A drainage basin is an extent or area of land where water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam from rain Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the condensation of atmospheric water vapor into drops of water heavy and melting snow In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many parts of the world, in some cases contributing high fractions of the annual runoff in a watershed or ice Ice, technically, is one of the 15 known crystalline phases of water. In non-scientific contexts, the term usually means ice Ih, which is known to be the most abundant of these solid phases. It can appear transparent or opaque bluish-white colour, depending on the presence of impurities or air inclusions. The addition of other materials such as drains downhill into a body of water, such as a river A river is a natural watercourse, usually freshwater, flowing toward an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including stream, creek, brook, rivulet, and rill; there is no, lake A lake is a terrain feature , a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is not global). Another definition is a body of fresh or salt water of considerable size that is surrounded by land. On Earth a body of water is considered a lake when it is inland,, reservoir A reservoir is an artificial lake used to store water. Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such a brickwork or cast concrete, estuary An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea, wetland A wetland is an area of land which soil is saturated with moisture either permanently or seasonally. Such areas may also be covered partially or completely by shallow pools of water. Wetlands include swamps, marshes, and bogs, among others. The water found in wetlands can be saltwater, freshwater, or brackish. The world's largest wetland is the, sea A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, the term refers to a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean. It is also used sometimes to describe a large saline lake that lacks a natural outlet, such as the Caspian Sea or ocean An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 1014 m2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide A drainage divide, water divide, divide or watershed is the line separating neighbouring drainage basins (catchments). In hilly country, the divide lies along topographical peaks and ridges, but in flat country (especially where the ground is marshy) the divide may be invisible – just a more or less notional line on the ground on either side of.^[1]

The drainage basin acts as a funnel A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur by collecting all the water within the area covered by the basin and channelling it into a waterway. Each drainage basin is separated topographically from adjacent basins by a geographical barrier such as a ridge A ridge is a geological feature that features a continuous elevational crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:, hill A hill is a landform that extends above the surrounding terrain. Hills often have a distinct summit, although in areas with scarp/dip topography a hill may refer to a particular section of flat terrain without a massive summit or mountain A mountain is a large landform that stretches above the surrounding land in a limited area usually in the form of a peak. A mountain is generally steeper than a hill. The adjective montane is used to describe mountainous areas and things associated with them. The study of mountains is called Orography, which is known as a water divide A drainage divide, water divide, divide or watershed is the line separating neighbouring drainage basins (catchments). In hilly country, the divide lies along topographical peaks and ridges, but in flat country (especially where the ground is marshy) the divide may be invisible – just a more or less notional line on the ground on either side of.

Other terms that are used to describe a drainage basin are catchment, catchment area, catchment basin, drainage area, river basin, water basin and watershed.^[2] In the technical sense, a watershed refers to a divide that separates one drainage area from another drainage area.^[3] However, in the United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language and Canada The land occupied by Canada was inhabited for millennia by various groups of Aboriginal peoples. Beginning in the late 15th century, British and French expeditions explored, and later settled, along the Atlantic coast. France ceded nearly all of its colonies in North America in 1763 after the Seven Years' War. In 1867, with the union of three, the term is often used to mean a drainage basin or catchment area itself. Drainage basins drain into other drainage basins in a hierarchical pattern, with smaller sub-drainage basins combining into larger drainage basins.^[4]

The United States Environmental Protection Agency The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged to protect human health and the environment, by writing and enforcing regulations based on laws passed by Congress. The EPA was proposed by President Richard Nixon and began operation on December 2, 1970, when its establishment was passed launched the website Watershed Central for the US public to exchange information and locate resources needed to restore local drainage basins in that country.

Major drainage basins of the world

Map

Drainage basins of the principal oceans and seas of the world. Grey areas are endorheic An endorheic basin is a closed drainage basin that retains water and allows no outflow to other bodies of water such as rivers or oceans. Normally, water that has accrued in a drainage basin eventually flows out through rivers or streams on Earth's surface or by underground diffusion through permeable rock, ultimately ending up in the oceans basins that do not drain to the ocean.

