A rainwater harvesting system is any type of device for the collection and storage of rainwater, and whose technical and economic viability depends on the rainfall of the collection area and the use given to the collected water.[1]​.

In places where the available groundwater is outside the limits established to consider it drinkable (especially if it contains heavy metals such as lead, mercury, chromium or other substances harmful to health), rainwater harvesting can be used for restricted consumption, that is, for drinking and cooking food. Generally, it is considered that the needs for these purposes are limited to 4 to 6 liters per inhabitant per day, while the total water consumption is much higher, even exceeding one hundred liters per inhabitant per day.

Catchment area

Where:.

In other words, the necessary catchment area will be:.

Cisterns in the Roman Empire

In ancient times, in the territories of the Roman Empire, where the climate was semi-arid, such as in the Iberian Peninsula and the Italian Peninsula; Large underground reservoirs were built where rainwater was stored for later consumption, whether human or agricultural.[2]​[3]​ Many of these large hydraulic works continue to appear and show the techniques of Roman engineering,[4]​ which had replaced the sandstone used by the Greeks with limestone rock.

In Gibraltar

The plates that cover a section of the eastern slope of the Rock, a place especially affected by the rains, constituted the system to obtain drinking water, since Gibraltar lacks its own rivers or springs; Rainwater was channeled from there to enormous buried tanks. This system, although effective, became insufficient, so currently most of the water for human consumption is obtained through the desalination of seawater.