Rainwater harvesting is the collection of rainwater, which falls freely from the sky, for use as a substitute for potable water, which is costly and in increasingly short supply in many regions. The water is usually collected from building roof areas, and diverted from the roof drainage system to a storage tank, where it is held for use.
The simplest rainwater collection system is a “rain barrel”. This is simply a barrel (or some other tank-like receptacle) that is positioned under the downspout from a building – usually a house – that collects water to be dispensed as needed for watering the garden. A single barrel may be used, or several may be connected together. Rain barrels may be purchased, but it is quite easy for a home gardener to fashion one from almost any type of barrel or tank.
In homes and larger buildings, rainwater may also be used for many purposes for which pure drinking water is not required. Besides landscape maintenance and other outdoor uses such cleaning, these include flushing toilets, washing laundry, and operating mechanical heating and cooling equipment that use water. In these cases, a specialized plumbing system will be required to collect and distribute the rainwater. A simple diagram is shown.
The collected rainwater is usually stored in a tank in the basement (or underground) and then pumped to a higher level tank from which the water can flow by gravity for use. The high-level tank may also be supplied with fresh water, only used in case of long periods without rain.
Rainwater harvesting, even for drinking water, is as old as mankind, and certainly not rare in the world today. In less developed regions of the world without municipal water supply, rainwater collection may be the only source of safe drinking water.