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Rainwater harvesting is the gathering, or accumulating and storing, of rainwater. Traditionally, rainwater harvesting has been practiced in arid and semi-arid areas, and has provided drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish ground water levels.
 What is rainwater harvesting
The principle of collecting and using precipitation from a catchments surface.
An old technology is gaining popularity in a new way. Rain water harvesting is enjoying a renaissance of sorts in the world, but it traces its history to biblical times. Extensive rain water harvesting apparatus existed 4000 years ago in the Palestine and Greece. In ancient Rome, residences were built with individual cisterns and paved courtyards to capture rain water to augment water from city's aqueducts. As early as the third millennium BC, farming communities in Baluchistan and Kutch impounded rain water and used it for irrigation dams.
Artificial Recharge to Ground Water is a process by which the ground water reservoir is augmented at a rate exceeding that obtained under natural conditions or replenishment. Any man-made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.
Rain water harvesting is essential when:
- Surface water is inadequate to meet our demand and we have to depend on ground water.
- Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging of ground water has diminished.
There are two main techniques of rain water harvesting:
- Storage of rainwater on surface for future use.
- Recharge to ground water.
There are many types of systems to harvest rainwater. Notable systems are systems for runoff rainwater (e.g. hillside run-off) and rooftop rainwater harvesting systems. The type used depends greatly on the purpose (domestic or industrial use) and to some extent also on economics, physical and human considerations. Generally speaking, rooftop rainwater systems are most used as they are most economical (if there is more than 254mm of precipitation a year)
 System types
At the moment, 2 types of systems are generally used. These include DIY and commercial systems. Both of these systems are known under the term water harvesters and require only a limited amount of knowledge to set up (if basic systems are used). In both cases, the system consists of a storage tank to store the water and piping (to guide the water in). Additionally, extra pressuring equipment as pressure vessels, inline pump controllers or pressure sensitive pumps may also be required.  Finally, water purifying equipment as water-purifying plants, UV-lights or distillation equipment are sometimes (depending on local conditions ) added to purify the collected water. The system is then called a Greywater treatment system. Greywater systems are usually preferred over regular water harvesters as they allow the system to not only treat the rainwater, but water from other sources as well (eg the watercloset; if plants are used). However, this feature may also be averted by using a UV-lamp and composting toilet instead.
Depending on local circumstances, a gravity-fed system may already be enough to have a pressured water collection system. Gravity-fed system through height difference also sometimes are enough for pressured water collection system. In the latter case, no pumps/pressure vessels are thus required to have a pressured system. In practice, gravity-controlled systems are usually created by placing the water harvester on an elevation (e.g. rooftops).
 DIY domestic systems
As water conservation is becoming more and more popular, more people have begun to make their own homebrew installation. These systems range from traditional technologies like rain barrels to more complex greywater systems.
 Commercial domestic systems
Commercial systems are also made. They are offered by a variety of companies which include Rain Man
- Irrigation: Rainwater is the primary source of irrigation for crops around the world. Water harvesting techniques have been employed for thousands of years to get more water to the fields in order to improve crop production. This is the primary traditional use of rainwater harvesting.
- Sustaining Animals: Because of animals' higher tolerance for bacteria and other impurities, harvested rainwater is often used as the primary source of water for livestock. Since cattle and sheep are well adapted to drinking rainwater, safer groundwater is often saved for human drinking.
 System's operation
A mechanism can be used to send the initial water flow to waste, usually the first few liters. These are commonly known as diverters, and are used to increase the chance that the large-particle residue that might accumulate on the collection surface is washed away from (and not into) the storage tank. Such a system also compensates for the fact that the initial minutes of a rainfall can include airborne pollutants being washed from the sky, and likewise minimizes contamination of the captured supply. Simple but regular inspection and maintenance of such a device is usually necessary.
Not all catchment systems use such a feature. For example, rainwater in rural areas of Australia is traditionally used without such a system, and without treatment, but this may be unwise in different environments
 Industrial systems
Rainwater may also be used for groundwater recharge, where the runoff on the ground is collected and allowed to be absorbed, adding to the groundwater. In US, rooftop rainwater is collected and stored in sump. In India this includes Bawdis and johads, or ponds which collect the run-off from small streams in wide area. Recharging the groundwater in this way is claimed to not only improve the year-round availability of groundwater, but also lead to more richer vegetation.
