can change. Glacial retreat is the process in which glaciers melt faster
than their ice can be replaced by precipitation. Glacial retreat limits the amount of fresh
water available on Earth. We are experiencing the fastest rate of glacial retreat in
recorded history.
Summary of the Water Cycle

New!

Water cycle for kids poster.

What is the water cycle?

What is the water cycle? I can easily answer that—it is "me" all
over! The water cycle describes the existence and movement of water on, in,
and above the Earth. Earth's water is always in movement and is always
changing states, from liquid to vapor to ice and back again. The water cycle
has been working for billions of years and all life on Earth depends on it
continuing to work; the Earth would be a pretty stale place without it.

Where does all the Earth's water come from? Primordial Earth was an
incandescent globe made of magma, but all magmas contain water. Water
set free by magma began to cool down the Earth's atmosphere, and
eventually the environment became cool enough so water could stay on the
surface as a liquid. Volcanic activity kept and still keeps introducing water
into the atmosphere, thus increasing the surface-water and groundwater
volume of the Earth.

A quick summary of the water cycle

Here is a quick summary of the water cycle. The links in this paragraph go to
the detailed Web pages in our Web site for each topic. A shorter summary of
each topic can be found further down in this page, though.
The water cycle has no starting point, but we'll begin in the oceans, since
that is where most of Earth's water exists. The sun, which drives the water
cycle, heats water in the oceans. Some of it evaporates as vapor into the
air; a relatively smaller amount of moisture is added as ice and
snow sublimate directly from the solid state into vapor. Rising air currents
take the vapor up into the atmosphere, along with water
from evapotranspiration, which is water transpired from plants and
evaporated from the soil. The vapor rises into the air where cooler
temperatures cause it to condense into clouds.

Air currents move clouds around the globe, and cloud particles collide, grow,
and fall out of the sky as precipitation. Some precipitation falls as snow and
can accumulate as ice caps and glaciers, which can store frozen water for
thousands of years. Snowpacks in warmer climates often thaw and melt
when spring arrives, and the melted water flows overland as snowmelt. Most
precipitation falls back into the oceans or onto land, where, due to gravity,
the precipitation flows over the ground as surface runoff. A portion of runoff
enters rivers in valleys in the landscape, with streamflow moving water
towards the oceans. Runoff, and groundwater seepage, accumulate and
are stored as freshwater in lakes.

Not all runoff flows into rivers, though. Much of it soaks into the ground
as infiltration. Some of the water infiltrates into the ground and
replenishes aquifers (saturated subsurface rock), which store huge amounts
of freshwater for long periods of time. Some infiltration stays close to the
land surface and can seep back into surface-water bodies (and the ocean)
as groundwater discharge, and some groundwater finds openings in the land
surface and emerges as freshwater springs. Yet more groundwater is
absorbed by plant roots to end up as evapotranspiration from the leaves.
Over time, though, all of this water keeps moving, some to reenter the
ocean, where the water cycle "ends" ... oops - I mean, where it "begins."

Main components of the water cycle

 Water storage in oceans

 Evaporation
 Sublimation
 Evapotranspiration
 Water in the atmosphere
 Condensation
 Precipitation
 Water storage in ice and snow

 Snowmelt runoff to streams

 Surface runoff
 Streamflow
 Freshwater storage
 Infiltration
 Groundwater storage
 Groundwater discharge
 Springs

Global water distribution

Also, find out how much water exists on (and in) the Earth and where it is
located.

Water storage in oceans: Saline water existing in oceans and inland seas

The ocean as a storehouse of water

 The water cycle
sounds like it is describing how water moves above, on, and through the
Earth ... and it does. But, in fact, much more water is "in storage" for long
periods of time than is actually moving through the cycle. The storehouses
for the vast majority of all water on Earth are the oceans. It is estimated
that of the 332,600,000 cubic miles (mi3) (1,386,000,000 cubic kilometers
(km3)) of the world's water supply, about 321,000,000 mi3 (1,338,000,000
km3) is stored in oceans. That is about 96.5 percent. It is also estimated
that the oceans supply about 90 percent of the evaporated water that goes
into the water cycle.

During colder climatic periods more ice caps and glaciers form, and enough
of the global water supply accumulates as ice to lessen the amounts in other
parts of the water cycle. The reverse is true during warm periods. During the
last ice age glaciers covered almost one-third of Earth's land mass, with the
result being that the oceans were about 400 feet (122 meters) lower than
today. During the last global "warm spell," about 125,000 years ago, the
seas were about 18 feet (5.5. meters) higher than they are now. About
three million years ago the oceans could have been up to 165 feet (50
meters) higher.

Oceans in movement

If you have ever been seasick (we hope not), then you know how the ocean
is never still. You might think that the water in the oceans moves around
because of waves, which are driven by winds. But, actually, there are
currents and "rivers" in the oceans that move massive amounts of water
around the world. These movements have a great deal of influence on the
water cycle. The Kuroshio Current, off the shores of Japan, is the largest
current. It can travel between 25 and 75 miles (40 and 121 kilometers) a
day, 1-3 miles (1.4-4.8 kilometers) per hour, and extends some 3,300 feet
(1,000 meters) deep. The Gulf Stream is a well known stream of warm
water in the Atlantic Ocean, moving water from the Gulf of Mexico across the
Atlantic Ocean towards Great Britain. At a speed of 60 miles (97 kilometers)
per day, the Gulf stream moves 100 times as much water as all the rivers on
Earth. Coming from warm climates, the Gulf Stream moves warmer water to
the North Atlantic.

More about water storage in the oceans.

Evaporation: The process by which water is changed from liquid to a gas or

vapor

Evaporation and why it occurs

Evaporation is the process by which

water changes from a liquid to a gas or vapor. Evaporation is the primary
pathway that water moves from the liquid state back into the water cycle as
atmospheric water vapor. Studies have shown that the oceans, seas, lakes,
and rivers provide nearly 90 percent of the moisture in our atmosphere via
evaporation, with the remaining 10 percent being contributed by
plant transpiration.

