View source for Earth:Drainage basin

{{Short description|Land area where water converges to a common outlet}}
[[File:Major River Systems within the Mississippi River Basin.svg|thumb|upright=1.35|The Mississippi River drains the largest area of any {{wipe|United States}} 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.]]

A '''drainage basin''' is an area of land where all flowing [[Earth:Surface water|surface water]] converges to a single point, such as a [[Physics:River mouth|river mouth]], or flows into another [[Earth:Body of water|body of water]], such as a [[Earth:Lake|lake]] or [[Earth:Ocean|ocean]]. A basin is separated from adjacent basins by a perimeter, the ''[[Earth:Drainage divide|drainage divide]]'',<ref>{{cite web |url= http://www.uwsp.edu/geo/faculty/ritter/glossary/a_d/drainage_basin.html |title=drainage basin |work=The Physical Environment |publisher=University of Wisconsin–Stevens Point |archive-url=https://web.archive.org/web/20040321033433/http://www.uwsp.edu/geo/faculty/ritter/glossary/a_d/drainage_basin.html |archive-date=March 21, 2004 }}</ref> made up of a succession of elevated features, such as [[Earth:Ridge|ridge]]s and [[Earth:Hill|hill]]s. A basin may consist of smaller basins that merge at river [[Earth:Confluence|confluence]]s, forming a [[Strahler number|hierarchical pattern]].<ref name=whatis>{{cite web |url=http://www.wr.udel.edu/cb/whatwhycare.html |title=What is a watershed and why should I care? |publisher=University of Delaware |access-date=2008-02-11 |archive-url=https://web.archive.org/web/20120121122519/http://www.wr.udel.edu/cb/whatwhycare.html |archive-date=2012-01-21 |url-status=dead }}</ref>

Other terms for a drainage basin are '''catchment area''', '''catchment basin''', '''drainage area''', '''river basin''', '''water basin''',<ref>{{cite book |last=Lambert |first=David |title=The Field Guide to Geology |publisher=Checkmark Books |year=1998 |pages=[https://archive.org/details/fieldguidetogeol00lamb_0/page/130 130–13] |isbn=0-8160-3823-6 |url-access=registration |url=https://archive.org/details/fieldguidetogeol00lamb_0/page/130}}</ref><ref>{{cite journal |last1=Uereyen |first1=Soner |last2=Kuenzer |first2=Claudia |title=A Review of Earth Observation-Based Analyses for Major River Basins |journal=Remote Sensing |date=9 December 2019 |volume=11 |issue=24 |pages=2951 |doi=10.3390/rs11242951 |bibcode=2019RemS...11.2951U |doi-access=free }}</ref> and [[Engineering:Impluvium|impluvium]].<ref name=":1">{{cite journal |last1=Huneau|first1=F. |last2=Jaunat |first2=J. |last3=Kavouri |first3=K. |last4=Plagnes |first4=V.| last5=Rey |first5=F. |last6=Dörfliger |first6=N. |date= 2013-07-18 |publisher= Elsevier |title= Intrinsic vulnerability mapping for small mountainous [[Earth:Karst|karst]] aquifers, implementation of the new PaPRIKa method to Western Pyrenees (France) |journal= Engineering Geology |volume= 161 |pages=81–93 |quote= Efficient management is strongly correlated to the proper protection perimeter definition around springs and proactive regulation of land uses over the spring's catchment area ("impluvium"). |doi=10.1016/j.enggeo.2013.03.028 |bibcode=2013EngGe.161...81H }}</ref><ref>{{cite web |url= https://www.encyclopedie-environnement.org/en/water/natural-mineral-waters/#5_Protected_and_managed_over_time |title= Natural mineral waters |last=Lachassagne |first=Patrick |publisher=Encyclopédie de l'environnement |date=2019-02-07 |access-date=2019-06-10 |quote= In order to preserve the long-term stability and purity of natural mineral water, bottlers have put in place "protection policies" for the impluviums (or catchment areas) of their sources. The catchment area is the territory on which the part of precipitated rainwater and/or [[Earth:Snowmelt|snowmelt]] that infiltrates the subsoil feeds the mineral aquifer and thus contributes to the renewal of the resource. In other words, a precipitated drop on the impluvium territory may join the mineral aquifer; ...}}</ref><ref>{{cite journal |doi=10.1016/S0022-1694(00)00321-8 |last1=Labat |first1=D. |last2=Ababou |first2=R. |last3=Manginb |first3=A. |title=Rainfall–runoff relations for [[Earth:Karst|karst]]ic springs. Part I: convolution and spectral analyses |journal= Journal of Hydrology |volume=238 |issue=3–4 |pages=123–148 |date=2000-12-05 |quote= The non-karstic impluvium comprises all elements of the ground surface and soils that are poorly permeable, on a part of which water is running while also infiltrating on another minor part. This superficial impluvium, if it exists, constitutes the first level of organization of the drainage system of the karstic basin. |bibcode=2000JHyd..238..123L}}</ref> In North America, they are commonly called a '''watershed''', though in other English-speaking places, "watershed" is used only in its original sense, that of a drainage divide.

