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Newsletter and Technical Publications Case Study 5: Aflaj Systems in Oman The Sultanate of Oman is characterized by a situation of water scarcity. The annual precipitation is only about 75-100 mm. Perennial streams are nonexistent and renewable water resources do not exceed 2 billion m3/year, whereas 1.24 billion m3/year were withdrawn in 1996 (ACSAD, 1997). Estimates indicate that Oman, like many Arab countries, is on the brink of an imminent water crisis. The requirements of social and economic development are expected to create an imbalance between the available water resources and the demands placed on these resources. Water withdrawal increased from 500 million m3 in 1985 to 1,200 million m3 in 1996, and is expected to reach approximately 7,500 million m3/year by 2025 (ACSAD, 1997). The Sultanate of Oman traditionally has relied on Aflaj systems for sustaining agricultural production. Prior to the introduction of water pumps, therefore, Aflaj were the major irrigation water sources. Despite the limited role of hand-dug wells and their limited spread, they were essentially used to augment Aflaj discharges. It is noted that, even today, Aflaj water represents one of the major resources for meeting various water demands in Oman. Over 3,095 Aflaj are believed to exist in Oman, according to the 1997 Aflaj survey. On the other hand, the total number of Aflaj (operating and non-operating) is estimated to be 4,159. Based on these figures, it is clear that Aflaj play a large role in the economy of Oman’s rural areas. Accordingly, Oman authorities have given special attention to Aflaj systems, and the government is committed to their maintenance and development in order to enhance their efficiency. Technology Description Aflaj is the plural for Falaj and has several meanings. Linguistically, it means a gap between two sides, a small creek or a small water conveying channel. The word “Aflaj” is a term that represents an irrigation system and includes the economic, social and managerial aspects. Aflaj systems are remarkable engineering works illustrating the distinguished art of engineering present in the Sultanate of Oman at times when mechanical excavation, topographic leveling and modern equipment were absent. A Falaj is a channel that may be open, or partially or completely covered. The Falaj collects groundwater through natural infiltration conditions and conveys it to the ground surface by gravity. Construction of a falaj begins with excavation of a “mother well” extending 1-2 m into the free saturated zone. The channel is oriented approximately parallel to the groundwater flow direction, or at a small angle to this direction, in order to ensure water flow to the Falaj outlet (called the “sharia). Excavation also can be done in the opposite manner (i.e., start from the outlet in the direction of the groundwater aquifer). The covered parts of the Falaj are excavated through a series of vertical wells, with 1.5 m diameters and at 15-30 m spacing distances. These vertical wells are connected through a series of tunnels with diameters between 0.5-1 m. The tunnels and the vertical wells are lined if the excavated soil layers are loose, or susceptible to collapse. The Falaj utility area (“Manafea el Falaj”) where the water infiltrates into the Falaj are not lined, thereby ensuring a free flow of water into the tunnels. Despite the similar characteristics of most Aflaj systems and their similar manner of construction, some differences nevertheless exist among the systems on the basis of their lengths, quality and quantity of flowing water, developed aquifers, geological nature of the Falaj path, availability of water recharge, leakage, evaporation from the Falaj and the amounts of rain in the region. A Falaj system, however, is a perennial system providing flowing water day and night throughout the year. Based on these considerations, the Aflaj can be categorized into three types as follows:
These systems are usually open channels for their greater part, and usually exist in Wadi courses. The depth of Ghayli Aflaj does not normally exceed 4 m, and their lengths range from a few hundred meters to 2 km (Figure 61). This group is called Ghaylii because of the seasonal nature of their flow, which occurs only during certainly periods depending on the rainfall and presence of shallow groundwater. The system withdraws the water from Wadi beds and mountainous slopes through infiltration. During dry seasons, most Ghayli Aflaj are dry, due to their direct reliance on the rainwater collected in ponds at the lower regions of the valleys, as well as shallow groundwater. This type of Aflaj is common in several regions of northern Oman, and constitutes about 50% of total Aflaj in the Sultanate.