Heat (energy) is necessary for evaporation to occur. Energy is used to break

the bonds that hold water molecules together, which is why water easily
evaporates at the boiling point (212° F, 100° C) but evaporates much more
slowly at the freezing point. Net evaporation occurs when the rate of
evaporation exceeds the rate of condensation. A state of saturation exists
when these two process rates are equal, at which point, the relative
humidity of the air is 100 percent. Condensation, the opposite of
evaporation, occurs when saturated air is cooled below the dew point (the
temperature to which air must be cooled at a constant pressure for it to
become fully saturated with water), such as on the outside of a glass of ice
water. In fact, the process of evaporation removes heat from the
environment, which is why water evaporating from your skin cools you.
Evaporation drives the water cycle

Evaporation from the oceans is the primary mechanism supporting the

surface-to-atmosphere portion of the water cycle. After all, the large surface
area of the oceans (over 70 percent of the Earth's surface is covered by the
oceans) provides the opportunity for such large-scale evaporation to occur.
On a global scale, the amount of water evaporating is about the same as the
amount of water delivered to the Earth as precipitation. This does vary
geographically, though. Evaporation is more prevalent over the oceans than
precipitation, while over the land, precipitation routinely exceeds
evaporation. Most of the water that evaporates from the oceans falls back
into the oceans as precipitation. Only about 10 percent of the water
evaporated from the oceans is transported over land and falls as
precipitation. Once evaporated, a water molecule spends about 10 days in
the air.

More about evaporation.

Sublimation: The changing of snow or ice to water vapor without melting

Sublimation describes the process of snow and

ice changing into water vapor without first melting into water. Sublimation is
a common way for snow to disappear in certain climates.

It is not easy to actually see sublimation happen, at least not with ice. One
way to see the results of sublimation is to hang a wet shirt outside on a
below-freezing day. Eventually the ice in the shirt will disappear. Actually,
the best way to visualize sublimation is to not use water at all, but to use
carbon dioxide instead, as this picture shows."Dry ice" is solid, frozen carbon
dioxide, which sublimates, or turns to gas, at the temperature -78.5 °C (-
109.3°F). The fog you see in the picture is a mixture of cold carbon dioxide
gas and cold, humid air, created as the dry ice sublimates.

Sublimation occurs more readily when certain weather conditions are

present, such as low relative humidity and dry winds. It also occurs more at
higher altitudes, where the air pressure is less than at lower altitudes.
Energy, such as strong sunlight, is also needed. If I was to pick one place on
Earth where sublimation happens a lot, I might choose the south side of Mt.
Everest. Low temperatures, strong winds, intense sunlight, very low air
pressure - just what is needed for sublimation to occur.

More about sublimation.

Evapotranspiration: The process by which water vapor is discharged to the

atmosphere as a result of evaporation from the soil and transpiration by
plants.

Although some definitions of evapotranspiration

include evaporation from surface-water bodies, such as lakes and even the
ocean, on this Web site, evapotranspiration is defined as the water lost to
the atmosphere from the ground surface and the transpiration of
groundwater by plants through their leaves.

Transpiration: The release of water from plant leaves

 Transpiration is the process by which
moisture is carried through plants from roots to small pores on the
underside of leaves, where it changes to vapor and is released to the
atmosphere. Transpiration is essentially evaporation of water from plant
leaves. It is estimated that about 10 percent of the moisture found in the
atmosphere is released by plants through transpiration.

Plant transpiration is an invisible process—since the water is evaporating

from the leaf surfaces, you don't just go out and see the leaves "breathing".
During a growing season, a leaf will transpire many times more water than
its own weight. A large oak tree can transpire 40,000 gallons (151,000
liters) per year.

Atmospheric factors affecting transpiration

The amount of water that plants transpire varies greatly geographically and
over time. There are a number of factors that determine transpiration rates:

 Temperature:Transpiration rates go up as the temperature goes up,

especially during the growing season, when the air is warmer.
 Relative humidity: As the relative humidity of the air surrounding the
plant rises the transpiration rate falls. It is easier for water to
evaporate into dryer air than into more saturated air.
 Wind and air movement: Increased movement of the air around a
plant will result in a higher transpiration rate.
 Soil-moisture availability: When moisture is lacking, plants can
begin to senesce (premature ageing, which can result in leaf loss) and
transpire less water.
 Type of plant: Plants transpire water at different rates. Some plants
which grow in arid regions, such as cacti and succulents, conserve
precious water by transpiring less water than other plants.
 More about evapotranspiration.

Water storage in the atmosphere: Water stored in the

atmosphere as vapor, such as clouds and humidity

The atmosphere is full of water

The water cycle is all about storing water and

moving water on, in, and above the Earth. Although the atmosphere may
not be a great storehouse of water, it is the superhighway used to move
water around the globe. There is always water in the atmosphere. Clouds
are, of course, the most visible manifestation of atmospheric water, but even
clear air contains water—water in particles that are too small to be seen.
One estimate of the volume of water in the atmosphere at any one time is
about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3). That may
sound like a lot, but it is only about 0.001 percent of the total Earth's water
volume. If all of the water in the atmosphere rained down at once, it would
only cover the ground to a depth of 2.5 centimeters, about 1 inch.

More about water stored in the atmosphere.

Condensation: The process by which water is changed from vapor to liquid

Condensation is the process in which water vapor in the air is changed into
liquid water. Condensation is crucial to the water cycle because it is
responsible for the formation of clouds. These clouds may produce
precipitation, which is the primary route for water to return to the Earth's
surface within the water cycle. Condensation is the opposite of evaporation.
You don't have to look at something as far away as a cloud to notice
condensation, though. Condensation is responsible for ground-level fog, for
your glasses fogging up when you go from a cold room to the outdoors on a
hot, humid day, for the water that drips off the outside of your glass of iced
tea, and for the water on the inside of your home windows on a cold day.

Condensation in the air

Even though clouds are

absent in a crystal clear blue sky, water is still present in the form of water
vapor and droplets which are too small to be seen. Depending on
meteorological conditions, water molecules will combine with tiny particles of
dust, salt, and smoke in the air to form cloud droplets, which grow and
develop into clouds, a form of water we can see. Cloud droplets can vary
greatly in size, from 10 microns (millionths of a meter) to 1 millimeter
(mm), and even as large as 5 mm. This process occurs higher in the sky
where the air is cooler and more condensation occurs relative to
evaporation. As water droplets combine (also known as coalescence) with
each other, and grow in size, clouds not only develop, but precipitation may
also occur. Precipitation is essentially water cloud in its liquid or solid form
falling form the base of a cloud. This seems to happen too often during
picnics or when large groups of people gather at swimming pools.

You might ask ... why is it colder higher up?

As we said, clouds form in the atmosphere because air containing water

vapor rises and cools. The key to this process is that air near the Earth's
surface is warmed by solar radiation. But, do you know why the atmosphere
cools above the Earth's surface? Generally, air pressure, is the reason. Air
has mass (and, because of gravity on Earth, weight) and at sea level the
weight of a column of air pressing down on your head is about 14 ½ pounds
(6.6 kilograms) per square inch. The pressure (weight), called barometric
pressure, that results is a consequence of the density of the air above. At
higher altitudes, there is less air above, and, thus, less air pressure pressing
down. The barometric pressure is lower, and lower barometric pressure is
associated with fewer molecules per unit volume. Therefore, the air at higher
altitudes is less dense. Since fewer air molecules exist in a certain volume of
air, there are fewer molecules colliding with each other, and as a result,
there will be less heat produced. This means cooler air. Do you find this
confusing? Just think, clouds form all day long without having to understand
any of this.

