طاس

مسيسپي درياءَ ڪنهن به آمريڪي درياهه جو سڀ کان وڏو علائقو پاڻي نيڪال ڪري ٿو. ان جو گهڻو حصو زرعي علائقن تي مشتمل آهي. زرعي وهڪري ۽ ٻيو پاڻي آلودگي، جيڪو ٻاهر نڪرڻ تائين وهندو آهي، ميڪسيڪو جي نار ۾ هائپوڪسڪ يا ڊيڊ زون جو سبب آهي.

هڪ طاس، "سامونڊي طاس" يا "نيڪال جو طاس" (Drainage Basin) زمين جو اهڙو علائقو آهي جنهن ۾ سڀ وهندڙ مٿاڇري جو پاڻي هڪ نقطي تي گڏ ٿئي ٿو، جهڙوڪ درياهه جو وات يا پاڻيءَ جيڪا ٻئي جسمن، جهڙوڪ ڍنڍ يا سمنڊ ۾ وهندو آهي. هڪ طاس کي ويجهن طاسن کان هڪ دائري، نيڪال جي ورڇ (drainage divide)، جيڪا اچي جڳھن، جهڙوڪ ٽڪريون ۽ اچائين (Ridges) جي تسلسل مان ٺهيل آهي، جي ذريعي الڳ ڪيو ويندو آهي. هڪ وڏو طاس ننڍي طاسن، جيڪا دريائن جي سنگم تي گڏ ٿئي، هڪ درجي بندي وارو نمونو ٺاهيندا آهن، تي مشتمل ٿي سگهي ٿو.

نيڪال جي طاس لاءِ ٻئي اصطلاحن ۾ نيڪال جو علائقو، دريا جو طاس، ڪيچمينٽ بيسن، واٽر ڪيچمينٽ، پاڻي جو طاس ۽ امپلويئم شامل آهن. اتر آمريڪا ۾ انهن کي عام طور تي واٽر شيڊ سڏيو ويندو آهي، جڏهن ته ٻين انگريزي ڳالهائيندڙ جڳهن ۾ "واٽر شيڊ" صرف ان جي اصل معنيٰ، "نيڪال جي ورڇ واري لائن" لاء استعمال ٿيندو آهي.

هڪ نيڪال جي طاس جون حدون واٽرشيڊ جي حدبندين، جيڪا ماحولياتي انجنيئرنگ ۽ ماحولياتي سائنس جي وچ ۾ هڪ مشترڪ ٽاسڪ آهي، ذريعي طئي ڪيون وينديون آهن.

هڪ بند نيڪال جي طاس (Endorheic Basin) ۾، سمنڊ ڏانهن وهڻ جي بدران پاڻي طاس جي اندريان ئي، جيڪا نيڪاس (sink) جي نالي سان سڃاتو وڃي ٿو، گڏ ٿئي ٿو ۽ پاڻي جو هڪ جسم ٺاهيندو آهي، جيڪو هڪ مستقل ڍنڍ، هڪ خشڪ ڍنڍ يا هڪ نقطو، جتي مٿاڇري جو پاڻي زمين جي اندر گم ٿي ويندو آهي، ٿي سگهي ٿو.

نيڪال جا طاس، جيڪا نيڪال جا علائقا آهن، جيڪا حدبندي ڪيا ويا آهن ته جيئن هڪ گھڻ-سطحي درجي بندي واري نيڪال جي نظام ۾ گهيرو ڪري سگهجن، هڪجهڙا هوندا آهن، پر هائيڊرولاجي جي يونٽن مان هڪجهڙا نه آهن. هائڊرولاجي يونٽن کي ڪيترن ئي انليٽس، آئوٽليٽس يا سنڪ جي اجازت ڏيڻ لاءِ بيان ڪيو ويو آهي. سخت معنيٰ ۾، سڀئي ڊرينيج بيسن هائيڊروولوجيڪ يونٽ آهن، پر سڀئي هائيڊرولوجيڪ يونٽ ڊرينيج بيسن نه آهن.

دنيا جا مکيه نيڪال جا طاس

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 Atlantic Ocean.^{[حوالو گهربل]} In 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 South America east of the Andes also drains to the Atlantic, as does most of Western and 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 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 Mediterranean Sea basin, with the Black Sea, includes much of North Africa, east-central Africa (through the Nile River), Southern, Central, and Eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and Syria.

The Arctic Ocean drains most of Western Canada and Northern Canada east of the Continental Divide, northern 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 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 west of the Continental Divide (including most of Alaska), as well as western Central America and 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 parts of Australia.

Largest river basins

The five largest river basins (by area), from largest to smallest, are those of the Amazon (7 million km²), the Congo (4 million km²), the Nile (3.4 million km²), the Mississippi (3.22 million km²), and the Río de la Plata (3.17 million km²). The three rivers that drain the most water, from most to least, are the Amazon, Ganges, and Congo rivers.

Endorheic drainage basins

Endorheic basin in Central Asia

اصل مضمون جي لاءِ ڏسو 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 Endorheic lake or 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, dry lake, or alkali sink.

The largest endorheic basins are in Central 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 African Great Lakes, the interiors of 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 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.^{[حوالو گهربل]}