The Water Cycle. Illustration by John M. Evans, USGS, Colorado District

Transcription

1 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

2 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

3 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 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. Water source Oceans, Seas, & One estimate of global water distribution: Water volume, in cubic miles Water volume, in cubic kilometers Percent of freshwater Percent of total water 321,000,000 1,338,000,

4 Bays Ice caps, Glaciers, & Permanent Snow 5,773,000 24,064, Groundwater 5,614,000 23,400, Fresh 2,526,000 10,530, Saline 3,088,000 12,870, Soil Moisture 3,959 16, Ground Ice & Permafrost 71, , Lakes 42, , Fresh 21,830 91, Saline 20,490 85, Atmosphere 3,095 12, Swamp Water 2,752 11, Rivers 509 2, Biological Water 269 1, Total 332,500,000 1,386,000, 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 Water Cycle Home Water Science for Schools Water Resources USGS Home Accessibility FOIA Privacy Policies and Notices U.S. Department of the Interior U.S. Geological Survey URL: Page Contact Information: Howard Perlman Page Last Modified: Wednesday, 13-Aug :25:24 EDT

5 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

6 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

7 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!

8 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 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,

9 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

10 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]

11 Reservoirs Condensation is the transformation of water vapour to liquid water droplets in the air, producing clouds and fog. [8] 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] Reservoir Volume of water stored in the water cycle's reservoirs [9] Volume of water (10 6 km³) Percent of total Oceans Ice caps & glaciers Groundwater Lakes Soil moisture Atmosphere Streams & rivers Biosphere 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. Reservoir Oceans Glaciers Average reservoir residence times [9] Seasonal snow cover Soil moisture Groundwater: shallow Groundwater: deep Lakes (see lake retention time) Average residence time 3,200 years 20 to 100 years 2 to 6 months 1 to 2 months 100 to 200 years 10,000 years 50 to 100 years Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater Rivers Atmosphere 2 to 6 months 9 days 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

12 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 (mi 3 ) (1,386,000,000 km 3 ) of the world's water supply, about 321,000,000 mi 3 (1,338,000,000 km 3 ) 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.

13 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

14 1. ^ Arctic Climatology and Meteorology. Precipitation. Retrieved on ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on ^ National Weather Service Northwest River Forecast Center. Hydrologic Cycle. Retrieved on ^ Arctic Climatology and Meteorology. Evaporation. Retrieved on ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on ^ Arctic Climatology and Meteorology. Sublimation. Retrieved on ^ Arctic Climatology and Meteorology. Advection. Retrieved on ^ Arctic Climatology and Meteorology. Condensation. Retrieved on ^ a b PhysicalGeography.net. CHAPTER 8: Introduction to the Hydrosphere. Retrieved on ^ Environmental Literacy Council. Water Cycle. Retrieved on ^ a b USGS, The Water Cycle: Water Storage in Oceans - Retrieved on ^ 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 ^ Science at NASA. NASA Oceanography: The Water Cycle. Retrieved on ^ The Environmental Literacy Council. Biogeochemical Cycles. Retrieved on ^ The Environmental Literacy Council. Phosphorus Cycle. Retrieved on ^ Ohio State University Extension Fact Sheet. Nitrogen and the Hydrologic Cycle. Retrieved on ^ NASA's Earth Observatory. The Carbon Cycle. Retrieved on 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 " Categories: Hydrology Water Forms of water Soil physics This page was last modified on 19 September 2008, at 04:12.

15 All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.) Wikipedia is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) tax-deductible nonprofit charity. 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:

16 #1 = #2 = #3 = #4 = #5 =