The 'water cycle' is an issue closely related to climate geoengineering technologies. OE
Article from USGS science for a changing world - Summanry of the Water Cycle
Summary of the Water Cycle
What is the water cycle?
A quick summary of the water cycle
Main components of the water cycle
Global water distribution
Water storage in oceans: Saline water existing in oceans and inland seas
The ocean as a storehouse of water
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
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
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
More about evaporation.
Sublimation: The changing of snow or ice to water vapor without melting
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.
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
- 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.
Water storage in the atmosphere: Water stored in the atmosphere as vapor, such as clouds and humidity
The atmosphere is full of water
More about water stored in the atmosphere.
Condensation: The process by which water is changed from vapor to liquid
Condensation in the air
You might ask ... why is it colder higher up?
More about condensation.
Precipitation: The discharge of water, in liquid or solid state, out of the atmosphere, generally upon a land or water surface
How do raindrops form?
Precipitation rates vary geographically and over time
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
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
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.
Snowmelt runoff to streams: The movement of water as surface runoff from snow and ice to surface water
Contribution of snowmelt to streamflow
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
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 riverThe 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 riversRivers 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
Streamflow is always changingStreamflow 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
Surface water keeps life going
Usable freshwater is relatively scarce
More about freshwater storage.
Infiltration: The downward movement of water from the land surface into soil or porous rock
Groundwater begins as precipitation
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 ground-water 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 ground-water 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
To find water, look under the table ... the water table
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 ground-water storage.
Groundwater discharge: The movement of water out of the ground
There's more water than just what you can see
Groundwater flows undergroundSome 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 ground-water 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 ground-water storage, where it might stay for much longer periods.
More about ground-water discharge.
Spring: A place where a concentrated discharge of groundwater flows at the ground surface
What is a spring?
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
More about springs.
Global water distribution
|Water source||Water volume, in cubic miles||Water volume, in cubic kilometers||Percent of|
|Oceans, Seas, & Bays||321,000,000||1,338,000,000||--||96.5|
|Ice caps, Glaciers, & Permanent Snow||5,773,000||24,064,000||68.7||1.74|
|Ground Ice & Permafrost||71,970||300,000||0.86||0.022|
|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).|