Drainage basin

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Drainage basin. The dashed line is the main water divide of the hydrographic basin

A drainage basin is an extent of land where water from rain or snow melt drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. 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, separated from adjacent basins by a drainage divide,water Shed or Water table .^[1]

The drainage basin acts like a funnel - collecting all the water within the area covered by the basin and channeling it into a waterway. Each drainage basin is separated topographically from adjacent basins by a ridge, hill or mountain, which is known as a water divide. Other terms that can be used to describe the same concept are catchment, catchment area, catchment basin, drainage area, river basin and water basin.^[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 and Canada, the term is often used to mean a drainage basin or catchment area itself. Watersheds drain into other watersheds in a hierarchical form, larger ones breaking into smaller ones or sub-watersheds with the topography determining where the water flows. Understanding geomorphology is essential in understanding how watersheds interconnect.^[4]

[edit] Major drainage basins of the world

See also: List of drainage basins

[edit] Map

Drainage basins of the principal oceans and seas of the world. Grey areas are endorheic basins that do not drain to the ocean.

[edit] 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:

• The Atlantic Ocean drains approximately 47% of all land in the world. In North America, it directly drains the Saint Lawrence River and Great Lakes basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador. It also directly drains nearly all of South America east of the Andes, most of Western and Central Europe, and the greatest portion of western Sub-Saharan Africa. The three major mediterranean seas of the world also flow to the Atlantic:
  • The American Mediterranean Sea (the Caribbean Sea and Gulf of Mexico) basin includes most of the American interior between the Appalachian and Rocky Mountain ranges, a small part of the Canadian provinces of Alberta and Saskatchewan, eastern Central America, the islands of the Caribbean and the Gulf, and a small part of northern South America.
  • The European Mediterranean Sea basin includes much of northern Africa, east-central Africa (through the Nile), southern, central, and eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria.
  • The Arctic Ocean basin drains most of Western and Northern Canada east of the Continental Divide, 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.
• The Pacific Ocean drains just over 13% of the land in the world. 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 drains around 13% of the Earth's land. It drains the eastern coast of Africa, the coasts of the Red Sea and the Persian Gulf, Afghanistan and Pakistan, the Indian subcontinent, Burma, and most of Australia.
• The Southern Ocean drains Antarctica. Antarctica comprises approximately eight percent of the Earth's land.

[edit] Endorheic drainage basins

Endorheic basin in Central Asia

Main article: Endorheic basin

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.

[edit] Importance of drainage basins

[edit] Geopolitical boundaries

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Drainage basins have been important historically in determining 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 Indian Trade in the entire Hudson Bay watershed, an area called Rupert's Land. The company later acquired the North American watershed of the Arctic Ocean (the North-Western Territory). These lands later became part of Canada as the Northwest Territories, making up the vast majority of Canada's land area.

Currently the Sistan Basin stretches over the southern parts of Afghanistan and Iran.

Shangrila Lake in Skardu Pakistan.

Today, bioregional democracy can include agreements of states in a particular drainage basin to defend it. These include the Great Lakes Commission.

[edit] Hydrology

Drainage basin of the Ohio River, part of the Mississippi River drainage basin.

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.

[edit] Ecology

The Mississippi River drains the largest area of any U.S. river, much of it agricultural regions. Agricultural runoff and other water pollution that flows to the outlet is the cause of the dead zone in the Gulf of Mexico.

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.

[edit] Resource management

Further information: Watershed 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.

[edit] See also

• Drainage system
• Mainstem
• Reservoir

[edit] Footnotes

[edit] References

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

[edit] External links

• 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