Start with the roof, not the tank
A big cistern looks reassuring. Its capacity says little about how often it fills. The EPA describes rainwater harvesting as a linked system: a capture area, storage and an intended use. Screens, debris control and regular maintenance are part of that system, rather than optional finishing touches.
Sources: s1
A transparent upper-bound calculation
Consider an invented 100-square-metre roof receiving 10 millimetres of rain. Rain depth is 0.01 metres, so area multiplied by depth gives one cubic metre: 1,000 litres. That is the volume landing on the roof, before losses. A 5,000-litre tank cannot turn this rainfall event into 5,000 litres of supply.
If we assume, purely for illustration, that 80 percent reaches storage, the captured volume becomes 800 litres. A notional demand of 200 litres a day would use that amount in four days. These assumptions are arithmetic inputs, not a design recommendation or a measured household demand.
Timing changes the answer
The EPA notes that an abandoned or unused rain barrel can remain full, leaving no capacity for the next storm. At the other extreme, a long dry interval can exhaust storage even where annual rainfall seems generous. The useful historical question is therefore broader than 'Did this settlement have tanks?' It is 'What collected the water, when did rain arrive, how was storage maintained, and what demands could it meet over time?' A plausible reconstruction should show those assumptions.
Sources: s1