A drainage basin's boundaries are determined by [[Earth:Watershed delineation|watershed delineation]], a common task in environmental engineering and science. 

In a closed drainage basin, or [[Earth:Endorheic basin|endorheic basin]], rather than flowing to the ocean, water converges toward the interior of the basin, known as a [[Earth:Sink (geography)|sink]], which may be a permanent lake, a [[Earth:Dry lake|dry lake]], or a point where surface water is [[Earth:Losing stream|lost underground]].<ref name=virginia>{{cite web |url= http://www.dcr.virginia.gov/stormwater_management/hu.shtml |title= Hydrologic Unit Geography |publisher= Virginia Department of Conservation & Recreation |access-date= 21 November 2010 |url-status= dead |archive-url= https://archive.today/20121214100316/http://www.dcr.virginia.gov/stormwater_management/hu.shtml |archive-date= 14 December 2012 }}</ref>

Drainage basins are similar but not identical to [[Earth:Hydrological code|hydrologic unit code]], which are drainage areas delineated so as to nest into a multi-level hierarchical [[Earth:Drainage system (geomorphology)|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.<ref name=virginia/>

== Major drainage basins of the world ==
[[File:Ocean drainage.png|thumb|right|upright=3| Major continental divides, showing how terrestrial drainage basins drain into the oceans. Grey areas are endorheic basins that do not drain to the oceans]]

=== Ocean basins ===
About 48.71% of the world's land drains to the [[Earth:Atlantic Ocean|Atlantic Ocean]].{{citation needed|date=January 2017}} In [[Earth:North America|North America]], surface water drains to the Atlantic via the Saint Lawrence River and Great Lakes basins, the Eastern Seaboard of the United States, the Canadian Maritimes, and most of Newfoundland and Labrador. Nearly all of [[Earth:South America|South America]] east of the [[Place:Andes|Andes]] also drains to the Atlantic, as does most of [[Place:Western Europe|Western]] and [[Place:Central Europe|Central Europe]] and the greatest portion of western [[Place:Sub-Saharan Africa|Sub-Saharan Africa]], as well as Western Sahara and part of Morocco. 

The two major [[Earth:Mediterranean sea (oceanography)|mediterranean seas]] of the world also flow to the Atlantic. The Caribbean Sea and 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 and Saskatchewan, eastern Central America, the islands of the Caribbean and the Gulf, and a small part of northern South America. The [[Place:Mediterranean Sea|Mediterranean Sea]] basin, with the Black Sea, includes much of [[Place:North Africa|North Africa]], east-central [[Earth:Africa|Africa]] (through the Nile River), [[Place:Southern Europe|Southern]], Central, and [[Place:Eastern Europe|Eastern Europe]], [[Place:Turkey|Turkey]], and the coastal areas of [[Place:Israel|Israel]], [[Place:Lebanon|Lebanon]], and [[Place:Syria|Syria]].

