Watershed

Watershed has traditionally designated the dividing line, or drainage divide, between two drainage basins; that is, the boundary or ridge of high land separating regions that are drained by different river systems or bodies of water (lake, sea, etc.). Some uses of the term keep this meaning, but in North American geographical usage watershed has come to be used interchangeably with the definition for drainage basin. In other words, watershed often refers to the entire region or area where all the waters drain into the same body of water, rather than just the elevation separating the waters flowing into different basins. Both are accepted definitions.

The term watershed comes from an old term shedding, meaning "splitting" or "dividing." It is commonly used in the English language to refer to a turning point or an event that may mark a change of course, such as a "watershed moment in history."

Terminology

A drainage basin is a region of land where water from rain or snow melt 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.

Other terms that can be used to describe the same concept are catchment, catchment area, catchment basin, drainage area, river basin and water basin.

One can subdivide basins as well. The Upper Paraguay River Basin is the upper sub-basin of the Paraguay Basin, which is part of the 2.8 million square kilometer Parana Basin (or Parana-Paraguay Basin). The Parana Basin in turn is a sub-basin of the Rio de la Plata Basin (a region that includes the Uruguay Sub-Basin, draining the Uruguay River, and the Salado Sub-Basin).

Each drainage basin is separated topographically from adjacent basins by a ridge, hill, or mountain, which is known as a water divide or a watershed. Water that lands on one side of that divide flow into one drainage basin and water that lands on another side flows into the other basin. For example, there is a slightly pronounced rise in Brazil, the "Chapada dos Parecis," that divides the headwaters of the Paraguay River from the headwaters of some Amazon tributaries. Water on one side flows into the Paraguay Basin and the other side into the Amazon basin. Farther east, the Chapada dos Guimaraes forms another physical barrier between the Amazon and Paraguay River Basins.

Especially in North American usage, however, watershed refers to the drainage basin itself.

In defining watershed, Langbein and Iseri (1995) of the United States Geological Survey note this ambiguity:

"Watershed. The divide separating one drainage basin from another and in the past has generally been used to convey this meaning. However, over the years, use of the term to signify drainage basin or catchment area has come to predominate, although drainage basin is preferred. Drainage divide, or just divide, is used to denote the boundary between one drainage area and another.

Geography

• Watershed (in Britain and some other Commonwealth countries), a drainage water divide, the ridge of land that separates two adjacent drainage basins
• Watershed (in North American usage), a drainage basin or river catchment, a region of land whose water drains into a particular watercourse
• The European Watershed, the line dividing the drainage basins of the major rivers of Europe
• The Humber Watershed, a drainage basin of south-central Ontario, Canada
• Watersheds of Indiana, six distinct Indiana drainage basins draining into five major bodies of water
• Taunton River Watershed, a drainage basin in southeastern Massachusetts, USA.
• List of watersheds, a list of drainage basins
• Watershed management, the management of drainage basins

Science and Medicine

• Watershed (algorithm), an algorithm for image segmentation
• Watershed Area (medical), a region of the body with overlapping blood supply
• Watershed Stroke, an infarct of a portion of the brain with overlapping blood supply

Organizations and institutions

• Watershed district, one of a number of government entities in the US state of Minnesota which monitor and regulate the use of water in drainage basins
• The Watershed Media Centre, a media and arts centre in Bristol, England.
• Watershed College, a school situated on the outskirts of Marondera town in Zimbabwe
• Santa Fe Watershed Association, a non-profit organization based in Santa Fe, New Mexico

History & Culture

• Watershed, a moment or event separating two distinct periods of time, a momentous event that alters the course of history
• Watershed (television), in Canada, Ireland, South Africa and the United Kingdom, a particular time of day before which program content of a specified or implied kind may not be screened and after which it is permissible
• Watershed (album), a 2008 album by singer k d lang
• Watershed (South African band), a South African band featuring lead singer Craig W Hinds
• Watershed (Columbus Ohio band), an American band based in Columbus, Ohio

Major drainage basins of the world

Map

Major continental divides, showing drainage into the major oceans and seas of the world. Grey areas are endorheic basins that do not drain to the ocean.

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

Ocean basins

There are many drainage basins throughout the world. The following is a list 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 Newfoundland and Labrador. It also directly drains nearly all of South America (that portion east of the Andes), northern Europe, and the greatest portion of western Sub-Saharan Africa. The three major mediterranean seas of the world also flow to the Atlantic: The basin of the American Mediterranean Sea (the Caribbean Sea and Gulf of Mexico) includes all of the American interior between the Appalachian and Rocky mountains, eastern Central America, and a small part of northern South America. The basin of the European Mediterranean Sea includes much of northeastern Africa (including Egypt, Libya, Sudan and the other countries of the Nile basin), southern and eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria. The basin of the Arctic Sea drains most of northern Canada and 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 Pacific Islands, the northeast coast of Australia, most of Alaska, the western part of Canada, the United States, Central America, and the South America (the smaller portion west of the Andes). 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, the Indian subcontinent, Burma, and most of Australia. The Southern Ocean drains Antarctica. Antarctica comprises approximately eight percent of the Earth's land.

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.

Importance of drainage basins

As 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.

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.

In 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.

In 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 body.

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.

In 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.

See also

• Drainage system
• Mainstem
• Reservoir

References

ISBN links support NWE through referral fees

• [1]
• [2]

General references

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

External links

• 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 drainage basins/watersheds, WRI
• Surf Your Watershed, EPA
• [3]
• Cycleau - A project looking at approaches to managing catchments in North West Europe
• American Water Resources Association

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