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.
TRADITIONAL WATER CATCHMENT 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.
Mountain drainage galleries
Galleries excavated in the rock on mountain terracing to convey the drained residues of rainwater. The layout is rectilinear and rather short and it requires few ventilation shafts. In those Oases located at the foot of a mountain, the gallery is more winding, due to the heterogeneity of the ground and its course becomes longer – also to increase groundwater drainage – thus requiring more shafts to maintain the whole system. The widespread use of cisterns to harness the water grants a continuous production cycle in the Oasis, allowing for crop rotation typical of the traditional agricultural system.
When the galleries do not intercept mountain creeks that run downstream – as it is the case with Oman’s Aflaj called daoudi – water is harnessed from the phreatic aquifers present in the permeable rock layer situated above the Oasis.
Wadi drainage galleries
Excavated galleries branch out from the prehistoric riverbed, catching both the residual water flows, and the infiltrations and micro-flows which run underground into the wadis. In Wadi Oases, such the ones which are located in Yemen’s Hadramaut wadi, or Algeria’s Roufi and Saoura wadis or Morocco’s Dades, Draa and Ziz wadis, water is collected upstream, on a higher altitude than the terraced cultivations running parallel to the river course. Date palm groves are reached thanks to the natural slope, just exploiting runoff. During the period when surface flows are either limited or absent, water is extracted from wells, by pumping it from the aquifers replenished by the wadi. This is made possible by the accomplishment of underground dykes in order to infiltrate the soil. In the past, the Saharan and Arabian areas saw the use of traditional bar wells with a counterweight mounted on huge earthen pillars (khottara, in Algeria; shaduf, in Egypt). Some of these still exist in Oman, where water is lifted with the help of donkeys and oxen.
When the scarce waterway is perennial, Oases are regularly supplied. More often than not, however, the river is dry for decades. In general the water, which is either drained by galleries or extracted from wells, can be found only in the underground sediments, both in the fossil riverbed and in the surrounding areas which host the micro-flows. Flash floods too can permeate the ground and replenish superficial phreatic aquifers. That is why the underground dykes, built deep into the wadi’s bed for this purpose, must regularly be maintained in times of drought.
Erg drainage galleries
Drainage galleries (foggara in Algeria and Libya) may be even up to 20 km long. These are wide, gently sloping tunnels dug out in the limestone substratum of the big dunes, in order to allow underground water flow with a minimum bed erosion. The excavation starts in the farming area of the Oasis and continues uphill along the dune in the open desert. Sometimes the course is winding in order to favour a constant flow discharge speed. The galleries are connected to the surface via ventilation shafts perpendicular to their trail – placed at a distance of 5-22 meters and up to 20 meters in depth, according to ground inclination. In such way, water is supplied both to the villages (where underground settling basins store water for households) and to the palm groves, located further downhill. The shafts, which make the track visible due to distinctive small craters formed on the surface by excavation rubble, play an essential role in the sophisticated mechanism of the foggara: besides ensuring ventilation and exploiting condensation processes, they preserve an internal pressure equal to the atmospheric one – thus favouring water flow – and allow access for the frequent maintenance operations.
The water is harnessed from the great Saharan Aquifers which are located not far from the surface, in extensive areas of the desert. The traditional drainage gallery system is designed to exploit only an amount of water which will not lower groundwater table level. The gallery collects also micro-flows which drain erg sands’ moisture (resulting from the sporadic rainfall) but mainly the condensation absorbed by the soil is due to the high thermal range, a significant contribution to the system’s flow rate.
Sebkha drainage galleries
Drainage galleries (like the Timimoun foggara in Algeria) very similar to the erg ones, but excavated starting near the highest point of elliptic, wide and deep depressions, which have saline outcrop at the bottom: the ancient, dried-up, prehistoric lakes (sebkha or chott). Overlooking the sebkha‘s shores, not unlike traditional coastal settlements, are the villages, while palm groves stand further down. The gallery layout begins where Oases farmed plots are located – and winds back up towards the plateau through extremely deep tunnels. The aim they are designed for, is to intercept and drain the underground micro-flows that rainfalls have infiltrated in the bedrock of desert’s highlands. Once they have reached the prehistoric hydrographic network, such flows will slowly converge underground into the endpoints represented by depressions. Sebkha galleries must collect the flows before they can reach the great basin’s underground bed and before they evaporate, releasing a thick layer of salt. The structure that harnesses the moisture from sand is the classical one, common to all the drainage galleries, with typical vertical shafts, used both for ventilation and maintenance, thus enhancing condensation and percolation processes. Once again, the excavation’s slope is minimal, in order to allow gravitational leading and avoid erosion.
Sebkha drainage galleries are situated in hyper arid areas. Along the bore path, water is collected underground from the abundant micro-flows descending towards the natural impluvium, where all the waters from desert highlands converge. The length of the gallery supplying water via drainage, condensation and percolation, has therefore vital importance: the greater the amount of water collected, the wider portion of land can be reclaimed from the desert and become farmable. Extending the gallery towards the sebkha‘s bottom, however, has a limit: the soil salinization. Due to evaporation, it increases exponentially when moving closer to the depression’s centre. Consequently, when the Oasis’ population grows, new foggaras need to be excavated at a certain distance from the existing ones, in order to supply water to new villages which will be built, separately, on the shores of the same sebkha.
TRADITIONAL WATER DISTRIBUTION SYSTEM
Above ground, at the outlet of hydraulic devices, is located the water adduction point within the Oasis. This is where water capacity is originally measured through a small, perforated copper plate (hallafa) in order to equally distribute it. Whether the agreed-upon rules of long lasting traditions remain in force, flow allocations are permanently transferred to a great comb-shaped stone divider (kesria), then regularly conveyed, at given shifts or time slots, through one or more earthen conduits, in the exact amount each beneficiary is entitled to receive. These rights were assigned the time the construction was completed and determined according the contribution given to the enterprise. The water share is inherited and can be divided through marriages and sales. For this reason, water flows within the Oasis through an extremely complex system of open-air earthen conduits (seguia) which are further subdivided by additional comb dividers. Every conduit leads to a personal storage basin (majen), located in the cultivated plot, where each landowner collects the water to irrigate his own “garden”, in sections and according to both seasonal and agricultural needs.