Ocean basins

There are numerous drainage basins throughout the world in all sorts of places. The following is a list of some of the major ones:

• About 47% of all land in the world drains to the Atlantic Ocean The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106,400,000 square kilometres , it covers approximately twenty percent of the Earth's surface and about twenty-six percent of its water surface area. The first part of its name refers to the Atlas of Greek mythology, making the Atlantic the ". In North America, it directly drains the Saint Lawrence River The Saint Lawrence River is a large river flowing approximately from southwest to northeast in the middle latitudes of North America, connecting the Great Lakes with the Atlantic Ocean. It is the primary drainage of the Great Lakes Basin. It traverses the Canadian provinces of Quebec and Ontario and forms part of the international boundary between and Great Lakes The Great Lakes are a collection of freshwater seas located in eastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron , Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface and volume. The total surface is 208,610 km2 (80,545 sq mi), and the total volume basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador A former colony and dominion of the United Kingdom, it became the tenth province to enter the Canadian Confederation on 31 March 1949, named simply as Newfoundland. Since 1964, the province's government has referred to itself as the Government of Newfoundland and Labrador, and on 6 December 2001, an amendment was made to the Constitution of Canada. It also directly drains nearly all of South America South America is the southern continent of America, situated in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest east of the Andes The Andes are the world's longest continental mountain range. It is a continuous chain of highland along the western coast of South America. The range is over 7,000 km long, 200 km (120 mi) to 700 km (430 mi) wide (widest between 18° to 20°S latitude), and of an average height of about 4,000 m (13,000 ft), most of Western and Central Europe Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus region (Specification of borders) and the Black Sea to the southeast. Europe is bordered by the, and the greatest portion of western Sub-Saharan Africa Sub-Saharan Africa is a geographical term used to describe the area of the African continent which lies south of the Sahara, or those African countries which are fully or partially located south of the Sahara. It contrasts with North Africa, which is considered a part of the Arab world. The three major mediterranean seas In oceanography, a mediterranean sea is a mostly enclosed sea that has limited exchange of deep water with outer oceans and where the water circulation is dominated by salinity and temperature differences rather than winds of the world also flow to the Atlantic:
  • The American Mediterranean Sea (the Caribbean Sea The Caribbean Sea is a sea of the Atlantic Ocean situated in the tropics of the Western hemisphere. It is bounded to the southwest by the Central American countries of Panama, to the west by Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and Mexico, to the north by The Greater Antilles , and to the east by the Lesser Antilles and Gulf of Mexico The Gulf of Mexico is the eleventh largest body of water in the world. Considered a smaller part of the Atlantic Ocean, it is an ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and) basin includes most of the American ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language interior between the Appalachian and Rocky Mountain ranges, a small part of the Canadian The land occupied by Canada was inhabited for millennia by various groups of Aboriginal peoples. Beginning in the late 15th century, British and French expeditions explored, and later settled, along the Atlantic coast. France ceded nearly all of its colonies in North America in 1763 after the Seven Years' War. In 1867, with the union of three provinces of Alberta Alberta is located in western Canada, bounded by the provinces of British Columbia to the west and Saskatchewan to the east, the Northwest Territories to the north, and the U.S. state of Montana to the south. Alberta is one of three Canadian provinces and territories to border only a single U.S. state . It is also one of only two Canadian and Saskatchewan Other major cities, in order of size, are Prince Albert, Moose Jaw, Yorkton, Swift Current and North Battleford. The province's name comes from the Saskatchewan River, whose name comes from its Cree designation: ᑭᓯᐢᑳᒋᐘᓂ ᓯᐱᐩ , meaning "swift flowing river", eastern Central America Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. Central America is considered to be part of the Mesoamerican biodiversity hotspot, excluding the southern portions of Panama, the islands of the Caribbean and the Gulf, and a small part of northern South America South America is the southern continent of America, situated in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest.
  • The European Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by Africa, and on the east by the Levant. The sea is technically a part of the Atlantic Ocean, although it is usually identified as a completely separate basin includes much of northern Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population, east-central Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population (through the Nile The Nile is a major north-flowing river in Africa, generally regarded as the second longest river in the world), southern, central, and eastern Europe Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus region (Specification of borders) and the Black Sea to the southeast. Europe is bordered by the, Turkey Turkey , known officially as the Republic of Turkey ( Türkiye Cumhuriyeti (help·info)), is a Eurasian country situated in the Anatolian peninsula, located in Western Asia, and Eastern Thrace, located in southeastern Europe. Turkey is one of the six independent Turkic states. Turkey is bordered by eight countries: Bulgaria to the northwest;, and the coastal areas of Israel Israel , officially the State of Israel (Hebrew: מְדִינַת יִשְׂרָאֵל (help·info), Medīnat Yisrā'el; Arabic: دَوْلَةُ إِسْرَائِيلَ‎, Dawlat Isrā'īl), is a country in Western Asia located on the eastern shore of the Mediterranean Sea. It borders Lebanon in the north, Syria in the northeast, Jordan and the, Lebanon Lebanon (pronounced /ˈlɛbənɒn/ or /ˈlɛbənən/; Arabic: لُبْنَان‎ Lubnān; French: Liban), officially the Republic of Lebanon[nb 1] (Arabic: اَلْجُمْهُورِيَّة اَللُّبْنَانِيَّة al-Jumhūrīyah al-Lubnānīyah; French: République libanaise), is a country on the eastern shore of the Mediterranean, and Syria Syria , officially the Syrian Arab Republic (Arabic: الجمهورية العربية السورية‎), is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest.
  • The Arctic Ocean The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest, and shallowest of the world's five major oceanic divisions. The International Hydrographic Organization recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying basin drains most of Western and Northern Canada The land occupied by Canada was inhabited for millennia by various groups of Aboriginal peoples. Beginning in the late 15th century, British and French expeditions explored, and later settled, along the Atlantic coast. France ceded nearly all of its colonies in North America in 1763 after the Seven Years' War. In 1867, with the union of three east of the Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not connected to the open sea. The endpoints where a continental divide meets the coast are not always, the north shore of Alaska and parts of North Dakota, South Dakota, Minnesota, and Montana in the United States, the north shore of the Scandinavian peninsula in Europe, and much of central and northern Russia.
• Just over 13% of the land in the world drains to the Pacific Ocean. Its basin includes much of China, southeastern Russia, Japan, Korea, most of Indonesia and Malaysia, the Philippines, all of the Pacific Islands, the northeast coast of Australia, and the western parts of Canada, the United States (including most of Alaska), Central America, and South America.
• The Indian Ocean's drainage basin also comprises about 13% of Earth's land. It drains the eastern coast of Africa, the coasts of the Red Sea and the Persian Gulf, the Indian subcontinent, Burma, and most of Australia.
• The Southern Ocean drains Antarctica. Antarctica comprises approximately eight percent of the Earth's land.

