Qilba

A qilba is a gently sloping underground channel to transport water from an aquifer or water well to the surface for drinking and irrigation, acting as an underground aqueduct. Constructed across Abayad, they were an ancient system of water supply from a deep well, making use of a series of vertical access shafts. The qilba still create a reliable supply of water for Abayadis in the country’s semiarid and arid regions, but the value of the system today is directly related to the quality, volume, and regularity of water flow. Traditionally, qilba have been built by skilled laborers, called qilbis. The profession historically paid very well, and was handed down by men to their maternal nephews. According to most sources, the technology to build qilba was developed in the Early Classical era of Abayad’s history in the country’s western part, spread elsewhere slowly eastward and southward. Under King Fathi I, several new qilba were constructed to promote agricultural production.

Name
Qilba comes from the Old Imbu word for “channel.” Common variations of the word include qilbat, qilban, qilbanat, and qilbundat.

Origins
Qilba technology was invented in Abayad sometime around 1400 BC and spread from there slowly to the rest of the peninsula. However, more primitive versions of the same systems have been found to be even older; a qilba found in northern Abayad has been dated to be 2700 years old. Differences in the way that they were constructed has led to archaeologists disagreeing as to whether the “Ishraqi qilba” as it has been called represents a true qilba, as the qilba is a well that has been turned into an artificial spring, whereas the Ishraqi qilba site represents the development of a natural spring to renew or increase flow following a recession of the water table. Whatever the discourse, consensus has been established that they were established and used by 1400 BC.

Technical Features
Qilba are a series of well-like vertical shafts, connected by gently sloping tunnels. They efficiently deliver large amounts of subterranean water to the surface without any need for pumping. The water drains instead by gravity, typically from an upland aquifer, with the destination lower than the source. Qilba allow water to be transported over long distances in hot, semiarid climates without much water loss to evaporation. It is very common for qilba to start below the foothills of mountains, where the water table is closest to the surface. From this source, the qilba tunnel slopes gently downward, slowly converging with the steeper slope. Of the land’s surface above, and the water finally flows out above around where the two levels meet. To connect a populated or agricultural area with an aquifer, qilbas must often extend for long distances. Qilba are sometimes split into an underground network of smaller channels called karz. Like qilba, these smaller channels are below ground to avoid contamination and evaporation. In some cases, water from a qilba is stored in a reservoir, typically with night flow stored for daytime use. The qilba system has the advantage of being resistant to natural disasters such as earthquakes or floods, and to deliberate destruction in war. It is almost insensitive to varying levels of precipitation, delivering a flow with only gradual variations from wet years to dry. From a perspective of sustainability, qilba are powered only by gravity, and thus have low operation and maintenance costs once built. They transfer freshwater from the region’s mountains to lower-lying plains with saltier soil, also helping to control soil salinity and prevent desertification. The value of the individual qilba is directly related to the quality, volume, and regularity of its water flow. Much of Abayad’s inland population not connected to rivers or springs has historically depended on water from qilba; areas of high population density there generally correspond to areas with historically productive qilba. Although expensive to construct, its long-term value to the community (and the group investing to build and maintain it) is substantial.

Features common to regions utilizing qilba
Qilba are used most extensively in areas with the following characteristics:
 * An absence of larger rivers with year-round flows sufficient to support irrigation
 * Proximity of potentially fertile areas to precipitation-rich mountains or mountain ranges
 * Arid or semiarid climate with high surface evaporation rates so that surface reservoirs and canals would result in high losses
 * An aquifer at the potentially fertile area which is too keep for convenient use of simple wells

Impact of qilba on settlement patterns
A typical town or city in Abayad where the qilba is used typically has more than one. Fields and gardens are located both over the qilba a short distance before they emerge from the ground and below the surface outlet. Water from the qilba defines both the social regions in towns and cities and their layouts. Water is freshest, cleanest, and coolest in the upper reaches, meaning that the more prosperous community members normally lived at the outlet or immediately upstream from it. When the qilba is still below ground, the water is drawn to the surface via wells or animal-driven wells known as Tinifghani wells. Private subterranean reservoirs could supply houses and buildings for domestic use and garden irrigation as well. Further, air flow from the qilba is often used to cool underground summer rooms called burfatgarda (cool chambers) found in many older houses and buildings. Downstream from the outlet, water runs through surface canals which run downhill, with lateral branches to carry it to the community’s neighborhoods, gardens, and fields. The streets normally parallel these canals and their lateral branches, and as a result, inland cities and towns are oriented consistent with the gradient of the land. This is a practical response to efficient water distribution over varying terrain. The lower reaches of the canals are less desirable both for residences and agriculture. The water grows progressively more polluted as it passes downstream, and in dry years, the lower reaches are the first to see substantial reductions in flow.

Construction
Traditionally, qilba are built by skilled laborers, called qilbis, by hand. The profession has historically been a prestigious and well-paying one, guarded jealously and handed down from a man to his maternal nephews.

Preparation
The initial step in qilba construction is identification of an appropriate water source. The search begins at the point where an alluvial fan meets the mountains or foothills; water is more abundant in the mountains because orographic lifting, and excavation in the alluvial fan is relatively easy. The qilbis follow the track of the main water courses coming from the mountains or foothills to identify evidence of subsurface water, such as deep-rooted vegetation or seasonal seeps. A trial well is then dug to determine the location of the water table and to determine whether a sufficient flow is available to justify construction. If these prerequisites are met, the route is laid out above ground. The equipment for the process must be assembled on-site, but is fairly straightforward: containers (usually leather bags), ropes, reels to raise the container to the surface at the shaft head, hatchets and shovels for excavation, lights, spirit levels or plumb bobs, and string. Depending on the soil type, qailba liners (usually fired clay hoops) may be required. Although construction methods are simple, the construction of a qilba requires a detailed understanding of subterranean geology and a degree of engineering sophistication. The gradient of the qilba must be carefully controlled: two shallow a gradient yields no flow, and too step will result in excessive erosion, eventually causing it to collapse. Misreading the soil conditions also leads to collapses, which at best require extensive rework, and at work are fatal for crews.

