Hydrogeology and Water Supply

On the basis of the geological and hydrogeological setting of Thrace, it is possible to hypothesise that the scarcity of springs with significant discharge during the dry season could explain the necessity to extend the catchment to the perennial sources of higher discharge in the distant area of Vize. For the first 100km of the supply line, from the principal sources in the region of Vize to modern Binkiliç the main water channel is mostly supplied by karstic springs related to aquifers located on the southern slopes of the Istranja massif and belonging to the Ergene River Basin. This was also the case for the spring sources close to the Anastasian Wall such as Papuç and Pinarca. The implication of this feature for the water supply is that there is a very significant difference in the discharge of the springs between the winter and summer months, with a particularly rapid response following heavy rain.

In terms of scale and complexity, the closest parallel for the water supply of Constantinople is the system that evolved in Rome between the 1st and the 3rd centuries AD. Yet Rome benefitted from a much greater availability of water resources within a shorter distance of the city, with more substantial hydraulic potentials and more stable discharge. The Thracian water scheme was instead conceived to overcome problems related to a general shortage of ground-water resources, particularly during droughts. It is likely that survival strategies were in place to cope with the cyclical drought periods that must have occurred due to natural decline of the source discharge as a result of low precipitation in the summer season.

The dramatic seasonal variability of the Thracian supply was noted by the historian Procopius in his account of the construction of the Basilica cistern (Yerebatan Saray) in 6th-century Constantinople, for which 'the emperor Justinian made a suitable storage reservoir for the summer season, to contain the water which had been wasted because of its very abundance during the other seasons' (Buildings I, xi, 13-15). The construction of the Basilica cistern is attributed by Malalas, another 6th-century chronicler, to Longinus, the Prefect of Constantinople under Justinian also mentioned in an an inscription found by us at Elkafdere, who was presumably therefore involved in restorations and improvements to the system both in the city and the hinterland (Malalas, 482B). Although it may be doubted that Justinian was the first to recognise the problem of seasonal variability, a consideration of these hydrogeological issues may explain why Istanbul possesses so many open and covered cisterns of Byzantine date. At least 70 cisterns are known from archaeological and literary sources in the city, ranging from small private establishments, to enormous open-air reservoirs.

Rome's aqueduct discharge was therefore perhaps 10 times more prolific than that of Constantinople during the dry season (summer -early autumn). Although at different times, both these systems were devised to supply populations in excess of half a million. In the context of such a quantification, the distribution, quantity and scale of the cisterns recorded in Istanbul perhaps finds explanation.

For a more detailed account of the hydrogeology of Thrace and a comparison with that of Rome, see Crow, Bono and Bayliss 2001.