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A drainage basin is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. The drainage basin includes all the surface water from rain runoff, snowmelt, and nearby streams that run downslope towards the shared outlet, as well as the groundwater underneath the earth's surface.[1] Drainage basins connect into other drainage basins at lower elevations in a hierarchical pattern, with smaller sub-drainage basins, which in turn drain into another common outlet.[2] Other terms used interchangeably with drainage basin are catchment area, catchment basin, drainage area, river basin, and water basin.[3] In North America, the term watershed is commonly used to mean a drainage basin, though in other English-speaking countries, it is used only in its original sense, that of a drainage divide. In a closed drainage basin, or endorheic basin, the water converges to a single point inside the basin, known as a sink, which may be a permanent lake, a dry lake, or a point where surface water is lost underground.[4] The drainage basin acts as a funnel by collecting all the water within the area covered by the basin and channelling it to a single point. Each drainage basin is separated topographically from adjacent basins by a perimeter, the drainage divide, making up a succession of higher geographical features (such as a ridge, hill or mountains) forming a barrier. Drainage basins are similar but not identical to hydrologic units, which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system. Hydrologic units are defined to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins.[4]

Contents

1 Major drainage basins of the world

1.1 Map 1.2 Ocean basins 1.3 Largest river basins 1.4 Endorheic drainage basins

2 Importance of drainage basins

2.1 Geopolitical boundaries 2.2 Hydrology 2.3 Geomorphology 2.4 Ecology 2.5 Resource management

3 Catchment factors

3.1 Topography 3.2 Shape 3.3 Size 3.4 Soil
Soil
type 3.5 Land use

4 See also 5 References

5.1 Citations 5.2 Sources

6 External links

Major drainage basins of the world[edit] See also: List of drainage basins by area Map[edit]

Drainage basins of the principal oceans and seas of the world. Grey areas are endorheic basins that do not drain to the oceans. Ocean basins[edit] The following is a list of the major ocean basins:

About 48.7% of the world's land drains to the Atlantic Ocean.[citation needed] In North America, surface water drains to the Atlantic via the Saint Lawrence River
River
and Great Lakes
Great Lakes
basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador. Nearly all of South America
South America
east of the Andes
Andes
also drains to the Atlantic, as does most of Western and Central Europe
Central Europe
and the greatest portion of western Sub-Saharan Africa, as well as Western Sahara and part of Morocco. The two major mediterranean seas of the world also flow to the Atlantic:

The Caribbean Sea
Caribbean Sea
and Gulf of Mexico
Gulf of Mexico
basin includes most of the U.S. interior between the Appalachian and Rocky Mountains, a small part of the Canadian provinces of Alberta
Alberta
and Saskatchewan, eastern Central America, the islands of the Caribbean and the Gulf, and a small part of northern South America. The Mediterranean Sea
Mediterranean Sea
basin includes much of North Africa, east-central Africa
Africa
(through the Nile River), Southern, Central, and Eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria.

The Arctic Ocean
Arctic Ocean
drains most of Western and Northern Canada
Northern Canada
east of the Continental Divide, northern Alaska
Alaska
and parts of North Dakota, South Dakota, Minnesota, and Montana
Montana
in the United States, the north shore of the Scandinavian peninsula
Scandinavian peninsula
in Europe, central and northern Russia, and parts of Kazakhstan
Kazakhstan
and Mongolia
Mongolia
in Asia, which totals to about 17% of the world's land. Just over 13% of the land in the world drains to the Pacific Ocean. Its basin includes much of China, eastern and southeastern Russia, Japan, the Korean Peninsula, most of Indochina, Indonesia and Malaysia, the Philippines, all of the Pacific Islands, the northeast coast of Australia, and Canada and the United States
United States
west of the Continental Divide (including most of Alaska), as well as western Central America
Central America
and South America
South America
west of the Andes. 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
Southern Ocean
drains Antarctica. Antarctica
Antarctica
comprises approximately eight percent of the Earth's land.

Largest river basins[edit] See also: List of drainage basins by area The five largest river basins (by area), from largest to smallest, are the basins of the Amazon (7M km2), the Congo (4M km2), the Nile (3.4M km2), the Río de la Plata
Río de la Plata
(3.2M km2), and the Mississippi (3M km2). The three rivers that drain the most water, from most to least, are the Amazon, Ganga, and Congo rivers.[5] Endorheic drainage basins[edit]