In India, reservoirs called tankas were used to store water; typically they were shallow with mud walls. Ancient tankas still exist in some places
 Advantages in urban areas
Rainwater harvesting in urban areas can have manifold reasons: to provide supplemental water for the city's requirement, to increase soil moisture levels for urban greenery, to increase the ground water table through artificial recharge, to mitigate urban flooding, and to improve the quality of groundwater. In urban areas of the developed world, at a household level, harvested rainwater can be used for flushing toilets and washing laundry. Indeed in hard water areas it is superior to mains water for this. It can also be used for showering or bathing. It may require treatment prior to use for drinking.
In New Zealand, many houses away from the larger towns and cities routinely rely on rainwater collected from roofs as the only source of water for all household activities. This is almost inevitably the case for many holiday homes.
As rainwater may be contaminated, it is often not considered suitable for drinking without suitable treatment.
Rainwater harvested from roofs can contain animal and bird feces, mosses and lichens, windblown dust, particulates from urban pollution, pesticides, and inorganic ions from the sea (calcium, magnesium, sodium, potassium, chlorine, sulfate), and dissolved gases (CO2, NOx, SOx). High levels of pesticide have been found in rainwater in Europe with the highest concentrations occurring in the first rain immediately after a dry spell, the concentration of these and other contaminants are reduced significantly by diverting the initial flow of water to waste as described above. The water may need to be analysed properly, and used in a way appropriate to its safety.
- In China's Gansu province for example, harvested rainwater is boiled in parabolic solar cookers before being used for drinking
- In Brazil alum and chlorine is added to disinfect water before consumption.
Appropriate technology methods, such as solar water disinfection, provide low-cost disinfection options for treatment of stored rainwater for drinking. Traditionally rain water has been one of the few water sources known to usually be clean and safe to drink. Today water harvesters must be wary of pesticide contamination, high mineral levels, bacteria and other impurities in their runoff water. Filtering technologies have been used in the past and today to purify it by passing it through a series of rocks, gravels, and sands to scrub out contaminants. These methods have proven to be very effective.
 Around the world
- Currently in China and Brazil, rooftop rainwater harvesting is being practiced for providing drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish ground water levels. Gansu province in China and semi-arid north east Brazil have the largest rooftop rainwater harvesting projects ongoing.
- In Bermuda, the law requires all new construction to include rainwater harvesting adequate for the residents.
- The U.S. Virgin Islands have a similar law.
- In the Indus Valley Civilization, Elephanta Caves and Kanheri Caves in Mumbai rainwater harvesting alone had been used to supply in their water requirements.
- In Senegal/Guinea-Bissau, the houses of the Diola-people are frequently equipped with homebrew rainwater harvesters made from local, organic material.
- In the United Kingdom water butts are oft-found in domestic gardens to collect rainwater which is then used to water the garden.
- In Colorado, water rights laws severely restrict rainwater harvesting -- a property owner who captures rainwater is effectively stealing it from those who have rights to take water from the watershed.
- Water harvesting can be traced back through human history almost as far as the origins of agriculture. Water harvesting is defined as the redirection and productive use of rainfall. In some incarnations it resembles a primitive type of irrigation. Unlike conventional irrigation, however, rainwater harvesting does not depend on a constant flow of water; it is totally dependent on rain. Basically, harvesting involves a variety of methods used to get as much water as possible out of each rainfall. Uses were nearly limitless when rain could assumed to be clean. These ancient practices sustained ancient people when conditions would have otherwise totally prevented agriculture. Many peoples in the world have continued to rely on water harvesting practices. Others have returned to it in order to relieve pressure on overburdened underground water tables or municipal water systems. Some Western groups are discovering ways that rainwater harvesting can be used in our society to relieve stress on the environment, save money, and recharge groundwater tables.
 See also
Design and installation of rainwater collection systems