Super Team

Rain Water Harvesting

What is the maximum quantity of rain water that can be collected using rain water harvesting tank?

There are three main factors to be considered:

  1. Area of the roof in squire meters.
  2. Annual rainfall of the area in millimeters.
  3. Runoff Coefficient of rain water after exceeding the retention capacity of the roofing materials. (Runoff Coefficient of Tile & Asbestos- 0.8 and for Aluminum sheets - 0.9 )

 Maximum amount of                       Area of                     Annual rain fall                      Runoff
    Water that can be           =          the roof        X           of the area                   X      Coefficient   

For example when considering an annual rainfall of 1000 millimeters, a total of 40000 liters of rain water can be collected from an area of 50 sq.meters of tile roof. It is very much encouraging if we can save this whole quantity of rain water during a year. However, there are other factors that have to be taken into consideration when practicing rain water harvesting process.

  • Difference of annual rainfall recorded at the nearest metrological station and actual rainfall received at the location in question.
  • Rainfall estimates are based on 30 year monthly average and this period is too long considering the present day climate. There may be prolong dry periods without experiencing any rain.
  • Even in a situation of a heavy rainfall the amount of rain water that can be saved is limited to the capacity of the rain water harvesting tank.

In view of the above the actual quantity of rain water that can be saved is very much less compared to the rain fall received in a given year.



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Janathakshan Project

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No 05, Lionel Edirisinghe Mawatha,

Colombo-5, Sri Lanka.


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