Excavation
Construction of a qilba is usually performed by a crew of 3-4 qilbis. For a shallow qilba, one worker typically digs the horizontal shaft, one raises the excavated earth from the shaft, and one distributed the excavated earth at the top. The crew typically begins from the destination to which the water will be delivered into the soil and works toward the source (the test well). Vertical shafts are excavated along the route, separated at a distance of 20-35 m. The separation of the shafts is a balance between the amount of work required to excavate them and the amount of effort required to excavate the space between them, as well as the ultimate maintenance effort. In general, the shallower the qilba, the closer the vertical shafts. If the qilba is long, excavation may begin from both ends at once. Tributary channels are often constructed to supplement the flow of water.

Most qilba run less than 5 km, while some have measured up to 70 km. The vertical shafts usually range from 20 to 200 m in depth, although qilba in Abayad’s northeast have been recorded with vertical shafts of up to 275 m. The vertical shafts support construction and maintenance of the underground channel, as well as air interchange. Deep shafts require intermediate platforms to simplify the process of removing soil. Construction speed depends upon the depth and nature of the ground itself. If the earth is soft and easy to work, at 20 m depth a crew of four can excavate a horizontal length of 40 m a day. When the vertical shaft reaches 40 m, they can excavate only 20 m a cay, and at 60 m in depth, this drops below 5 horizontal meters a day. In many parts of the northeast, a common speed is just 2 m per day at a depth of 15 m. Deep, long qilba (which many are) require years, or even decades, to build. Excavated material is usually transported by means of leather bags up the vertical shafts. It is mounded around the vertical shaft exist, providing a barrier preventing windblown or rain-driven debris from entering the shaft. These mounds are often covered to provide further protection of the qilba. From the air, qilba shafts often resemble a string of bomb craters.

The qilba’s water carrying channel must have a sufficient downward slope that water flows easily. However, the downward gradient must not be so great as to create conditions under which the water transitions between supercritical and subcritical flow; if this occurs, the waves of water that are produced can result in severe erosion that can damage or destroy the qilba. The choice of the slope is a trade off between erosion and sedimentation. Highly sloped tunnels are subject to more erosion as water flows at higher speeds, while on the other hand, less sloped tunnels require frequent maintenance due to the problem of sedimentation. A lower downward gradient also contributes to reducing the solid contents and contamination in water. In shorter qilba, the downward gradient varies between 1:1000 and 1:1500, while in longer ones it may be nearly horizontal. Such precision is routinely obtained with a spirit level and string. In cases where the gradient is steeper, underground waterfalls may be constructed with appropriate design features (usually linings) to absorb the energy with minimal erosion. In some cases, the water power has been harnessed to drive underground mills. If it is not possible to bring the outlet of the qilba out near the settlement, it is necessary to run a canal overground. This is avoided when possible to limit pollution, warming, and water loss due to evaporation.

Maintenance and restoration
Vertical shafts in most places are covered to minimize blown-in dirt of sand, or tampering. The channels of qilba must be periodically inspected for erosion or cave-ins, cleaned of mud and sand, and otherwise repaired. For safety, air flow is assured before entry by qilbis. Some damaged qilba used historically have been restored in the modern day, spearheaded by Abayad’s republican government in 1456 AC. To be sustainable, restoration needs to take into account many nontechnical factors, beginning with the process of selecting the qilba to be restored. In southern Abayad, three sites have recently been chosen based on a national inventory conducted in 1591. Selection criteria include the availability of a steady groundwater flow and social cohesion and willingness to contribute of the community using the qilba.

Applications
The primary applications for qilba are for drinking water supply, irrigation, and providing livestock with water. However, numerous other applications have been found for the systems as well.

Cooling
Qilbas used in conjunction with a wind tower can provide cooling as well as a water supply. A wind tower is a chimney-like structure positioned above the building; of its four openings, the one opposite the wind direction is opened to move air out of the house. Incoming air is pulled from a qilba below the house. The air flow across the vertical shaft opening creates a lower pressure and draws cool air up from the qilba tunnel, mixing with it. The air from the qilba is drawn into the tunnel at some distance away and is cooled both by contact with the cool tunnels walls/water and by the transfer of latent heat of evaporation as water evaporates into the air stream. In drier semiarid or arid climates, this can result in reduction in air temperature greater than 15 C coming from the qilba; the mixed air still feels dry, so the basement is cool and only comfortably humid. Wind tower and qilba cooling have been used in Abayad for over 1,000 years.

Ice storage
By 800 BC, Abayadi engineers had mastered the technique of storing ice, even in the middle of the Summer. Ice would be brought in during the winters from nearby mountains, but in a more usual and sophisticated method, they built a wall in the east-west direction near the ice pit. In winter, the qilba water would be channeled to the north side of the wall, whose shade made the water freeze more quickly, increasing the ice formed during the day. Then, the ice was stored in large underground spaces with thick, insulated walls called tamakhid. The tamakhid was connected to the qilba, and a system of wind towers was used to draw cool subterranean air from the qilba to maintain temperatures inside the space at low levels, even during hot summer days. As a result, the ice melted very slowly, and was available year-round.