Newsletter and Technical Publications
Alternative Technologies for Freshwater Augmentation
in West Asia>
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were withdrawn in 1996 (ACSAD, 1997).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.
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:
- Alluvial Aquifers in Wadis
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.
- Alluvial Plains Aquifers Underlying
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
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).
- Intermediate groundwater flow systems:
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).
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.