The Water Cycle
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 to live without it.
Illustration by John M. Evans, USGS, Colorado District
A quick summary of the water cycle
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,
until it could stay on the surface as a liquid. Volcanic activity kept and
still keeps introducing water in the atmosphere, thus increasing the
surface- and ground-water volume of the Earth.
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. Ice and snow can sublimate directly into water
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, 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 water infiltrates deep 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 ground-water discharge, and some ground water
finds openings in the land surface and emerges as freshwater springs.
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."
Global water distribution
For a detailed explanation of where Earth's water exists, look at the
chart and data table 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 96 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.
One estimate of global water distribution:
Water
source
Oceans,
Seas, &
Water
volume, in
cubic miles
Water
volume, in
cubic
kilometers
321,000,000 1,338,000,000
Percent
Percent of
of total
freshwater
water
--
96.5
Bays
Ice caps,
Glaciers, &
Permanent
Snow
5,773,000
24,064,000
68.7
1.74
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
--
0.94
3,959
16,500
0.05
0.001
Ground Ice
&
Permafrost
71,970
300,000
0.86
0.022
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
Water
2,752
11,470
0.03
0.0008
Rivers
509
2,120
0.006
0.0002
Biological
Water
269
1,120
0.003
0.0001
-
100
Soil Moisture
Total
332,500,000 1,386,000,000
Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of
Climate and Weather, ed. by S. H. Schneider, Oxford University Press,
New York, vol. 2, pp.817-823.
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The Water Cycle:
Evaporation
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 the atmosphere via
evaporation, with the remaining 10 percent being contributed by plant
transpiration.
A very small amount of water vapor enters the atmosphere through
sublimation, the process by which water changes from a solid (ice or
snow) to a gas, bypassing the liquid phase. This often happens in the
Rocky Mountains as dry and warm Chinook winds blow in from the
Pacific in late winter and early spring. When a Chinook takes effect
local temperatures rise dramatically in a matter of hours. When the
dry air hits the snow, it changes the snow directly into water vapor,
bypassing the liquid phase. Sublimation is a common way for snow to
disappear quickly in arid climates. (Source: Mount Washington
Observatory)
Why evaporation occurs
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 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. The process of evaporation is so great that without
precipitation runoff, and ground-water discharge from aquifers, oceans
would become nearly empty.
Less evaporation takes place during periods of calm winds than during
windy times. When the air is calm, evaporated water tends to stay
close to the water body, as the picture above shows; when winds are
present, the more moist air close to the water body is moved away
and replaced by drier air which favors additional evaporation.
People make use of evaporation
If you ever find yourself
stranded on an island in need of some salt, just grab a bowl, add some
seawater, and wait for the sun to evaporate the water. In fact, much
of the world's table salt is produced within evaporation ponds, a
technique used by people for thousands of years.
Salt is not the only product that people obtain using evaporation.
Seawater contains other valuable minerals that are easily obtained by
evaporation. The Dead Sea is located in the Middle East within a closed
watershed and without any means of outflow, which is abnormal for
most lakes. The primary mechanism for water to leave the lake is by
evaporation, which can be quite high in a desert—upwards of 1,300 1,600 millimeters per year. The result is that the waters of the Dead
Sea have the highest salinity and density (which is why you float
"higher" when you lay in the water) of any sea in the world, too high
to support life. The water is ideal for locating evaporation ponds for
the extraction of not only table salt, but also magnesium, potash, and
bromine. (Source: Overview of Middle East Water Resources, Middle East
Water Data Banks Project ).
Evaporative cooling: Cheap air
conditioning!
We said earlier that heat is
removed from the environment during evaporation, leading to a net
cooling; notice how cold your arm gets when a physician rubs it with
alcohol before pulling out a syringe with that scary-looking needle
attached. In climates where the humidity is low and the temperatures
are hot, an evaporator cooler, such as a "swamp cooler" can lower the
air temperature by 20 degrees F., while it increases humidity. As this
map shows, evaporative coolers work best in the dry areas of the
United States (red areas marked A) and can work somewhat in the
blue areas marked B. In the humid eastern U.S., normal air
conditioners must be used.
