Drainage basin

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Image:Drainage basin.gif
A drainage basin is the area within the drainage basin divide (yellow outline), and drains the surface runoff and river discharge (blue lines) of a contiguous area.

A drainage basin (commonly referred to in North America as a watershed and Australia as a water catchment) is a region of land where water from rain or snowmelt drains downhill into a body of water, such as a river, lake, dam, 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. 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 or sometimes a watershed (in those parts of the world where the drainage basin itself is not called a watershed).

In addition to drainage basin and watershed, the following terms can be used to describe the same concept:

  • catchment, catchment area, or catchment basin
  • drainage area
  • river basin
  • water basin
Drainage basin of the Ohio River, part of the Mississippi River drainage basin.

The largest drainage basin in the world is the Amazon, covering 6,144,727 km² (a third of the area of South America).


[edit] Ocean drainage basins

Most of the Earth's surface drains into one of the five oceans. Basins that do not drain into an ocean are called endorheic (see below).

[edit] Endorheic drainage basins

Endorheic drainage basins are inland basins that do not drain into an ocean. 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] Geopolitical boundaries

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.

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

[edit] 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.

[edit] Ecology

Image:Mississippi River basin.jpg
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 body.

[edit] 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 Minnesota, U.S.A., 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] References

[edit] External links

cs:Povodí da:Afvandingsareal de:Einzugsgebiet (Hydrologie) eo:Akvokolekta areo es:Cuenca hidrográfica et:Valgla fr:Bassin versant gl:Conca fluvial he:אגן ניקוז id:Batas air it:Bacino idrografico ja:流域面積 nl:Stroomgebied no:Tilsigsfelt pl:Dorzecze pt:Bacia hidrográfica ru:Речной бассейн sv:Avrinningsområde tr:Havza uk:Водозбірний басейн vi:Lưu vực zh:流域

Drainage basin

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