The [[Earth:Arctic Ocean|Arctic Ocean]] drains most of Western Canada and [[Place:Northern Canada|Northern Canada]] east of the Continental Divide, northern [[Earth:Alaska|Alaska]] and parts of North Dakota, South Dakota, Minnesota, and Montana in the United States, the north shore of the Scandinavian peninsula in Europe, central and northern Russia, and parts of Kazakhstan and Mongolia in [[Earth:Asia|Asia]], which totals to about 17% of the world's land.<ref name=":0">{{Cite journal|last1=Vörösmarty|first1=C. J.|last2=Fekete|first2=B. M.|last3=Meybeck|first3=M.|last4=Lammers|first4=R. B.|date=2000|title=Global system of rivers: Its role in organizing continental land mass and defining land-to-ocean linkages|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999GB900092|journal=Global Biogeochemical Cycles|language=en|volume=14|issue=2|pages=599–621|doi=10.1029/1999GB900092|bibcode=2000GBioC..14..599V|s2cid=129463497 |issn=1944-9224|access-date=2021-08-15|archive-date=2021-08-15|archive-url=https://web.archive.org/web/20210815070038/https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/1999GB900092|url-status=dead|doi-access=free}}</ref>

Just over 13% of the land in the world drains to the [[Earth:Pacific Ocean|Pacific Ocean]].<ref name=":0" /> 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 {{wipe|Australia}}, and Canada and the United States west of the Continental Divide (including most of Alaska), as well as western Central America and South America west of the Andes.

The [[Earth:Indian Ocean|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 [[Earth:Indian subcontinent|Indian subcontinent]], Burma, and most parts of {{wipe|Australia}}.<ref>{{cite news |title=Largest Drainage Basins in the World |url=https://www.worldatlas.com/articles/largest-drainage-basins-in-the-world.html |work=WorldAtlas |date=17 May 2018}}</ref>

=== Largest river basins ===
The five largest river basins (by area), from largest to smallest, are those of the Amazon (7M km{{sup|2}}), the Congo (4M km{{sup|2}}), the Nile (3.4M km{{sup|2}}), the Mississippi (3.22M km{{sup|2}}), and the {{Lang|es|Río de la Plata|italic=no}} (3.17M km{{sup|2}}). The three rivers that drain the most water, from most to least, are the Amazon, Ganges, and Congo rivers.<ref>Encarta Encyclopedia articles on Amazon River, Congo River, and Ganga Published by Microsoft in computers.</ref>

=== Endorheic drainage basins ===
[[File:Uureg Nuur.jpg|thumb|Endorheic basin in Central Asia]]
{{Main|Earth:Endorheic basin}}

[[Earth:Endorheic basin|Endorheic basin]] are inland basins that do not drain to an ocean. Endorheic basins cover around 18% of the Earth's land. Some endorheic basins drain to an [[Earth:Endorheic lake|Endorheic lake]] or [[Earth:Inland sea|Inland sea]]. Many of these lakes are ephemeral or vary dramatically in size depending on climate and inflow. If water evaporates or infiltrates into the ground at its terminus, the area can go by several names, such playa, salt flat, [[Earth:Dry lake|dry lake]], or [[Earth:Alkali sink|alkali sink]].  