Evaporative coolers are really quite simple devices, at least compared
to air conditioners. Swamp coolers pull in the dry, hot outdoor air and
pass it through an evaporative pad that is kept wet by a supply of
water. As a fan draws the air through the pad, the water in the pad
evaporates, resulting in cooler air which is pumped through the house.
Much less energy is used as compared to an air conditioner. (Source:
California Energy Commission)
Sources and more information
http://ga.water.usgs.gov/edu/watercycleevaporation.html
Water cycle
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The Earth's water is always in movement, and the water cycle, also known as the
hydrologic cycle, describes the continuous movement of water on, above, and below the
surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end.
Water can change states among liquid, vapor, and ice at various places in the water cycle,
with these processes happening in the blink of an eye and over millions of years.
Although the balance of water on Earth remains fairly constant over time, individual
water molecules can come and go in a hurry, but there is always the same amount of
water on the surface of the earth.
Contents
[hide]
•
•
•
•
•
•
•
•
•
1 Description
2 Reservoirs
3 Residence times
4 Changes over time
5 Effects on climate
6 Effects on biogeochemical cycling
7 See also
8 References
9 External links
Description
The water cycle.
The water cycle has no starting or ending point. The sun, which drives the water cycle,
heats water in the oceans. Some of it evaporates as vapor into the air. Ice and snow can
sublimate directly into water 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,
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 ground-water seepage,
accumulate and are 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 (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 ground-water discharge, and some
ground water finds openings in the land surface and emerges as freshwater springs. Over
time, the water continues flowing, some to reenter the ocean, where the water cycle
renews itself.
The different processes are as follows:
•
Precipitation is 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 of water fall as
precipitation each year, 398,000 km³ of it over the oceans.[2]
•
Canopy interception is the precipitation that is intercepted by plant
foliage and eventually evaporates back to the atmosphere rather than
falling to the ground.
•
Snowmelt refers to the runoff produced by melting snow.
•
Runoff includes 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 infiltrate into the ground, evaporate into the air, become stored
in lakes or reservoirs, or be extracted for agricultural or other human uses.
•
Infiltration is the flow of water from the ground surface into the ground.
Once infiltrated, the water becomes soil moisture or groundwater.[3]
•
Subsurface Flow is the flow of water underground, in the vadose zone
and aquifers. Subsurface water may return to the surface (eg. 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 is the transformation of water from liquid to gas phases as it
moves from the ground or bodies of water into the overlying
atmosphere.[4] 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 of
water, 434,000 km³ of which evaporates from the oceans.[5]
•
Sublimation is the state change directly from solid water (snow or ice) to
water vapor.[6]
•
Advection is the movement of water — in solid, liquid, or vapour states
— through the atmosphere. Without advection, water that evaporated over
the oceans could not precipitate over land.[7]
•
Condensation is the transformation of water vapour to liquid water
droplets in the air, producing clouds and fog.[8]
Reservoirs
In the context of the water cycle, a
reservoir represents the water
contained in different steps within the
cycle. The largest reservoir is the
collection of oceans, accounting for
97% of the Earth's water. The next
largest quantity (2%) is stored in solid
form in the ice caps and glaciers. This
small amount accounts for
approximately 75% of all fresh water
reserves on the planet. The water
contained within all living organisms
represents the smallest reservoir.
