Rainwater harvesting

Rainwater harvesting is the gathering, or accumulating and storing, of rainwater.
Rainwater harvesting has been used to provide drinking water, water for livestock,
water for irrigation or to refill aquifers in a process called groundwater recharge.
Rainwater collected from the roofs of houses, tents and local institutions, or from specially
prepared areas of ground, can make an important contribution to drinking water. In some cases,
rainwater may be the only available, or economical, water source. Rainwater systems are simple
to construct from inexpensive local materials, and are potentially successful in most habitable locations.
Roof rainwater can be of good quality and may not require treatment before consumption.

In average there is around 1000 liter water on each square meter a year.
So there is around 100 000 liter rainwater to collect a year on an average roof.
This means 270 Liter a day.