More about condensation.

Precipitation: The discharge of water, in liquid or solid state, out of the

atmosphere, generally upon a land or water surface

Precipitation is water released from clouds in the form of rain,

freezing rain, sleet, snow, or hail. It is the primary connection in the water
cycle that provides for the delivery of atmospheric water to the Earth. Most
precipitation falls as rain.

How do raindrops form?

The clouds floating overhead

contain water vapor and cloud droplets, which are small drops of condensed
water. These droplets are way too small to fall as precipitation, but they are
large enough to form visible clouds. Water is continually evaporating and
condensing in the sky. If you look closely at a cloud you can see some parts
disappearing (evaporating) while other parts are growing (condensation).
Most of the condensed water in clouds does not fall as precipitation because
their fall speed is not large enough to overcome updrafts which support the
clouds. For precipitation to happen, first tiny water droplets must condense
on even tinier dust, salt, or smoke particles, which act as a nucleus. Water
droplets may grow as a result of additional condensation of water vapor
when the particles collide. If enough collisions occur to produce a droplet
with a fall velocity which exceeds the cloud updraft speed, then it will fall out
of the cloud as precipitation. This is not a trivial task since millions of cloud
droplets are required to produce a single raindrop.

Precipitation rates vary geographically and over time

Precipitation does not fall in the same amounts throughout the world, in a
country, or even in a city. Here in Georgia, USA, it rains fairly evenly all
during the year, around 40-50 inches (102-127 centimeters (cm)) per year.
Summer thunderstorms may deliver an inch or more of rain on one suburb
while leaving another area dry a few miles away. But, the rain amount that
Georgia gets in one month is often more than Las Vegas, Nevada observes
all year. The world's record for average-annual rainfall belongs to Mt.
Waialeale, Hawaii, where it averages about 450 inches (1,140 cm) per year.
A remarkable 642 inches (1,630 cm) was reported there during one twelve-
month period (that's almost 2 inches (5 cm) every day!). Is this the world
record for the most rain in a year? No, that was recorded at Cherrapunji,
India, where it rained 905 inches (2,300 cm) in 1861. Contrast those
excessive precipitation amounts to Arica, Chile, where no rain fell for 14
years

The map below shows average annual precipitation, in millimeters and

inches, for the world. The light green areas can be considered "deserts". You
might expect the Sahara area in Africa to be a desert, but did you think that
much of Greenland and Antarctica are deserts?
On average, the 48 continental United States receives enough precipitation
in one year to cover the land to a depth of 30 inches (0.76 meters).

More about precipitation.

Water storage in ice and snow: Freshwater stored in frozen form, generally in
glaciers, icefields, and snowfields

ice caps around the world

Although the water cycle sounds like it is describing

the movement of water, in fact, much more water is in storage at any one
time than is actually moving through the cycle. By storage, we mean water
that is locked up in its present state for a relatively long period of time, such
as in ice caps and glaciers.

The vast majority, almost 90 percent, of Earth's ice mass is in Antarctica,

while the Greenland ice cap contains 10 percent of the total global ice-mass.
The Greenland ice cap is an interesting part of the water cycle. The ice cap
became so large over time (about 600,000 cubic miles (mi3) or 2.5 million
cubic kilometers (km3)) because more snow fell than melted. Over the
millenia, as the snow got deeper, it compressed and became ice. The ice cap
averages about 5,000 feet (1,500 meters) in thickness, but can be as thick
as 14,000 feet (4,300 meters). The ice is so heavy that the land below it has
been pressed down into the shape of a bowl. In many places, glaciers on
Greenland reach to the sea, and one estimate is that as much as 125
mi3 (517 km3) of ice "calves" into the ocean each year—one of Greenland's
contributions to the global water cycle. Ocean-bound icebergs travel with the
currents, melting along the way. Some icebergs have been seen, in much
smaller form, as far south as the island of Bermuda.

Ice and glaciers come and go

The climate, on a global scale, is always changing, although usually not at a

rate fast enough for people to notice. There have been many warm periods,
such as when the dinosaurs lived (about 100 million years ago) and many
cold periods, such as the last ice age of about 20,000 years ago. During the
last ice age much of the northern hemisphere was covered in ice and
glaciers, and, as this map from the University of Arizona shows, they
covered nearly all of Canada, much of northern Asia and Europe, and
extended well into the United States.

Some glacier and ice cap facts

  Glacial ice covers 10 - 11 percent of all land.
 According to the National Snow and Ice Data Center (NSIDC), if all
glaciers melted today the seas would rise about 230 feet (70 meters).
 During the last ice age (when glaciers covered more land area than
today) the sea level was about 400 feet (122 meters) lower than it is
today. At that time, glaciers covered almost one-third of the land.
 During the last warm spell, 125,000 years ago, the seas were about 18
feet (5.5 meters) higher than they are today. About three million
years ago the seas could have been up to 165 feet (50.3 meters)
higher.

More about water storage in ice and snow.

Snowmelt runoff to streams: The movement of water as surface runoff from

snow and ice to surface water

If you live in Florida or on the

French Riviera you might not wake up everyday wondering how melting
snow contributes to the water cycle. But, in the world-wide scheme of the
water cycle, runoff from snowmelt is a major component of the global
movement of water. In the colder climates much of the
springtime runoff and streamflow in rivers is attributable to melting snow
and ice. The effect of snowmelt on potential flooding, mainly during the
spring, is something that causes concern for many people around the world.
Besides flooding, rapid snowmelt can trigger landslides and debris flows.

Contribution of snowmelt to streamflow

A good way to visualize the contribution of snowmelt to streamflow in rivers

is to look at the hydrograph below, which shows daily mean streamflow
(average streamflow for each day) for four years for the North Fork
American River at North Fork Dam in California (real-time data streamflow).
The large peaks in the chart are mainly the result of melting snow, although
storms can contribute runoff also. Compare the fact that minimum mean-
daily streamflow during March of 2000 was 1,200 cubic feet per second (ft3),
while during August streamflows ranged from 55-75 ft3.

Note that runoff from snowmelt varies not only by season but also by year.
Compare the high peaks of streamflows for the year 2000 with the much
smaller streamflows for 2001. It looks like a major drought hit that area of
California in 2001. The lack of water stored as snowpack in the winter can
affect the availability of water (for streamflow) in streams the rest of the
year. This can have an effect on the amount of water in reservoirs located
downstream, which in turn can affect water available for irrigation and the
water supply for cities and towns.