The volume of water in the fresh water
reservoirs, particularly those that are
available for human use, are important
water resources.[10]
Volume of water stored in
the water cycle's reservoirs[9]
Reservoir
Volume of water Percent
(106 km³)
of total
Oceans
1370
97.25
Ice caps & glaciers
29
2.05
Groundwater
9.5
0.68
Lakes
0.125
0.01
Soil moisture
0.065
0.005
Atmosphere
0.013
0.001
Streams & rivers
0.0017
0.0001
Biosphere
0.0006
0.00004
Residence times
The residence time of a
reservoir within the
hydrologic cycle is the
average time a water
molecule will spend in that
reservoir (see the adjacent
table). It is a measure of the
average age of the water in
that reservoir, though some
water will spend much less
time than average, and some
much more.
Average reservoir residence times[9]
Reservoir
Average residence time
Oceans
3,200 years
Glaciers
20 to 100 years
Seasonal snow cover
2 to 6 months
Soil moisture
1 to 2 months
Groundwater: shallow
Groundwater: deep
Lakes (see lake retention time)
100 to 200 years
10,000 years
50 to 100 years
Rivers
2 to 6 months
Groundwater can spend over
10,000 years beneath Earth's
Atmosphere
9 days
surface before leaving.
Particularly old groundwater
is called fossil water. Water stored in the soil remains there very briefly, because it is
spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream
flow, or groundwater recharge. After evaporating, water remains in the atmosphere for
about 9 days before condensing and falling to the Earth as precipitation.
In hydrology, residence times can be estimated in two ways. The more common method
relies on the principle of conservation of mass and assumes the amount of water in a
given reservoir is roughly constant. With this method, residence times are estimated by
dividing the volume of the reservoir by the rate by which water either enters or exits the
reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir
to become filled from empty if no water were to leave (or how long it would take the
reservoir to empty from full if no water were to enter).
An alternative method to estimate residence times, gaining in popularity particularly for
dating groundwater, is the use of isotopic techniques. This is done in the subfield of
isotope hydrology.
Changes over time
The water cycle describes the processes that drive the movement of water throughout the
hydrosphere. However, 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 332,500,000 cubic miles (mi3)
(1,386,000,000 km3) of the world's water supply, about 321,000,000 mi3 (1,338,000,000
km3) is stored in oceans,or about 95%. It is also estimated that the oceans supply about 90
percent of the evaporated water that goes into the water cycle.[11]
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.[11]
The scientific consensus expressed in the 2007 Intergovernmental Panel on Climate
Change (IPCC) Summary for Policymakers[12] is for the water cycle to continue to
intensify throughout the 21st century, though this does not mean that precipitation will
increase in all regions. In subtropical land areas — places that are already relatively dry
— precipitation is projected to decrease during the 21st century, increasing the
probability of drought. The drying is projected to be strongest near the poleward margins
of the subtropics (for example, the Mediterranean Basin, South Africa, southern
Australia, and the Southwestern United States). Annual precipitation amounts are
expected to increase in near-equatorial regions that tend to be wet in the present climate,
and also at high latitudes. These large-scale patterns are present in nearly all of the
climate model simulations conducted at several international research centers as part of
the 4th Assessment of the IPCC.
Glacial retreat is also an example of a changing water cycle, where the supply of water to
glaciers from precipitation cannot keep up with the loss of water from melting and
sublimation. Glacial retreat since 1850 has been extensive.[13]
Human activities that alter the water cycle include:
•
•
•
•
•
•
•
agriculture
alteration of the chemical composition of the atmosphere
construction of dams
deforestation and afforestation
removal of groundwater from wells
water abstraction from rivers
urbanization
Effects on climate
The water cycle is powered from solar energy. 86% of the global evaporation occurs
from the oceans, reducing their temperature by evaporative cooling. Without the cooling
effect of evaporation the greenhouse effect would lead to a much higher surface
temperature of 67 °C, and a warmer planet.[14]
Effects on biogeochemical cycling
While the water cycle is itself a biogeochemical cycle,[15] flow of water over and beneath
the Earth is a key component of the cycling of other biogeochemicals. Runoff is
responsible for almost all of the transport of eroded sediment and phosphorus[16] from
land to waterbodies. The salinity of the oceans is derived from erosion and transport of
dissolved salts from the land. Cultural eutrophication of lakes is primarily due to
phosphorus, applied in excess to agricultural fields in fertilizers, and then transported
overland and down rivers. Both runoff and groundwater flow play significant roles in
transporting nitrogen from the land to waterbodies.[17] The dead zone at the outlet of the
Mississippi River is a consequence of nitrates from fertilizer being carried off agricultural
fields and funnelled down the river system to the Gulf of Mexico. Runoff also plays a
part in the carbon cycle, again through the transport of eroded rock and soil.[18]
See also
Water portal
•
•
Flood
Drought
References
1. ^ Arctic Climatology and Meteorology. Precipitation. Retrieved on 2006-10-24.
2. ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
3. ^ National Weather Service Northwest River Forecast Center. Hydrologic Cycle.
Retrieved on 2006-10-24.
4. ^ Arctic Climatology and Meteorology. Evaporation. Retrieved on 2006-10-24.
5. ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
6. ^ Arctic Climatology and Meteorology. Sublimation. Retrieved on 2006-10-24.
7. ^ Arctic Climatology and Meteorology. Advection. Retrieved on 2006-10-24.
8. ^ Arctic Climatology and Meteorology. Condensation. Retrieved on 2006-10-24.
9. ^ a b PhysicalGeography.net. CHAPTER 8: Introduction to the Hydrosphere. Retrieved on
2006-10-24.
10. ^ Environmental Literacy Council. Water Cycle. Retrieved on 2006-10-24.
11. ^ a b http://ga.water.usgs.gov/edu/watercycleoceans.html USGS, The Water Cycle: Water
Storage in Oceans - Retrieved on 2008-05-14
12. ^ Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical
Science Basis, WG1 Summary for Policymakers
13. ^ U.S. Geologic Survey. GLACIER RETREAT IN GLACIER NATIONAL PARK,
MONTANA. Retrieved on 2006-10-24.
14. ^ Science at NASA. NASA Oceanography: The Water Cycle. Retrieved on 2006-10-24.
15. ^ The Environmental Literacy Council. Biogeochemical Cycles. Retrieved on 2006-1024.
16. ^ The Environmental Literacy Council. Phosphorus Cycle. Retrieved on 2006-10-24.
17. ^ Ohio State University Extension Fact Sheet. Nitrogen and the Hydrologic Cycle.
Retrieved on 2006-10-24.
18. ^ NASA's Earth Observatory. The Carbon Cycle. Retrieved on 2006-10-24.
External links
•
•
•
•
The Water Cycle, United States Geological Survey
The water cycle, from Dr. Art's Guide to the Planet.
Water cycle slideshow, 1 Mb Flash multilingual animation highlighting the oftenoverlooked evaporation from bare soil, from managingwholes.com.
Will the wet get wetter and the dry drier? - Climate research summary from
NOAA Geophysical Fluid Dynamics Laboratory including text, graphics and
animations
Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle
Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle
Retrieved from "http://en.wikipedia.org/wiki/Water_cycle"
Categories: Hydrology | Water | Forms of water | Soil physics
•
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•
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http://en.wikipedia.org/wiki/Water_cycle
The Water Cycle
Evaporation, Condensation and Precipitation
The _________ evaporates _________ from lakes and oceans. As
the air rises, it cools. The water vapor condenses into tiny droplets
of _________
. The droplets crowd together and form a
_________
. Wind blows the _________ towards the land. The
tiny droplets join together and fall as precipitation to the
_________
. The water soaks into the ground and collects in
_________
. The _________ that never ends has started
again!
Use the diagram to identify the different parts of the water cycle:
•
•
•
•
•
#1 =
#2 =
#3 =
#4 =
#5 =
_________
_________
_________
_________
_________
http://education.jlab.org/reading/water_cycle.html