A massive Roman cistern in southern Spain could hold up to 2.18 million liters (576,000 gallons) of water, according to a new study. Researchers built a detailed three-dimensional model of the structure and used it to measure its size, capacity, and the sediment piling up inside.
The Great Cistern sits at the Roman site of Sierra Aznar, in Arcos de la Frontera, in the Cádiz province. The study, led by José-Antonio Calvillo-Ardila of the University of Cádiz, appears in the journal “Heritage.”
The cistern dates from the first century A.D. It rests at an altitude of 355 meters (1,165 feet) and has an almost square floor plan, roughly 20 meters by 19 meters (66 feet by 62 feet).
The structure runs about 5 meters (16 feet) deep and has rounded corners. One wall leans against the rock face, shaped to fit the slope of the land.
Two scanning methods mapped the ancient structure
To document it, researchers combined two methods. They used image-based photogrammetry with a Nikon camera and terrestrial laser scanning with a Leica device. A total station fixed the measurements in place.
The team took 513 photographs and set up the scanner at 11 positions. The whole field survey, including topographic control, took about four hours.
Both methods produced reliable results. However, the study found that photogrammetry was slightly more accurate overall, with errors staying within a few centimeters.
The model also revealed how much sediment has built up over the centuries. Researchers estimated the deposits at about 819.9 cubic meters (28,950 cubic feet). That fills roughly 37.5% of the original volume.
Sediment now fills one-third of the massive Roman cistern
In short, nearly one-third of the massive Roman cistern is now choked with sediment. The buildup reaches about 1.3 meters (4.3 feet) deep at its lowest point and about 2.3 meters (7.5 feet) at its highest.
The study also looked at how the cistern fits into the wider water system at the site. Nearby stand two sedimentation basins and a fountain.
The cistern sits at the highest point, followed by the basins and then the fountain at the bottom. Across the whole system, the land drops about 30 meters (98 feet).
A gravity-fed system built into the slope
This stepped layout would have let water flow downhill by gravity, the researchers said. The cistern stored water on a large scale. The basins likely cleaned or settled it. The fountain probably released or held water at the end of the line.
No pipes or channels survive to confirm the connection. Still, the team argued that the spacing, elevation, and sizes point to a single, planned system. The authors said future digs and water-flow modeling could sharpen that picture.
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