Largest river basins

The three largest river basins (by area), from largest to smallest, are the Amazon basin, the Congo basin, and the Mississippi basin.^[5] The three rivers that drain the most water, from most to least, are the Amazon, Congo, and Ganges Rivers.^[6]

Endorheic drainage basins

Endorheic drainage basins are inland basins that do not drain into an ocean; around 18% of all land drains to endorheic lakes or seas. The largest of these consists of much of the interior of Asia, and drains into the Caspian Sea and the Aral Sea. Other basins include the Great Basin in the United States, much of the Sahara Desert, the watershed of the Okavango River (Kalahari Basin), highlands near the African Great Lakes, the interiors of Australia and the Arabian Peninsula, and parts in Mexico and the Andes.

In endorheic bodies of standing water where evaporation is the primary means of water loss, the water is typically more saline than the oceans. An extreme example is the Dead Sea.

Importance of drainage basins

Geopolitical boundaries

Drainage basins have been historically important for determining territorial boundaries, particularly in regions where trade by water has been important. For example, the English crown gave the Hudson's Bay Company a monopoly on the fur trade in the entire Hudson Bay watershed, an area called Rupert's Land. Today, bioregional democracy can include agreements of states in a particular drainage basin to defend it. One example of this is the Great Lakes Commission.

Hydrology

In hydrology, the drainage basin is a logical unit of focus for studying the movement of water within the hydrological cycle, because the majority of water that discharges from the basin outlet originated as precipitation falling on the basin. A portion of the water that enters the groundwater system beneath the drainage basin may flow towards the outlet of another drainage basin because groundwater flow directions do not always match those of their overlying drainage network. Measurement of the discharge of water from a basin may be made by a stream gauge located at the basin's outlet.

Rain gauge data is used to measure total precipitation over a drainage basin, and there are different ways to interpret that data. If the gauges are many and evenly distributed over an area of uniform precipitation, using the arithmetic mean method will give good results. In the Thiessen polygon method, the watershed is divided into polygons with the rain gauge in the middle of each polygon assumed to be representative for the rainfall on the area of land included in its polygon. These polygons are made by drawing lines between gauges, then making perpendicular bisectors of those lines form the polygons. The isohyetal method involves contours of equal precipitation are drawn over the gauges on a map. Calculating the area between these curves and adding up the volume of water is time consuming.

