A Rock-Cut Underground Water-Supply System in Southern Yemen Report of the January 2001 Field Season of the Ghayl Bâ Wazîr Project

by Ingrid Hehmeyer
Royal Ontario Museum, Toronto

Yemen Update 44 (2002)

Ghayl Bâ Wazîr is an urban centre just inland from the southern coast of Yemen. The town is situated at the edge of a gypsum karst shield. Water seepage along fractures in the soluble bed-rock created an underground network of streams. Over time, large caverns were formed holding considerable volumes of water. When the cavern roofs collapsed, open reservoirs appeared like sink-holes in the otherwise dry, rocky landscape. In Arabic they are known as hawma, pl. huwam.

Use of the term ghayl in the town's name underlines the fact that the settlement's existence is intimately connected with the manipulation of underground water flow. In colloquial Yemeni use in an urban context, ghayl, pl.ghuyûl, means an engineered subterranean water-channel. The successful management of the underground water gave rise to a prosperous settlement based on irrigation agriculture.

This report summarizes the results of the January 2001 field season of the Ghayl Bâ Wazîr Project. The Project was co-funded by Shaykh Khâlid Buqshân (Jedda, Saudi Arabia), the American Institute for Yemeni Studies, and the Royal Ontario Museum Foundation, with logistical support from the German Institute of Archaeology, San‘â'. The archaeological permit was issued by the General Organization for Antiquities, Museums and Manuscripts, San‘â', courtesy of Dr. Yûsuf ‘Abdallâh, GOAMM President, under the licence of the Canadian Archaeological Mission of the Royal Ontario Museum.


1. Location of Ghayl Bâ Wazîr in Yemen

Exploiting the karst reservoirs

Tapping the open reservoirs was the most obvious way to exploit underground water in Ghayl Bâ Wazîr. But the gypsum karst in the immediate vicinity of the water-filled reservoirs inhibits soil formation; it is bare rock. The challenge in antiquity, as in the present, was to find a way to tap the water and direct it to a settlement or a field. To achieve this, tunnels were engineered through the bedrock, leading the water to its destination some distance away, using gravity flow. Vertical shafts sunk down from the surface provided access to the gallery below and facilitated removal of the excavated rock. The local term for these engineered water tunnels is ma‘yân, pl.ma‘âyîn.


2. Water-filled reservoir

Tapping ground-water

Where open reservoirs did not exist, alternate approaches were taken. The Project documented the digging of what is called a "mother (well)" in other parts of the Near East. In Ghayl Bâ Wazîr the term used is "father (well)": al-ab. A probe was sunk through the bedrock, creating a well-like shaft. On finding a good water flow, the same engineering principles were applied as in tapping the open reservoirs, namely: the digging of ama‘yân.

A variation on the "father well" theme in Ghayl Bâ Wazîr is found where the principle of digging a probe-shaft was applied in a wadi. Invariably, an established wadi is a barren gravel bed for most of the year. But, during the short-lived floods, some of the surface flow penetrates below ground, where the water persists longer than that above ground. In order to tap this subterranean source, a probe was sunk in the wadi bed. The shaft was ringed with masonry to keep out debris and sediment borne by the floods.

Water use

From tapping these various water sources, a dense network of channels criss-crosses the Ghayl Bâ Wazîr landscape. Mapping the underground water systems allowed observations to be made about the different water uses. In most cases, the channels were first directed to a settlement, for people to utilize the clean water. In inhabited areas a staircase provided access to the channel, for women to fetch water for household purposes. We also find public washing facilities, and facilities for bathing, straddling the channel.

The grey water then travels to market gardens outside the city. Fruit, vegetables and fresh fodder are cultivated here, as well as cash crops such as tobacco and henna. The principle of water being delivered first to the settlement, and then to the fields as irrigation water, is an efficient conservation device for a scarce resource. Notwithstanding, before the modern era, the town's inhabitants preferred well-water for drinking, because the water flowing from the karst had a high calcium content. The exception to this rule was a source in the gravelly area west of the gypsum shield which had sweet water. Allocations from this channel for drinking water were made at prescribed hours of the night, to ensure minimal contamination by animals or humans.


3. Stepped floor of rock-cut channel

Timing of the water allocation

In all instances, the underground water-flow was constant. But the flow needed to be directed to different places, whether for irrigation, religious, or domestic use. Allocation was timed by star calendar at night, and sundial principle by day. The star calendar has been in use in Ghayl Bâ Wazîr for at least the past 150 years. It is based on 28 marker stars, of which 14 are visible during any given night of the year. The passing of time is indicated by the succession of these 14 stars. Timetables based on the star calendar are still published by the local authorities in Ghayl Bâ Wazîr. They are widely used in the mosques to determine prayer time.

Age of the underground water system

An obvious question is when were the water sources first exploited by engineering underground channels? According to local legend, a certain Shaykh ‘Umar is credited with having discovered the subterranean water sources in the 14th century by firing an arrow in the air. From the spot where it landed, a spring immediately flowed forth. The story is clearly apocryphal; similar stories abound in the Middle East. But it reflects the geological reality of the terrain, and the fact that in the karst landscape of Ghayl Bâ Wazîr seepage along faults and joints was highly conducive to sudden spring formation. Digging open the spring to increase and regularize the flow could easily have furnished the perpetrator with legendary status.


4. Opening of debris removal shaft

Conclusion

The overall similarity of the Ghayl Bâ Wazîr ma‘yân to the so-called qanât, pl. qanawât, systems of Iran is obvious. Some explorers have gone one step further and credited construction of the Ghayl Bâ Wazîr ma‘yânsystem to Iranian qanât engineers. However, a qanât is by definition dug into an alluvial fan that is more or less compacted, but nevertheless an originally loose - and therefore porous - sediment. The engineering principle in Ghayl Bâ Wazîr is different, because it was based on a rock-cutting technique. The last living ma‘yân digger of Ghayl Bâ Wazîr asserted that digging through alluvium was only done if circumstances made it unavoidable; the preferred practice was cutting channels through rock.

It can be argued that cutting bed-rock is a typically Yemeni way of engineering water systems. If we turn to Mârib, examples abound where the bed-rock was cut to make either a water-channel or foundation for a solid irrigation structure. Another famous Yemeni example is in Baynûn, where tunnels were cut to carry water right through the mountain. The Project concludes that the application of rock cutting in the Ghayl Bâ Wazîr karst landscape reflects in essence a Yemeni technology.

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