what is rain water harvesting ?

1. Rainwater harvesting is a technique used for collecting, storing, and using rainwater for landscape irrigation and other uses. The rainwater is collected from various hard surfaces such as roof tops and/or other types of man made above ground hard surfaces.

Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to runoff. Uses include water for garden, water for livestock, water for irrigation, water for domestic use with proper treatment, and indoor heating for houses etc. In many places the water collected is just redirected to a deep pit with percolation. The harvested water can be used as drinking water as well as for storage and other purpose like irrigation.

Rainwater Harvesting Basic Components

Rainwater systems come in all shapes and sizes, from simple catchment system under a downspout to large above and/or underground cisterns with complex filtration systems that can store thousands of gallons of water. Most rainwater collection systems are comprised of the following basic components:

• Catchment surface - rooftop or other raised solid surface. The best catchment systems have hard, smooth surfaces such as metal roofs or concrete areas. The amount of water harvested depends on the quantity of rainfall, and the size of the surface and the slope of the catchment area.
• Gutters and downspouts - also known as distribution systems that channel the water from the catchment area to a holding container such as a barrel, cistern, planted area, etc.
• Filters - a Filter that removes or catches debris

• First Flush - a device that diverts the "first flush" of rain before it enters the storage tank. Most rainwater suppliers recommend that the "first flush" of water is diverted to an outside area of the storage system, since the catchment surface may accumulate bird droppings, debris and other pollution.
  
  (Act-Wisy filters are equipped with this feature and German made filters are now available in India through Act-Wisy )

• Storage tanks - In general, the storage tank is the most expensive component of a rainwater harvesting system. There are numerous types and styles of storage tanks available. Storage can be above-ground or underground. Storage containers can be made from galvanized steel, wood, concrete, clay, plastic, fiberglass, polyethylene, masonry, etc. Examples of above-ground storage include; cisterns, barrels, tanks, garbage cans, above ground swimming pools, etc. Storage tank prices vary based on different variables such as size, material and complexity. To inhibit the growth of algae, storage tanks should be opaque and preferably placed away from direct sunlight. The tanks should also be placed close to the areas of use and supply line to reduce the distance over which the water is delivered. Also consider placing the storage at an elevated area to take advantage of gravity flow. The tank should always be placed on a stable and level area to prevent it from leaning and possibly collapsing.
• Delivery systems - gravity-fed or pumped to the landscape or other end use areas.