Endorheic basin
Endorheic basin
in Central Asia

Main article: Endorheic basin Endorheic drainage basins are inland basins that do not drain to an ocean. Around 18% of all land drains to endorheic lakes or seas or sinks. The largest of these consists of much of the interior of Asia, which drains into the Caspian Sea, the Aral Sea, and numerous smaller lakes. Other endorheic regions include the Great Basin
Great Basin
in the United States, much of the Sahara Desert, the drainage basin of the Okavango River
River
(Kalahari Basin), highlands near the African Great Lakes, the interiors of Australia
Australia
and the Arabian Peninsula, and parts in Mexico and the Andes. Some of these, such as the Great Basin, are not single drainage basins but collections of separate, adjacent closed basins. 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 of this is the Dead Sea. Importance of drainage basins[edit] Geopolitical boundaries[edit] 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
Bay
Company a monopoly on the fur trade in the entire Hudson Bay basin, an area called Rupert's Land. Bioregional political organization today includes agreements of states (e.g., international treaties and, within the U.S.A., interstate compacts) or other political entities in a particular drainage basin to manage the body or bodies of water into which it drains. Examples of such interstate compacts are the Great Lakes
Great Lakes
Commission and the Tahoe Regional Planning Agency. Hydrology[edit]

Drainage basin
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
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
Thiessen polygon
method, the drainage basin 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. Isochrone maps can be used to show the time taken for runoff water within a drainage basin to reach a lake, reservoir or outlet, assuming constant and uniform effective rainfall.[6][7][8][9] Geomorphology[edit] Drainage basins are the principal hydrologic unit considered in fluvial geomorphology. A drainage basin is the source for water and sediment that moves from higher elevation through the river system to lower elevations as they reshape the channel forms. Ecology[edit]

The Mississippi River
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 hypoxic, or dead zone in the Gulf of Mexico.

Drainage basins are important in ecology. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants. With the water, they are 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 use of artificial fertilizers, containing nitrogen, phosphorus, and potassium, has affected the mouths of drainage basins. The minerals are carried by the drainage basin to the mouth, and may accumulate there, disturbing the natural mineral balance. This can cause eutrophication where plant growth is accelerated by the additional material. Resource management[edit] Further information: Watershed management Because drainage basins are coherent entities in a hydro-logical sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state
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 the territorial division of Brazilian water management. When a river basin crosses at least one political border, either a border within a nation or an international boundary, it is identified as a transboundary river. Management of such basins becomes the responsibility of the countries sharing it. Nile Basin Initiative, OMVS for Senegal River, Mekong River
River
Commission are a few examples of arrangements involving management of shared river basins. Management of shared drainage basins is also seen as a way to build lasting peaceful relationships among countries.[10] Catchment factors[edit] 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[edit] Generally, topography plays a big part in how fast runoff will reach a river. Rain
Rain
that falls in steep mountainous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient). Shape[edit] 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[edit] Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It also determined on the basis of length and width of the drainage basin. Soil
Soil
type[edit] Soil
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. If the surface is impermeable the precipitation will create surface run-off which will lead to higher risk of flooding; if the ground is permeable, the precipitation will infiltrate the soil. Land use[edit] 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[edit]

Wetlands portal

Integrated catchment management Interbasin transfer International Journal of River
River
Basin Management (JRBM) International Network of Basin Organizations Main stem River
River
Basin Management Plans River
River
bifurcation Tenaja Time of concentration

References[edit] Citations[edit]

^ "drainage basin". The Physical Environment. University of Wisconsin–Stevens Point. Archived from the original on March 21, 2004.  ^ "What is a watershed and why should I care?". university of delaware. Retrieved 2008-02-11.  ^ Lambert, David (1998). The Field Guide to Geology. Checkmark Books. pp. 130–13. ISBN 0-8160-3823-6.  ^ a b "Hydrologic Unit Geography". Virginia Department of Conservation & Recreation. Archived from the original on 14 December 2012. Retrieved 21 November 2010.  ^ Encarta Encyclopedia articles on Amazon River, Congo River, and Ganges
Ganges
Published by Microsoft in computers. ^ Bell, V. A.; Moore, R. J. (1998). "A grid-based distributed flood forecasting model for use with weather radar data: Part 1. Formulation" (PDF). Hydrology
Hydrology
and Earth System Sciences. Copernicus Publications. 2 (2/3): 265–281. doi:10.5194/hess-2-265-1998.  ^ Subramanya, K (2008). Engineering Hydrology. Tata McGraw-Hill. p. 298. ISBN 0-07-064855-7.  ^ "EN 0705 isochrone map". UNESCO. Archived from the original on November 22, 2012. Retrieved March 21, 2012.  ^ "Isochrone map". Webster's Online Dictionary. Retrieved March 21, 2012. [permanent dead link] ^ "Articles". www.strategicforesight.com. 

Sources[edit]

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

External links[edit]

Science week catchment factsheet Catchment Modelling Toolkit Water Evaluation And Planning System (WEAP) - modeling hydrologic processes in a drainage basin New Mexico
Mexico
State University - Water Task Force Recommended Watershed Terminology Watershed Condition Classification Technical Guide United States Forest Service 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 flash animation of how rain falling onto the landscape will drain into a river depending on the terrain StarHydro – software tool that covers concepts of fluvial geomorphology and watershed hydrology EPA Surf your watershed Florida Watersheds and River
River
Basins - Florida DEP

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