More about snowmelt runoff to streams.

Surface runoff: Precipitation runoff which travels over the soil surface to the
nearest stream channel

Surface runoff is precipitation runoff over the landscape

Many people probably have an overly-simplified idea that precipitation falls

on the land, flows overland (runoff), and runs into rivers, which then empty
into the oceans. That is "overly simplified" because rivers also gain and lose
water to the ground. Still, it is true that much of the water in rivers comes
directly from runoff from the land surface, which is defined as surface runoff.

When rain hits saturated or impervious

ground it begins to flow overland downhill. It is easy to see if it flows down
your driveway to the curb and into a storm sewer, but it is harder to notice it
flowing overland in a natural setting. During a heavy rain you might notice
small rivulets of water flowing downhill. Water will flow along channels as it
moves into larger creeks, streams, and rivers. This picture gives a graphic
example of how surface runoff (here flowing off a road) enters a small creek.
The runoff in this case is flowing over bare soil and is depositing sediment
into the river (not good for water quality). The runoff entering this creek is
beginning its journey back to the ocean.

As with all aspects of the water cycle, the interaction between precipitation
and surface runoff varies according to time and geography. Similar storms
occurring in the Amazon jungle and in the desert Southwest of the United
States will produce different surface-runoff effects. Surface runoff is affected
by both meteorological factors and the physical geology and topography of
the land. Only about a third of the precipitation that falls over land runs off
into streams and rivers and is returned to the oceans. The other two-thirds
is evaporated, transpired, or soaks into groundwater. Surface runoff can also
be diverted by humans for their own uses.

More about surface runoff.

Streamflow: The movement of water in a natural channel, such as a river

The U.S. Geological Survey (USGS) uses the term "streamflow" to refer to
the amount of water flowing in a river. Although USGS usually uses the term
"stream" when discussing flowing water bodies, in these pages we'll use
"rivers" more often to describe flowing creeks, streams, and rivers, since
that is probably what you are more familiar with.

Importance of rivers

Rivers are invaluable to not

only people, but to life everywhere. Not only are rivers a great place for
people (and their dogs) to play, but people use river water for drinking-
water supplies and irrigation water, to produce electricity, to flush away
wastes (hopefully, but not always, treated wastes), to transport
merchandise, and to obtain food. Rivers are indeed major aquatic
landscapes for all manners of plants and animals. Rivers even help keep the
aquifers underground full of water by discharging water downward through
their streambeds. And, we've already mentioned that the oceans stay full of
water because rivers and runoff continually refreshes them.

Watersheds and rivers

When looking at the location of rivers and also the amount of streamflow in
rivers, the key concept to know about is the river's "watershed". What is a
watershed? Easy, if you are standing on the ground right now, just look
down. You're standing, and everyone is standing, in a watershed. A
watershed is the area of land where all of the water that falls in it and drains
off of it goes into the same place. Watersheds can be as small as a footprint
in the mud or large enough to encompass all the land that drains water into
the Mississippi River where it enters the Gulf of Mexico. Smaller watersheds
are contained in bigger watersheds. It all depends of the outflow point—all of
the land above that drains water that flows to the outflow point is the
watershed for that outflow location. Watersheds are important because the
streamflow and the water quality of a river are affected by things, human-
induced or not, happening in the land area "above" the river-outflow point

Streamflow is always changing

 Streamflow is always changing,
from day to day and even minute to minute. Of course, the main influence
on streamflow is precipitation runoff in the watershed. Rainfall causes rivers
to rise, and a river can even rise if it only rains very far up in the
watershed—remember that water that falls in a watershed will eventually
drain by the outflow point. The size of a river is highly dependent on the size
of its watershed. Large rivers have watersheds with lots of surface area;
small rivers have smaller watersheds. Likewise, different size rivers react
differently to storms and rainfall. Large rivers rise and fall slower and at a
slower rate than small rivers. In a small watershed, a storm can cause 100
times as much water to flow by each minute as during baseflow periods, but
the river will rise and fall possibly in a matter of minutes and hours. Large
rivers may take days to rise and fall, and flooding can last for a number of
days. After all, it can take days for all the water that fell hundreds of miles
upstream to drain past an outflow point.

More about streamflow.

Freshwater storage: Freshwater existing on the Earth's surface

One part of the water cycle that is obviously essential to all life on Earth is
the freshwater existing on the land surface. Just ask your neighbor, a
tomato plant, a trout, or that pesky mosquito. Surface water includes the
streams (of all sizes, from large rivers to small creeks), ponds, lakes,
reservoirs (man-made lakes), and freshwater wetlands. The definition of
freshwater is water containing less than 1,000 milligrams per liter of
dissolved solids, most often salt.

The amount of water in our rivers and lakes is always changing due to
inflows and outflows. Inflows to these water bodies will be from
precipitation, overland runoff, groundwater seepage, or tributary inflows.
Outflows from lakes and rivers include evaporation and discharge to
groundwater. Humans get into the act also, as people make great use of
diverted surface water for their needs. So, the amount and location of
surface water changes over time and space, whether naturally or with
human help. Certainly during the last ice age when glaciers and snowpacks
covered much more land surface than today, life on Earth had to adapt to
different hydrologic conditions than those which took place both before and
after. And the layout of the landscape certainly was different before and
after the last ice age, which influenced the topographical layout of many
surface-water bodies today. Glaciers are what made the Great Lakes not
only "great," but also such a huge storehouse of freshwater

Surface water keeps life going

As this satellite picture of the Nile Delta in

Egypt shows, life can even bloom in the desert if there is a supply of surface
water (or groundwater) available. Water on the land surface really does
sustain life, and this is as true today as it was millions of years ago. I'm sure
dinosaurs held their meetings at the local watering hole 100 million years
ago, just as antelopes in Africa do today. And, since groundwater is supplied
by the downward percolation of surface water, even aquifers are happy for
water on the Earth's surface. You might think that fish living in the saline
oceans aren't affected by freshwater, but, without freshwater to replenish
the oceans they would eventually evaporate and become too saline for even
the fish to survive.

Usable freshwater is relatively scarce

Freshwater represents only about three percent of all water on Earth and
freshwater lakes and swamps account for a mere 0.29 percent of the Earth's
freshwater. Twenty percent of all fresh surface water is in one lake, Lake
Baikal in Asia. Another twenty percent is stored in the Great Lakes (Huron,
Michigan, and Superior). Rivers hold only about 0.006 percent of total
freshwater reserves. You can see that life on Earth survives on what is
essentially only a "drop in the bucket" of Earth's total water supply!