Geomorphology

Drainage basins are the principal hydrologic unit considered in fluvial geomorphology. A drainage basin is the source for water and sediment that moves through the river system and reshapes the channel.

Ecology

Drainage basins are important elements to consider also in ecology. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants. Like the water, they get transported towards the outlet of the basin, and can affect the ecological processes along the way as well as in the receiving water source.

Modern usage of artificial fertilizers, containing nitrogen, phosphorus, and potassium, has affected the mouths of watersheds. The minerals will be carried by the watershed to the mouth and accumulate there, disturbing the natural mineral balance. This can cause eutrophication where plant growth is accelerated by the additional material.

Resource management

Because drainage basins are coherent entities in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state of Minnesota, governmental entities that perform this function are called watershed districts. In New Zealand, they are called catchment boards. Comparable community groups based in Ontario, Canada, are called conservation authorities. In North America this function is referred to as watershed management. In Brazil, the National Policy of Water Resources, regulated by Act n° 9.433 of 1997, establishes the drainage basin as territorial division of Brazilian water management.

Catchment factors

The catchment is the most significant factor determining the amount or likelihood of flooding.

Catchment factors are: topography, shape, size, soil type and land use (paved or roofed areas). Catchment topography and shape determine the time taken for rain to reach the river, while catchment size, soil type and development determine the amount of water to reach the river.

Topography

Topography determines the speed with which the runoff will reach a river. Clearly rain that falls in steep mountainous areas will reach the river faster than flat or gently sloping areas.

Shape

Shape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment.

Size

Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding.

Soil type

Soil type will help determine how much water reaches the river. Certain soil types such as sandy soils are very free draining and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground.

Land use

Land use can contribute to the volume of water reaching the river, in a similar way to clay soils. For example, rainfall on roofs, pavements and roads will be collected by rivers with almost no absorption into the groundwater.

See also

• Drainage system
• Drainage system (Geomorphology)
• Mainstem
• Reservoir
• Integrated catchment management
• International Journal of River Basin Management (JRBM)
• Catchment Management Authority
• River Basin Management Plans
• Time of Concentration

Footnotes

1. ^ "drainage basin". www.uwsp.edu. http://www.uwsp.edu/geo/faculty/ritter/glossary/a_d/drainage_basin.html. Retrieved 2008-02-21.
2. ^ Lambert, David (1998). The Field Guide to Geology. Checkmark Books. pp. 130–13. ISBN 0-8160-3823-6.
3. ^ "Recommended Watershed Terminology". watershed.org. http://watershed.org/news/fall_94/terminology.html. Retrieved 2008-02-10.
4. ^ "What is a watershed and why should I care?". university of delaware. http://www.wr.udel.edu/cb/whatwhycare.html. Retrieved 2008-02-11.
5. ^ Coupe Jr.; Goolsby, Richard; Donald; Donald A. Goolsby (June 7, 1999). "NASQAN Mississippi River Basin Fact Sheet". Geology. NASQAN. pp. 1. http://water.usgs.gov/nasqan/progdocs/factsheets/missfacts/missfs.html. Retrieved 2008-06-24.
6. ^ Encarta Encyclopedia articles on Amazon River, Congo River, and Ganges Published by Microsoft in computers.

References

• DeBarry,Paul A. (2004). Watersheds: Processes, Assessment and Management. John Wiley & Sons.

External links

• Science week catchment factsheet
• Catchment Modelling Toolkit
• eWater CRC
• New Mexico State University - Water Task Force
• Recommended Watershed Terminology
• A detailed map of watersheds in North America
• Science in Your Watershed, USGS
• Studying Watersheds: A Confluence of Important Ideas
• Water Sustainability Project Sustainable water management through demand management and ecological governance, with the POLIS Project at the University of Victoria
• Map of the Earth's primary watersheds, WRI
• What is a watershed and why should I care?
• Cycleau - A project looking at approaches to managing catchments in North West Europe
• American Water Resources Association
• Map of Watersheds.gif Watersheds of North America
• flash animation of how rain falling on to the landscape will drain into a river depending on the terrain
• StarHydro – software tool that covers concepts of fluvial geomorphology and watershed hydrology
• USGS Surf your watershed
• Florida Watersheds and River Basins - Florida DEP

Categories: Watersheds | Hydrology | Rivers | Fluvial landforms | Geomorphology

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