These are mainly open channels that derive their water directly from springs at the mountain foothills (Figure 62). Fracture zones act as conduits through which deep aquifers may sometimes flow. In such cases, the water is typically warm and sulfurous. There are many hot Ainy Aflaj in Oman, the most popular being those Ain Al-Thawara, Ain Al-Kasfa and Ain Arzat in Jabal Al-Qra in Zufar Governorate.
These are sub-surface tunnels (Figure 63) with a width of 0.5-1 m, and height of 0.5-2.0 m. Their maximum depth below the ground surface is about 50 m. These systems are found in upstream alluvial plains of Dhahira, Batina Sharqiya and Gharbiya. The Daudi Aflaj are characterized by their extended lengths (ranging between 3-12 km) and their perennial flow. These tunnels have water-feeding tributaries pouring into them, which can sometimes reach into the hundreds. The Daudi Aflaj constitute about 45 % of the total Aflaj systems in Oman.
Figure 61. Ghayli Falaj
Figure 62. Ainy Falaj
Figure 63. Daudi Falaj Aquifers feeding the Aflaj systems in Oman are of the following types:
These are renewable aquifers of limited water storage volumes. Aflaj that derive their water from such systems typically exhibit significant variations in annual discharge.
These aquifers consist of sediment alluvial fans transported through flushing floods generating at the steep slopes of the Omani mountains. The hydrographs of Aflaj supplied with waters from these aquifers are characterized by steep limbs. These steep limbs are characteristic of surface runoff, with limited water storage and high water velocity through highly-permeable soil layers.
These Aflaj systems derive their water from Alluvial plains, such as the coastal area of Al-Batina or the plains of the inner regions, and are characterized by their high and stable discharge. This is because of the large aquifer extension and their large volumes of stored water.
Aflaj systems deriving their water from extensive regional aquifers have constant flow throughout the year and over long periods. Studies of the hydrodynamic regime in Oman indicate that several aquifer systems can be distinguished. The above-noted types of aquifers control the boundary conditions of these identified aquifer systems, including the following:
The flow patterns in wadi channels and alluvial fans can be classified as local flow systems. They occur in the upper and middle reaches of wadis of Batina, Dhahira, and Sharkiya (Dhufar).
These are represented by groundwater flow in the alluvial plains, including the coastal plains and the alluvial plains of the interior (Dakhlia, Sharqiya and Dhahira). The latter extend from the mountain foothills to the desert edges. The coastal flow systems may discharge into the sea, but often flow into coastal sabkha zones where their water is drained through evaporation.
These systems are characterized by their extended areas and large storage volumes. The most important are found in the inner regions along the mountains, extending to the desert. Some Aflaj (e.g., Al-Awabi Falaj) derive their waters from the local flow systems. Other Aflaj derive their waters from local aquifer systems (e.g., Al-Hamra Falaj in wadi Ghol, where the discharge decreases by 50% during the dry season). Such fluctuations in Falaj flow also are observed in Al-Hamra Falaj and Mitha Falaj in Wadi Bahla. In contrast, Aflaj which derive their water from intermediate aquifer systems are characterized by uniform flows, examples being Falaj Dares and Falaj Adam. Recent surveys of Aflaj systems in Oman indicate that their discharge decreases to about 50% of their average discharge during the 3-6 month dry season, or to about 10% of their maximum discharge during the long dry spells (as happened in 1975). This is true for the Aflaj systems in the upper and middle reaches of Wadi Halfin, Wadi Nazwa, Wadi Bahla and Wadi Sifam (interior Oman). The Aflaj fed by intermediate-flow systems take longer before their discharge decreases by one-half. The Daudi Aflaj usually derive their water from regional aquifers systems. These include unconfined or confined aquifers leaking into upper aquifer systems. Falaj Al-Ghabi in Wilayat Badia is an example of such systems (depth up to 30 m; length about 15 km). The large uniform discharge of these Aflaj reflects the hydrodynamic nature of the feeding aquifer system. Water from regional flow systems may rise along fracture zones to feed hot or worm Ghayli Aflaj in Oman. Ainy Aflaj are characterized by