More about freshwater storage.

Infiltration: The downward movement of water from the land surface into soil
or porous rock

Groundwater begins as precipitation

Anywhere in the world, a portion of the

water that falls as rain and snow infiltrates into the subsurface soil and rock.
How much infiltrates depends greatly on a number of factors. Infiltration of
precipitation falling on the ice cap of Greenland might be very small,
whereas, as this picture of a stream disappearing into a cave in southern
Georgia, USA shows, a stream can act as a direct funnel right into
groundwater!

Some water that infiltrates will remain in the shallow soil layer, where it will
gradually move vertically and horizontally through the soil and subsurface
material. Eventually it might enter a stream by seepage into the stream
bank. Some of the water may infiltrate deeper, recharging groundwater
aquifers. If the aquifers are shallow or porous enough to allow water to
move freely through it, people can drill wells into the aquifer and use the
water for their purposes. Water may travel long distances or remain in
groundwater storage for long periods before returning to the surface or
seeping into other water bodies, such as streams and the oceans.

In places where the water table (the top of the saturated zone) is close to
the land surface and where the water can move through the aquifer at a
high rate, aquifers can be replenished artificially

More about infiltration.

Groundwater storage: Water existing for long periods below the Earth's
surface

Stored water as part of the water cycle

Large amounts of water are

stored in the ground. The water is still moving, possibly very slowly, and it is
a part of the water cycle. Most of the water in the ground comes from
precipitation that infiltrates downward from the land surface. The upper
layer of the soil is the unsaturated zone, where water is present in varying
amounts that change over time, but does not saturate the soil. Below this
layer is the saturated zone, where all of the pores, cracks, and spaces
between rock particles are saturated with water. The term groundwater is
used to describe this area. Another term for groundwater is "aquifer,"
although this term is usually used to describe water-bearing formations
capable of yielding enough water to supply peoples' uses. Aquifers are a
huge storehouse of Earth's water and people all over the world depend on
groundwater in their daily lives.

To find water, look under the table ... the water table

I hope you appreciate my spending an

hour in the blazing sun to dig this hole at the beach. It is a great way to
illustrate the concept of how at a certain depth the ground, if it is permeable
enough to hold water, is saturated with water. The top of the pool of water
in this hole is the water table. The breaking waves of the ocean are just to
the right of this hole, and the water level in the hole is the same as the level
of the ocean. Of course, the water level here changes by the minute due to
the movement of the tides, and as the tide goes up and down, the water
level in the hole moves, too.

In a way, this hole is like a dug well used to access groundwater, albeit
saline in this case. But, if this was freshwater, people could grab a bucket an
supply themselves with the water they need to live their daily lives. You
know that at the beach if you took a bucket and tried to empty this hole, it
would refill immediately because the sand is so permeable that water flows
easily through it, meaning our "well" is very "high-yielding" (too bad the
water is saline). To access freshwater, people have to drill wells deep
enough to tap into an aquifer. The well might have to be dozens or
thousands of feet deep. But the concept is the same as our well at the
beach—access the water in the saturated zone where the voids in the rock
are full of water.

More about groundwater storage.

Groundwater discharge: The movement of water out of the ground

There's more water than just what you can see

You see water all around you every day

as lakes, rivers, ice, rain and snow. There are also vast amounts of water
that are unseen—water existing in the ground. And even though
groundwater is unseen, it is moving below your feet right now. As part of the
water cycle, groundwater is a major contributor to flow in many streams and
rivers and has a strong influence on river and wetland habitats for plants
and animals. People have been using groundwater for thousands of years
and continue to use it today, largely for drinking water and irrigation. Life on
Earth depends on groundwater just as it does on surface water.

Groundwater flows underground

Some of the
precipitation that falls onto the land infiltrates into the ground to become
groundwater. Once in the ground, some of this water travels close to the
land surface and emerges very quickly as discharge into streambeds, but,
because of gravity, much of it continues to sink deeper into the ground. If
the water meets the water table (below which the soil is saturated), it can
move both vertically and horizontally. Water moving downward can also
meet more dense and water-resistant non-porous rock and soil, which
causes it to flow in a more horizontal fashion, generally towards streams,
the ocean, or deeper into the ground.

As this diagram shows, the direction and speed of groundwater movement is

determined by the various characteristics of aquifers and confining layers
(which water has a difficult time penetrating) in the ground. Water moving
below ground depends on the permeability (how easy or difficult it is for
water to move) and on the porosity (the amount of open space in the
material) of the subsurface rock. If the rock has characteristics that allow
water to move relatively freely through it, then groundwater can move
significant distances in a number of days. But groundwater can also sink into
deep aquifers where it takes thousands of years to move back into the
environment, or even go into deep groundwater storage, where it might stay
for much longer periods.
 More about groundwater discharge.

Spring: A place where a concentrated discharge of groundwater flows at the

ground surface

What is a spring?

A spring is a water resource formed

when the side of a hill, a valley bottom or other excavation intersects a
flowing body of groundwater at or below the local water table. A spring is
the result of an aquifer being filled to the point that the water overflows onto
the land surface. They range in size from intermittent seeps, which flow only
after much rain, to huge pools with a flow of hundreds of millions of liters
per day.

Springs may be formed in any sort of rock, but are more prevalent in
limestone and dolomite, which fracture easily and can be dissolved by
rainfall that becomes weakly acidic. As the rock dissolves and fractures,
spaces can form that allow water to flow. If the flow is horizontal, it can
reach the land surface, resulting in a spring.

Spring water is not always clear

 Water from springs usually is
remarkably clear. Water from some springs, however, may be "tea-colored."
This picture shows a natural spring in southwestern Colorado. Its red iron
coloring and metals enrichment are caused by groundwater coming in
contact with naturally occurring minerals present as a result of ancient
volcanic activity in the area. In Florida, many surface waters contain natural
tannic acids from organic material in subsurface rocks, and the color from
these streams can appear in springs. If surface water enters the aquifer near
a spring, the water can move quickly through the aquifer and discharge at
the spring vent. The discharge of highly colored water from springs can
indicate that water is flowing quickly through large channels within the
aquifer without being filtered through the limestone.

Thermal springs

Thermal springs are ordinary springs

except that the water is warm and, in some places, hot, such as in the
bubbling mud springs in Yellowstone National Park, Wyoming. Many thermal
springs occur in regions of recent volcanic activity and are fed by water
heated by contact with hot rocks far below the surface. Even where there
has been no recent volcanic action, rocks become warmer with increasing
depth. In such areas water may migrate slowly to considerable depth,
warming as it descends through rocks deep in the Earth. If it then reaches a
large crevice that offers a path of less resistance, it may rise more quickly
than it descended. Water that does not have time to cool before it emerges
forms a thermal spring. The famous Warm Springs of Georgia and Hot
Springs of Arkansas are of this type. And, yes, warm springs can even
coexist with icebergs, as these happy Greenlanders can tell you.

