COLLECTION OF WATER
A. Trevor Hodge
Before water can be either used or conveyed, by aqueduct or otherwise, it has to be collected. Sometimes indeed, though rarely, it was collected directly from the ground surface and channeled into cisterns or aqueducts,1 on the same general principles that one sees today in the paved catchment areas serving Gibraltar and a good deal of Bermuda. Otherwise it may be collected naturally, forming springs, streams and lakes, which are then tapped by conduits, or artificially, by dams, reservoirs, and cisterns, not to mention wells, which will be dealt with in the following section.
The cistern differs from the other methods of collection mentioned in that its water does not require transport, for the cistern is located at the point of use. It is essentially an underground tank, built of masonry and lined with waterproof cement to prevent leaks, and fed by rainwater, either as surface water running along the ground, or, with domestic cisterns, as run-off from the roof of the houses they serve. The largest cisterns are those found in industry and agriculture. They can be practically any shape that is found locally convenient, though the circular and the rectangular are the commonest, and, as compared with their surface area, are relatively shallow, while house cisterns, where space is at a premium, are much deeper.
Set into the ground, they may go down to a depth of two or three meters below it, and are either open to the air, or, sometimes, covered by a wooden roof to restrict evaporation; the roof is supported by a central stone pier. Often there is, in one corner of the cistern, a flight of stone steps leading down to the bottom and giving access for cleaning and maintenance, and, doubtless, drawing water when the level was low. One is liable to encounter such cisterns almost anywhere, but local economics and climate (and the
1 Hodge 1992, 79.