The largest endorheic basins are in Central [[Earth:Asia|Asia]], including the Caspian Sea, the Aral Sea, and numerous smaller lakes. Other endorheic regions include the Great Basin in the United States, much of the Sahara Desert, the drainage basin of the Okavango River (Kalahari Basin), highlands near the [[Place:African Great Lakes|African Great Lakes]], the interiors of {{wipe|Australia}} and the [[Place:Arabian Peninsula|Arabian Peninsula]], and parts in Mexico and the [[Place:Andes|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 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 [[Earth:Dead Sea|Dead Sea]].{{citation needed||date=April 2023}}

== Importance ==

=== 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 {{wipe|England}} crown gave the Hudson's Bay Company a monopoly on the fur trade in the entire Hudson Bay basin, an area called Rupert's Land. [[Earth:Bioregionalism|Bioregional]] political organization today includes agreements of states (e.g., international treaties and, within the US, 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 Commission and the Tahoe Regional Planning Agency.

=== Hydrology ===
[[File:Ohiorivermap.png|thumb|right|Drainage basin of the Ohio River, part of the Mississippi River drainage basin]]
In [[Earth:Hydrology|hydrology]], the drainage basin is a logical unit of focus for studying the movement of water within the hydrological cycle. The process of finding a drainage boundary is referred to as [[Earth:Watershed delineation|watershed delineation]]. Finding the area and extent of a drainage basin is an important step in many areas of science and engineering. 

The majority of water that discharges from the basin outlet originated as precipitation falling on the basin.<ref>{{Cite web|title=drainage basin Definition, Example, & Facts|url=https://www.britannica.com/science/drainage-basin|access-date=2021-10-22|website=Encyclopedia Britannica|language=en}}</ref> A portion of the water that enters the [[Earth:Groundwater|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 [[Earth:Stream gauge|stream gauge]] located at the basin's outlet. Depending on the conditions of the drainage basin, as rainfall occurs some of it seeps directly into the ground. This water will either remain underground, slowly making its way downhill and eventually reaching the basin, or it will permeate deeper into the soil and consolidate into groundwater aquifers.<ref>{{Cite web|title=Watersheds and Drainage Basins|url=https://www.usgs.gov/special-topic/water-science-school/science/watersheds-and-drainage-basins?qt-science_center_objects=0#qt-science_center_objects|access-date=2021-10-22|website=www.usgs.gov}}</ref>

As water flows through the basin, it can form tributaries that change the structure of the land. There are three different main types, which are affected by the rocks and ground underneath. Rock that is quick to erode forms dendritic patterns, and these are seen most often. The two other types of patterns that form are trellis patterns and rectangular patterns.<ref>{{Cite web|last=Earle|first=Steven|date=2015-09-01|title=13.2 Drainage Basins|url=https://opentextbc.ca/geology/chapter/13-2-drainage-basins/|language=en}}</ref> 

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|arithmetic mean]] method will give good results. In the 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.

[[Engineering:Isochrone map|Isochrone map]]s 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.<ref name="hydrolearthsystsci bell 1998">{{cite journal | title=A grid-based distributed flood forecasting model for use with weather radar data: Part 1. Formulation | author=Bell, V. A. | journal=Hydrology and Earth System Sciences | year=1998 | volume=2 | issue=2/3 | pages=265–281 | doi=10.5194/hess-2-265-1998 | publisher=[[Company:Copernicus Publications|Copernicus Publications]] | author2=Moore, R. J.| bibcode=1998HESS....2..265B | url=https://hal.archives-ouvertes.fr/hal-00304542/file/hess-2-265-1998.pdf | doi-access=free }}</ref><ref name="subramanya 2008">{{cite book | title=Engineering Hydrology | publisher=Tata McGraw-Hill | author=Subramanya, K | year=2008 | pages=298 | isbn=978-0-07-064855-5 | url=https://books.google.com/books?id=LROqKvHaRyMC&q=Clark%E2%80%99s+IUH+time-area+method+isochrone&pg=PA298}}</ref><ref name="unesco ">{{cite web | url=http://webworld.unesco.org/water/ihp/db/glossary/glu/EN/GF0705EN.HTM | title=EN 0705 isochrone map | publisher=[[Organization:UNESCO|UNESCO]] | access-date=March 21, 2012 | url-status=dead | archive-url=https://web.archive.org/web/20121122145250/http://webworld.unesco.org/water/ihp/db/glossary/glu/EN/GF0705EN.HTM | archive-date=November 22, 2012 }}</ref><ref>{{Cite web |url=https://www.petropedia.com/definition/2126/isochron-map |title=Isochron Map |access-date=2021-09-03 |archive-date=2021-09-03 |archive-url=https://web.archive.org/web/20210903103537/https://www.petropedia.com/definition/2126/isochron-map |url-status=dead }}</ref>