More about springs.

Global water distribution

For an estimated explanation of where Earth's water exists, look at the chart
below. By now, you know that the water cycle describes the movement of
Earth's water, so realize that the chart and table below represent the
presence of Earth's water at a single point in time. If you check back in a
thousand or million years, no doubt these numbers will be different!

Notice how of the world's total water supply of about 332.5 million cubic
miles of water, over 97 percent is saline. And, of the total freshwater, over
68 percent is locked up in ice and glaciers. Another 30 percent of freshwater
is in the ground. Fresh surface-water sources, such as rivers and lakes, only
constitute about 22,300 cubic miles (93,100 cubic kilometers), which is
about 1/150th of one percent of total water. Yet, rivers and lakes are the
sources of most of the water people use everyday.
Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H.
Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water
Resources (Oxford University Press, New York).

One estimate of global water distribution

(Percents are rounded, so will not add to 100)

Water source Water volume, Water volume, in Percent of Percent

in cubic miles cubic kilometers freshwater of
total
water
Oceans, 321,000,000 1,338,000,000 -- 96.5
Seas, & Bays
Ice caps, 5,773,000 24,064,000 68.7 1.74
Glaciers, &
Permanent
Snow
 One estimate of global water distribution
(Percents are rounded, so will not add to 100)

Water source Water volume, Water volume, in Percent of Percent

in cubic miles cubic kilometers freshwater of
total
water
Groundwater 5,614,000 23,400,000 -- 1.7
Fresh 2,526,000 10,530,000 30.1 0.76
Saline 3,088,000 12,870,000 -- 0.93
Soil Moisture 3,959 16,500 0.05 0.001
Ground Ice & 71,970 300,000 0.86 0.022
Permafrost
Lakes 42,320 176,400 -- 0.013
Fresh 21,830 91,000 0.26 0.007
Saline 20,490 85,400 -- 0.006
Atmosphere 3,095 12,900 0.04 0.001
Swamp 2,752 11,470 0.03 0.0008
Water
Rivers 509 2,120 0.006 0.0002
Biological 269 1,120 0.003 0.0001
Water
Source: Igor Shiklomanov's chapter "World fresh water resources" in Peter H.
Gleick (editor), 1993, Water in Crisis: A Guide to the World's Fresh Water
Resources (Oxford University Press, New York).

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Diagram of the Water Cycle

The water cycle

Earth's water cycle

As the Earth's surface water evaporates, wind moves water in the air from the sea to the land, increasing the
amount of freshwater on land.
 Water vapor is converted to clouds that bring fresh water to land in the form of rain snow and sleet

Precipitation falls on the ground, but what happens to that water depends greatly on the geography of the land
at any particular place.

The water cycle, also known as the hydrological cycle or the hydrologic cycle, describes the
continuous movement of water on, above and below the surface of the Earth. The mass of water on
Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of
ice, fresh water, saline water and atmospheric water is variable depending on a wide range
of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or
from the ocean to the atmosphere, by the physical processes
of evaporation, condensation, precipitation, infiltration, surface runoff, and subsurface flow. In doing
so, the water goes through different forms: liquid, solid (ice) and vapor.
The water cycle involves the exchange of energy, which leads to temperature changes. When water
evaporates, it takes up energy from its surroundings and cools the environment. When it condenses,
it releases energy and warms the environment. These heat exchanges influence climate.
The evaporative phase of the cycle purifies water which then replenishes the land with freshwater.
The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping
the geological features of the Earth, through processes including erosion and sedimentation. The
water cycle is also essential for the maintenance of most life and ecosystems on the planet.

Contents

 1Description
o 1.1Processes
 2Residence times
 3Changes over time
 4Effects on climate
 5Effects on biogeochemical cycling
 6Slow loss over geologic time
 7History of hydrologic cycle theory
o 7.1Floating land mass
o 7.2Hebrew Bible
 o 7.3Precipitation and percolation
o 7.4Precipitation alone
 8See also
 9References
 10Further reading
 11External links

Description
The sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water
vapor into the air. Some ice and snow sublimates directly into water vapor. Evapotranspiration is
water transpired from plants and evaporated from the soil. The water molecule H
2O has smaller molecular mass than the major components of the atmosphere, nitrogen and

oxygen, N
2 and O

2, hence is less dense. Due to the significant difference in density, buoyancy drives humid air higher.

As altitude increases, air pressure decreases and the temperature drops (see Gas laws). The lower
temperature causes water vapor to condense into tiny liquid water droplets which are heavier than
the air, and fall unless supported by an updraft. A huge concentration of these droplets over a large
space up in the atmosphere become visible as cloud. Some condensation is near ground level, and
called fog.
Atmospheric circulation moves water vapor around the globe, cloud particles collide, grow, and fall
out of the upper atmospheric layers as precipitation. Some precipitation falls as snow or hail, sleet,
and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years.
Most water falls back into the oceans or onto land as rain, where the water flows over the ground
as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow
moving water towards the oceans. Runoff and water emerging from the ground (groundwater) may
be stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground
as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which can store
freshwater for long periods of time. Some infiltration stays close to the land surface and can seep
back into surface-water bodies (and the ocean) as groundwater discharge. Some groundwater finds
openings in the land surface and comes out as freshwater springs. In river valleys and floodplains,
there is often continuous water exchange between surface water and ground water in the hyporheic
zone. Over time, the water returns to the ocean, to continue the water cycle.