their warm water, due to their water flow system reaching relatively great depths. Their water rises under the effect of high pressure, allowing some of the water to penetrate close to the ground surface to feed the Aflaj and springs. Extent of Use The Aflaj systems still have a high profile in the Sultanate of Oman. They are used extensively in two main geographic regions, including (1) the eastern coastal region, including the eastern foothills of Oman mountains and the coastal plains, and (2) the interior region, including the western foothills of Oman mountains and the plains extending between the foothills and desert. Since the mass of the Oman mountains consist mainly of impervious ophulites rocks, a large part of the rainfall (in the range of 150-250 mm as an average) forms runoff flowing eastwards and westwards to recharge the coastal and inner plains. Because of the decreasing particle gradation in the direction of these plains, most of their waters infiltrate to it at the bed of the loose alluvial layers. The water does not reach the sea except during intense rainstorms (estimated to be about 15-40% of actual discharges flowing out of the mountains). Most of the rest infiltrates to the crushed sandstone layers to feed the underground reservoirs in the alluvial plains. The free aquifer systems constitute the principal water source to the Aflaj systems in the alluvial plains. In certain localities, carbonate rocks protrude to form a belt of scattered hills separating the mountainous and plain zones. These carbonate rocks are highly fractured and decayed. Oman has utilized these fracture formations controlling the groundwater flow paths above the impervious ophulites surfaces. Thus, Aflaj systems were established to intersect these groundwater paths, to allow conversion of part of the water to agricultural and drinking use. In the Batina plain, for example, Aflaj are widespread in the higher lands. In the case of the coastal plains, where the ground surface slope is very mild, these plains are developed mainly through utilizing wells. The spatial distribution of Aflaj, based on the geo-climatic characteristics (Figure 64), includes (1) eastern area, (2) Sharqia region, (3) interior region, (4) Al-Dhahira, (5) Al-Batina, and (6) Zafar governorate (Ayoun). Eastern Area Aflaj provide most of the water needs of this region, and are spread in three sub-regions, including (1) Wadi Al-Bat-ha, (2) Al-Modiby region (Wadi Endam and Wadi Samad), and (3) Wadi Mejlas and Wadi Daikah In the area drained by wadi Al-Bat-ha, Aflaj are common in the upper and lower reaches of the wadi, with a total yield of about 2,000 L/s. The main aquifers feeding the Aflaj are the alluvial plains aquifers, with thicknesses ranging between 15-30 m. They are characterized by high permeability and recharge from surface runoff. The upper reaches are characterized with steep slopes, and consist mainly of impervious rocks that generate large amounts of surface runoff during rainstorms. Most of the storms are intense and of short duration, as reflected in the Aflaj discharge, which varies between a few liters and several hundred liters per second. These Aflaj generally can be categorized into two categories, including (1) constant discharge or low variability, and (2) fluctuating discharges throughout different seasons of the year. The latter are located mainly in the higher regions of the wadi (Figure 65). Aflaj water quality in the Al-Bah-ha basin is generally good, with salinity not exceeding 1,500 micromohz/cm. In the lower and southern reaches, uniform flows up to 50 and 170 L/s have been measured in Aflaj Al-Kamel and Al-Wafi, respectively. It is noted that about 42 Aflaj are found in this region, and appear to derive their water from wadi Bany Khaled (one of Al-Bat-ha branches). The Aflaj of the Al-Kamel region are considered a model for the Aflaj system and their water distribution among the users (Figure 66). Studies have shown that, in the case of the rational utilization of the water resources, this region can be developed through the Afjlaj and well systems. Some Aflaj of Al-Modibi State are characterized by their high yield (more than 100 L/s). The Al-Modibi Falaj discharge, for example, reaches about 150 L/s. On the other hand, the Aflaj of wadi Daikah and Wadi Mejlas generally have low yields (except for some Aflaj such as Heyal Al-Ghaf), and they also are substantially affected by dry seasons.
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