Water Supply Sources
CURRENT HYDROGEOLOGICAL BASINS
Saharan and Arabian deserts present very deep confined – partially self-regenerating – Artesian Aquifers preserved within two layers of impermeable sediments, large fossil water supplies dating from prehistoric times: the Arabian Platform Aquifer System in Oman, Yemen and Saudi Arabia; the world’s largest, the Nubian Sandstone Aquifer System (NSAS) in Libya, Egypt and Sudan; the Mourzouk Basin in Libya; the North Western Sahara Aquifer System (NWSAS), comprising the Albien Aquifer and the Grand Erg Occidental Aquifer in Algeria, Tunisia and Libya; the Taoudeni Basin between Mali, Mauritania and Algeria and the Iullemmeden Aquifer System in Niger, Mali and Algeria.
There is hardly any rainfall in desert environments. However, the water flow of sporadic rainfalls on the Saharan and Arabian altitudes is stored underground, in different ways and places. When free water is not harnessed upstream through dykes and cisterns – such as the Resaf in Yemen – or when flash floods don’t convey it into the beds of prehistoric rivers, rainfall permeates through the open fractures of mountain massifs and it is lodged in the porosity of permeable rocks on the highlands. This is how water tables are formed as well as microflows that migrate for thousands of kilometres and slowly seep into the Erg’s great sandy masses or run underground – following the courses of ancient river valleys and of extensive alluvial surfaces – until they reach the basins of the great depressions, the Sebkha: the terminal of the hydrographic network, dating from Prehistory, which lost its sea outlet. Rainfalls thus feed, although discontinuously, desert groundwater and regenerate them gradually through a process which can last hundreds of years.
Water vapour is also present, although in limited quantities, in the desert’s hot air. At night time, two factors, dry air and the lack of vegetation, determine high thermal variations with a sudden drop in temperatures. Upon touching the ground, by now very cold, the water vapour in the air is condensed and turns into small droplets, which lay on the ground’s surface. This is the so-called condensation process.
Sahara and Arabian deserts were once fertile and water-rich lands. Indeed, still today, we can identify the course of prehistoric rivers, called Wadi, the ancient hydrographic network, carved into sandstone or limestone rocks, forming deep canyons. In some wadis, rainfall water from mountain ranges, still flows above ground although in small quantities. Therefore, on those ancient shores, we find Oases able to harness such water resources through dykes, water diverters, sluices and dams. In most cases, wadis (drained millennia ago) are dry most of the year. However, water slowly keeps flowing in the surrounding alluvial undergrounds and under their beds. Thanks to subsoil water flows which are the remains of rainfalls on very distant mountains, surface groundwater is thus regenerated. This is the hidden water heritage harnessed – with drainage galleries and shafts – by other Oases located all along the wadi. Even after decades, however, dwellers come to observe the arrival of sudden flash floods which can abruptly fill riverbeds, and even cause full-scale flood with catastrophic consequences.
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Water Catchment → Traditional Systems
Drainage galleries, which still partially provide water supply to Oases, are traditional water catchment systems: large underground tunnels equipped with ventilation shafts. Useful to the maintenance of the whole system, shafts are dug out deep in the ground at regular intervals. Down the lightly sloping trail leading to the cultivated area of the Oasis, water flows thanks to gravity, within wide galleries which can extend for tens of kilometres. Drainage galleries can be found in the whole Saharan and Arabian area, but not exclusively. The earliest examples are the Qanats or Kariz built in Iran before 1000 BC and used to exploit groundwater mainly at the foothills: 20,000 still working structures covering over 250,000 km. In Africa and in the Arabian Peninsula, similar systems can be found, presenting common features and serving the same purpose, while the sources of supply and catchment methods vary due to environmental conditions: the Ngoula or Kriga in Tunisia and the Shridj in Yemen, although in both countries few units still survive; the Khettara in Morocco, based on a mechanism similar to that of the Qanat, though only a few hundred units are left; the Foggara in Algeria, built more than a millennium ago, are over a thousand, half of which are still active. According to a proven theory, the name Foggara also used to describe the Libyan Oases’ drainage galleries comes from the Arabic fakara (to dig). Other authors reckon that the term originates from the Arab word el fokr (poverty). As a matter of fact, it was believed that whoever took part in digging for years a foggara, was doomed to fall into poverty given the excessive investment in time and money required for its completion. The usefulness of drainage galleries, however, is testified by the more than 3000 Aflaj dating back as early as 500 BC and still operating today, which cater for most of Oman’s irrigation needs, as well as being a Unesco World Heritage Site (2006). Aflaj can be of three types: the Ghaili, which harness the water of the wadis; the Aini, which manage perennial water resources; the Daoudi, which – like the Iranian Qanat and the Moroccan Khettara – filter the water from confined water tables at the foot of the mountains. Usually, the flow rate of drainage galleries increases thanks to the wise exploitation of the condensation process, which starts at sunset: when the water vapour contained in the air touches the ground – rapidly cooled due to the desert environment – it moistures not only the surrounding ground, but also the inner walls of the ventilation shafts along the gallery. The air, entering the shafts, produces tiny drops of condensation which collect into the tunnel, thereby increasing the volume of water which flows driven by gravity into the tunnel towards the cultivated area. This process, however, does not take place only at night. In the early hours of the morning, inside of the tunnel, when the external temperature rises again, the air movement is reversed. The desert ground, now warmer, pumps the damp air of the palm grove – where humidity is preserved by the presence of vegetation – out of the shafts, in the opposite direction of the water flow. Hence, also during the day, the water vapour continues to be condensed on the gallery’s walls.
Water Distribution → Traditional Systems