Processes
Many different processes lead to movements and phase changes in water

Precipitation
Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain,
but also includes snow, hail, fog drip, graupel, and sleet.[1]Approximately
505,000 km3 (121,000 cu mi) of water falls as precipitation each year,
398,000 km3(95,000 cu mi) of it over the oceans.[2][better source needed] The rain on land contains
107,000 km3 (26,000 cu mi) of water per year and a snowing only
1,000 km3 (240 cu mi).[3]78% of global precipitation occurs over the ocean.[4]
Canopy interception
The precipitation that is intercepted by plant foliage eventually evaporates back to the
atmosphere rather than falling to the ground.
Snowmelt
The runoff produced by melting snow.
Runoff
The variety of ways by which water moves across the land. This includes both surface runoff
and channel runoff. As it flows, the water may seep into the ground, evaporate into the air,
become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
Infiltration
The flow of water from the ground surface into the ground. Once infiltrated, the water
becomes soil moisture or groundwater.[5] A recent global study using water stable isotopes,
however, shows that not all soil moisture is equally available for groundwater recharge or for
plant transpiration.[6]
Subsurface flow
The flow of water underground, in the vadose zone and aquifers. Subsurface water may
return to the surface (e.g. as a spring or by being pumped) or eventually seep into the
oceans. Water returns to the land surface at lower elevation than where it infiltrated, under
the force of gravity or gravity induced pressures. Groundwater tends to move slowly and is
replenished slowly, so it can remain in aquifers for thousands of years.
Evaporation
The transformation of water from liquid to gas phases as it moves from the ground or bodies
of water into the overlying atmosphere.[7] The source of energy for evaporation is
primarily solar radiation. Evaporation often implicitly includes transpiration from plants,
though together they are specifically referred to as evapotranspiration. Total annual
evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water,
434,000 km3 (104,000 cu mi) of which evaporates from the oceans.[2] 86% of global
evaporation occurs over the ocean.[4]
Sublimation
The state change directly from solid water (snow or ice) to water vapor by passing the liquid
state.[8]
Deposition
This refers to changing of water vapor directly to ice.
Advection
The movement of water through the atmosphere.[9] Without advection, water that evaporated
over the oceans could not precipitate over land.
Condensation
The transformation of water vapor to liquid water droplets in the air, creating clouds and
fog.[10]
Transpiration
The release of water vapor from plants and soil into the air.
Percolation
Water flows vertically through the soil and rocks under the influence of gravity.
Plate tectonics
Water enters the mantle via subduction of oceanic crust. Water returns to the surface via
volcanism.
The water cycle involves many of these
processes.
Answer:
Evaporation
Condensation (Storage)
Precipitation
Percolation (Infiltration)
Sublimation
Transpiration (Evapotranspiration)
Explanation:
Evaporation - is the process of water escaping from the surface of a body of water
into the atmosphere.
Condensation (Storage) - is the process by which water molecules gather from
evaporated gas into stored water in clouds, or precipitation into collecting bodies of
water
Precipitation - is the process of collected water in the clouds returning to the earth's
surface in rain, sleet, snow....
Percolation (Infiltration) - is the process that allows water on the earth's surface to seep
into the earth's crust and become ground water.
Sublimation - is the process that allows solid water in the form of ice to escape as gas
without turning into liquid water.
Transpiration (Evapotranspiration) - is the process that allows water released from the
process of photosynthesis in plants to be released into the environment.
THE WATER CYCLE: A GUIDE FOR STUDENTS
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 Water Cycle
(Mouse over to see larger picture)

Water is the basic element of nature. It covers 70% of the earth’s surface. It provides life,
eases out heat, drains harmful substances and mediates many day-to-day works. Water
needs to be replenished, purified and circulated again and again so that it can perform its
functions. Nature does this job through a process called the water cycle. Also known as
hydrologic cycle, the water cycle is a phenomenon where water moves through the three
phases (gas, liquid and solid) over the four spheres (atmosphere, lithosphere, hydrosphere
and biosphere) and completes a full cycle. The water cycle has many effects: it regulates
the temperature of the surroundings. It changes weather and creates rain. It helps in
conversion of rocks to soil. It circulates important minerals through the spheres. It also
creates the many geographical features present on earth like the ice caps of mountains,
icebergs, the rivers and the valleys, lakes, and more. Hence it is quite important to
understand and learn the processes of the water cycle.

Step 1: Evaporation

The water cycle starts with evaporation. It is a process where water at the surface turns
into water vapors. Water absorbs heat energy from the sun and turns into vapors. Water
bodies like the oceans, the seas, the lakes and the river bodies are the main source of
evaporation. Through evaporation, water moves from hydrosphere to atmosphere. As water
evaporates it reduces the temperature of the bodies.

Step 2: Condensation

As water vaporizes into water vapor, it rises up in the atmosphere. At high altitudes the
water vapors changes into very tiny particles of ice /water droplets because the
temperature at high altitudes is low. This process is called condensation. These particles
come close together and form clouds and fogs in the sky.

Step 3: Sublimation

Apart from evaporation, sublimation also contributes to water vapors in the air. Sublimation
is a process where ice directly converts into water vapors without converting into liquid
water. This phenomenon accelerates when the temperature is low or pressure is high. The
main sources of water from sublimation are the ice sheets of the North Pole and the South
Pole and the ice caps on the mountains. Sublimation is a rather slower process than
evaporation.

Step 4: Precipitation

The clouds (condensed water vapors) then pour down as precipitation due to wind or
temperature change. This occurs because the water droplets combine to make bigger
droplets. Also when the air cannot hold any more water, it precipitates. At high altitudes the
temperature is low and hence the droplets lose their heat energy. These water droplets fall
down as rain. If the temperature is very low (below 0 degrees), the water droplets fall as
snow. Water also precipices in the form of drizzle, sleet and hail. Hence water enters
lithosphere.

Step 5: Transpiration

As water precipitates, some of it is absorbed by the soil. This water enters into the process
of transpiration. Transpiration is a process similar to evaporation where liquid water is
turned into water vapor by the plants. The roots of the plants absorb the water and push it
toward leaves where it is used for photosynthesis. The extra water is moved out of leaves
through stomata (very tiny openings on leaves) as water vapor. Thus water enters the
biosphere and exits into gaseous phase.

Step 6: Runoff

As the water pours down (in whatever form), it leads to runoff. Runoff is the process where
water runs over the surface of earth. When the snow melts into water it also leads to runoff.
As water runs over the ground it displaces the top soil with it and moves the minerals along
with the stream. This runoff combines to form channels and then rivers and ends up into
lakes, seas and oceans. Here the water enters hydrosphere.

Step 7: Infiltration

Some of the water that precipitates does not runoff into the rivers and is absorbed by the
plants or gets evaporated. It moves deep into the soil. This is called infiltration. The water
seeps down and increases the level of ground water table. It is called pure water and is
drinkable. The infiltration is measured as inches of water-soaked by the soil per hour.

Look below for more information in understanding the phenomenon of the water cycle.
Evaporation and Transpiration
The water cycle can start anywhere but most time it starts in the ocean where the heat from sun,
heats up the water. The heat then causes the water to turn into water vapour. And this process is
called " Evaporation". Studies have shown that the oceans, seas, lakes, and rivers provide nearly
90 percent of the moisture in the atmosphere by evaporation, with the remaining 10 percent
being moisturised by plant transpiration. Transpiration can also be called " Evpotranspiration". Its
the sum of Evaporation and Transpiration.