=== Geomorphology ===
Drainage basins are the principal hydrologic unit considered in fluvial [[Physics:Geomorphology|geomorphology]]. A drainage basin is the source for water and [[Earth:Sediment|sediment]] that moves from higher elevation through the river system to lower elevations as they reshape the channel forms.

=== Ecology ===
[[File:Hydrographic basin.svg|thumb|upright=1.35|Top-down illustration of a [[Earth:Drainage system (geomorphology)|dendritic]] drainage basin. The dashed line is the main water divide of the hydrography basin.]]
[[File:EN Bazinul hidrografic al Raului Latorita, Romania.jpg|thumb|upright=1.35|Digital [[Relief mapping (computer graphics)|terrain map]] of the Latorița River's drainage basin in Romania]]

Drainage basins are important in [[Earth:Ecology|ecology]]. As water flows over the ground and along rivers it can pick up nutrients, sediment, and [[Earth:Water pollution|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 [[Earth:Eutrophication|eutrophication]] where plant growth is accelerated by the additional material.

=== Resource management ===
{{Further|Earth: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".<ref>{{Cite web|date=2010-09-07|title=Twin Cities Metropolitan Area (TCMA) Watersheds|url=https://www.pca.state.mn.us/water/twin-cities-metropolitan-area-tcma-watersheds|access-date=2021-09-22|website=Minnesota Pollution Control Agency|language=en}}</ref> 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 "[[Earth:Watershed management|watershed management]]".
In {{wipe|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, [[Organization:Mekong River Commission|Mekong 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.<ref>{{cite web|url=http://www.strategicforesight.com/inner-articles.php?id=310#.WA9Ia-V97GI|title=Water Cooperation for a Secure World|website=www.strategicforesight.com|date = 25 November 2013|last1= bin Talal|first1 = Hassan|last2= Waslekar|first2= Sundeep}}</ref>

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

Catchment factors are: [[Earth:Topography|topography]], shape, size, [[Chemistry:Soil|soil]] type, and land use (paved or [[Engineering:Roof|roof]]ed 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 ===
Generally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep [[Earth:Mountain|mountain]]ous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient).

=== 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. Water sheds are great for the environment

=== Size ===
Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It is also determined on the basis of length and width of the drainage basin.

=== Soil type ===
Soil type will help determine how much water reaches the river. The runoff from the drainage area is dependent on the soil type. 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 [[Earth:Clay|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.<ref name=":1" />

=== 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, [[Engineering:Sidewalk|pavements]], and roads will be collected by rivers with almost no absorption into the [[Earth:Groundwater|groundwater]].
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean.

== See also ==
* {{annotated link|Earth:Integrated catchment management}}
* {{annotated link|Earth:Interbasin transfer}}
* {{annotated link|Earth:Main stem}}
* {{annotated link|Earth:River bifurcation}}
* {{annotated link|Earth:Tenaja}}
* {{annotated link|Earth:Time of concentration}}
*[[Earth:Catchment hydrology|Catchment hydrology]]

== References ==
{{Reflist|30em}}

{{river morphology}}
{{Rivers, streams and springs}}
{{Wetlands}}

[[Category:Fluvial landforms]]
[[Category:Geomorphology]]
[[Category:Hydrology]]
[[Category:Rivers]]
[[Category:Water and the environment]]
[[Category:Water streams]]

{{Sourceattribution|Drainage basin}}