Wadi

Wadi ( ) is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry (ephemeral) riverbed that contains water only during times of heavy rain or simply an intermittent stream. 
==Etymology ==
The term is very widely found in Arabic toponyms.
Some Spanish toponyms are derived from Andalusian Arabic where was used to mean a permanent river, for example: Guadalcanal from wādī al-Qanal = "river of refreshment stalls", Guadalajara from wādī al-hidjārah = "river of stones",Official municipal tourist guide, p.5. Retrieved 17 August 2013. or Guadalquivir from al-wādī al-kabīr = "the great river". Seasonal streams, frequent in south-east Spain, are called rambla instead.
In the Maghreb, the term wadi (wad in Maghrebi Arabic, sometimes transcribed Oued) is applied to all rivers including regular ones.
==Hydrological action==
Modern English usage differentiates a wadi from another canyon or wash by the action and prevalence of water. Wadis, as drainage courses, are formed by water, but are distinguished from river valleys or gullies in that surface water is intermittent or ephemeral. Wadis are generally dry year round, except after a rain. The desert environment is characterized by sudden but infrequent heavy rainfall, often resulting in flash floods. Crossing wadis at certain times of the year can be dangerous as a result.
Wadis tend to be associated with centers of human population because sub-surface water is sometimes available in them. Nomadic and pastoral desert peoples will rely on seasonal vegetation found in wadis, even in regions as dry as the Sahara, as they travel in complex transhumance routes.
The centrality of wadis to water—and human life—in desert environments gave birth to the distinct sub-field of "Wadi Hydrology" in the 1990s.Review of Wheater, Howard ; Al Weshah, Radwan, Hydrology of Wadi systems -IHP Regional Network on Wadi Hydrology in the Arab Region, UNESCO- Technical documents in hydrology vol 55, SC.2002/WS/33,(2002).
===Deposits===
Deposition in a wadi is rapid because of the sudden loss of stream velocity and seepage of water into the porous sediment. Wadi deposits are thus usually poorly sorted gravels and sands. These sediments are often reworked by eolian processes.Mohamed Achite and Sylvain Ouillon, Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973–1995) Journal of Hydrology Volume 343, Issues 3-4, 20 September 2007, pp. 187-202.
Over time, wadi deposits may become "Inverted Wadis" where the presence at one time of underground water caused vegetation and sediment to fill in the Wadi's eroded channel to the point that previous washes appear as ridges running through desert regions.
==References==
===Bibliography===
*Summary: Drainage Courses, Wadis. United States Army Corps of Engineers. Desert Processes Working Group; Knowledge Sciences, Inc. (n.d), retrieved 2008-08-26.
*Summary: Summary: Drainage Courses, Wadis - Inverted. United States Army Corps of Engineers. Desert Processes Working Group; Knowledge Sciences, Inc. (n.d), retrieved 2008-08-26.
==External links==
*IHP REGIONAL WADI HYDROLOGY NETWORK, International Hydrological Programme, UNESCO.
*Arab Center for Studies of Arid Zones and Dry lands (ACSAD): Water resources division.
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