Condensation

After Evaporation and Transpiration, the water cycle goes on to "Condensation". Condensation is
the process where the water vapour in the air is changed into liquid water again. Condensation is
very important in the water cycle because it is responsible for the formation of clouds. Basically
they liquid water that has been changed from water vapour is stored in clouds.

As you can see in this picture, those clouds are filled with liquid water or droplets. Its a sign of a rain storm coming
through.

Precipitation

The next step in the water cycle is "Precipitation". Precipitation is when the water released from
clouds in the form of rain, snow, or hail. Precipitation happens when the liquid droplets grow so
big that the cloud cannot hold it any more so it precipitates.
Runoff

After Precipitation most people think that's when the water cycle ends but that is not true. There
are other parts to the water cycle like "Runoff". Runoff is when the water along the surface of the
ground flows to creeks, rivers,streams and oceans.

This picture shows the surface runoff from the mountains to lake.

Sublimation

Another step in the water cycle is "Sublimation". Sublimation is the phase of when solid changes
to gas without changing to liquid first. When snow falls on mountains or the polar region. Since
its so cold the snow and ice at the surface can directly change into water vapour. Rather than the
snow or ice melting first then evaporating.

This is the Mt.Everest today, as you can see there isn't much snow on the top because sublimation has happened.

Infiltration

One step in the water cycle is called "Infiltration". Its a process where a portion of the water that
falls as rain or snow infiltrates into the soil and rock. Some water that infiltrates will remain in the
shallow soil layer. Eventually ot might enter a stream seepage into the stream bank. Some water
can also infiltrate deeper into the aquifers.

In this picture the water on the surface is slowly infiltrating into ground water.
Process of Water Cycle
The water cycle explains the continuous movement of water on, above, and below the
surface of the earth. It is also referred to as the Hydrological Cycle. The cycle describes
the properties of water that make it undergo the various movements on the planet. The
water cycle has nine main physical processes that create a continuous water movement on
the planet.

Intricate sequences include the transition of water from the gaseous composition of the
atmosphere; through the water bodies such as oceans, lakes, rivers; passage through the soil,
rocks and underground waters; and later returns into the atmosphere. Simply put, the
hydrological cycle has neither a beginning nor an end, it’s an incessant process.

The water cycle processes involve evaporation, condensation, precipitation, interception,

infiltration, percolation, transpiration, runoff, and storage.
 1. Evaporation

Evaporation takes place when water changes from its liquid state to vapor or gaseous state. A
substantial heat amount is exchanged during the process, roughly 600 calories of energy per
gram of water. In most cases, the solar radiation and additional causes such as the wind, vapor
pressure, atmospheric pressure, and air temperature influence the amount of natural evaporation
in different geographical regions.
Evaporation occurs over the surfaces of the water bodies such as oceans, streams, and
lakes. It can also occur on raindrops, rocks, snow, soil or vegetation. When evaporation
happens, anything present in the water such as salts and minerals is left behind. Thus,
evaporation purifies the water. The evaporated moisture then rises into the atmosphere
from the evaporation sources as water vapor or in a gaseous state. At any particular
moment, some water vapor is present in the atmosphere.

2. Condensation

Condensation is the process whereby the water vapor changes from its gaseous physical state to
liquid or crystal solid. The water vapor condenses on minute air particles due to the cooling of
the air, freezing temperatures, or increased vapor amounts to the point of saturation in the upper
stratospheres.
The condensed vapor then forms fog, dew or clouds. When the condensed clouds, dew,
and fogs become too large and heavy to remain suspended in the atmosphere, they fall
back on earth as precipitation due to gravity. The 600 calories of energy per gram of
water needed during evaporations are released into the environment.

3. Precipitation

Precipitation takes place whenever any or all forms of water particles fall from the
atmosphere and reach the earth surface. Precipitation occurs when the liquid or solid
particles in the clouds, dew, and fog drops to the ground because of frictional drag and
gravity.

One falling particle leaves behind a turbulent wake, causing faster and continued drops.
The crystallized ice may reach the ground as ice pellets or snow or may melt and change
into raindrops before reaching the surface of the earth depending on the atmospheric
temperatures.

Precipitation falls on water bodies or on ground surface where it disperses in various

ways. For sometime, precipitation can remain on the surface as runoff or overland flow.
It may be carried into waterways, intercepted by plants, or infiltrate into the soil. A good
percentage of precipitation goes back to the atmosphere as evaporation.

4. Interception

Interception is whereby the water movement is interrupted in the various paths during
transportation events over the land surface. Interception takes place when the water is
absorbed by vegetation cover and trees, absorbed into the ground, or stored in puddles
and land formations such as furrows and streamlets. These waters can either infiltrate into
the soil or return to the atmosphere through evapotranspiration or evaporation.

5. Infiltration

Infiltration is the physical process involving the slow passage of water through the soil.
This phenomenon is influenced by the soil surface conditions such as permeability and
porosity of the soil profile. Other factors include soil texture, soil moisture content, and
soil structure. The infiltrated water is stored in the soil and can later return to the
atmosphere via evapotranspiration.

6. Percolation

Percolation is the flow of water through the soil and rocks by the influence of capillary
and gravity forces. All water on the earth’s surface move by the forces of gravity and
capillarity to rest beneath the earth as groundwater. Once beneath the earth, below the
water table, the water mostly moves horizontally rather than downwards based on the
geologic boundary formations.

This area normally acts as reservoirs for storing water. Some geologic formations may
conduct this water back to the surface such as springs.

7. Transpiration

Transpiration is a process in all plants that normally takes place during the day, giving off
water vapor from the leaves openings. Plants transpire to move nutrients to the upper
sections of the plant and to cool the plants. Most of the water absorbed by the plants are
transpired into the atmosphere until a water deficit point is reached whereby the plant
resorts to releasing water vapor at a much slower rate. Transpiration is important in the
water cycle because plants absorb the moisture from the soil and releases it into the
atmosphere as water vapor.

8. Runoff

Runoff is the occurrence of excess water from watershed or drainage basin that flows on
the surface. The flow is as a result of precipitation above waterways, groundwater runoff
from deep percolations, subsurface runoff that infiltrates the surface soils, and surface
runoff that flows on the land surface. As the water flows, it can be used for agricultural
and domestic purposes, it may seep into the ground, stored in reservoirs or water bodies,
or evaporate into the atmosphere.

9. Storage

Storage refers to the various water reservoirs in the planetary water or hydrological cycle.
The water is primarily stored in the atmosphere, the surface of the earth, and in the
ground. Storage in the atmosphere is in the form of water vapor. Storage on the surface of
the earth includes lakes, oceans, rivers, glaciers, and reservoirs. Storage in the ground
pertains to the soils, rock formations, and aquifers.