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Arid Lands and Deserts Loose More Water Than They Gain


Sahara Desert

Sahara Desert, Algeria, Africa

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In the 1970s, the Sahel strip that stretches along the southern fringe of the Sahara Desert in Africa experienced a devastating drought and land which was formerly used for grazing turned to desert in a process known as desertification. Approximately one quarter of the land on earth is threatened by desertification. The United Nations held a conference to discuss desertification in 1977. The result is the United Nations Convention to Combat Desertification, an international treaty established in 1996 to combat desertification.

Deserts, also known as arid lands, are regions that receive less precipitation then their potential evapotranspiration (evaporation from the soil and plants plus transpiration from plants equals evapotranspiration, abbreviated as ET). Deserts are located around the world.

The little precipitation and rain that falls in deserts is usually erratic and varies from year to year. While a desert might have an annual average of 5 inches of precipitation, that precipitation may come in the form of 3 inches one year, none the next, 15 inches the third, and 2 inches the fourth. Thus, in arid environments, the annual average tells little about actual rainfall.

Rain in the desert is often intense and since the ground is often impermeable (meaning that water isn't absorbed into the ground easily), the water runs quickly right into streams that only exist during rainfalls. The swift water of these ephemeral streams are responsible for most of the erosion that takes place in the desert. Desert rain often never makes it to the ocean, the streams usually end in lakes that dry up or the streams themselves just dry up. Almost all of the rain that falls in Nevada never makes it to a perennial river or to the ocean.

Permanent streams in the desert are usually the result of "exotic" water, meaning that the water in the streams comes from outside of the desert. The Nile River flows through a desert but the river's source in high in the mountains of Central Africa.

Deserts aren't always located in hot places. Although the world's highest temperature was recorded in the Sahara Desert (136 degrees F or 58 degrees C at Azizia, Libya on September 13, 1922), deserts can also be cold places. Not only do temperatures at night drop to near freezing levels due to the lack of moisture in the air (which usually holds heat in) there are deserts in cold places around the world. Antarctica is actually the world's driest continent, on average it receives less than two inches of precipitation annually.

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