Rain Water Harvesting Potential of the Andaman & Nicobar Islands

Some Basic Calculations on Roof top Rainwater Harvesting Systems for Port Blair

By Peeyush Sekhsaria

Rainfall data was kindly provided by John Swamy, a member of this e group.

Month	Average Rainfall for Port Blair in mm (based on 5 to 27 years average from 1930 - 1960)	Rain water harvesting potential for a 100 sq m/1000 sq feet roof	Number of days water rainwater can provide (based of 400 litres/day for a family of 4)
January	27	2700	6.75
February	26	2600	6.5
March	20	2000	5
April	82	8200	20.5
May	355	35500	88.75
June	564	56400	141
July	417	41700	104
August	432	43200	108
September	514	51400	128.5
October	309	30900	77.25
November	193	19300	42.25
December	140	14000	35
		307900	769.75

Rain Water Harvesting can alone Provide for the water needs of a family of four for 9 months of the year

Notes

1. Rainfall data for Port Blair is based on 5 to 27 years average from 1930 - 1960

2. Sample calculations done for a 100 square metre / 1000 square feet roof area

3. Average consumption assumed at 100 lpppd (100 litres per person per day) - This data is based on national standards and may not represent ground reality where in actual consumption may be found lower.

4. Family size assumed of 4 members, hence average daily consumption comes to 400 litres /day

5. Depending on the roof type there may be a maximum correction of 0.2 to the effective rainwater harvested to take care of cleaning, loss to absorption, etc

6. The cost of setting up a Rain Water Harvesting System vary a great deal depending on the individual site conditions, roof type, capacity of storage to be created.

7. Rain Water Harvesting systems can be mostly easily retrofitted on existing buildings

8. The calculations here are keeping in mind a harvest - store and a use as you store system. Meaning to say water is not stored only to be used during lean months

9. With the rainfall pattern of Port Blair it can be safely said that rain water alone can completely suffice for 8 months of the year, play a major part in the month of April. For the months of January it is possible to take advantage of the rainfall in December. Also depending on the storage capacity of the system rain water may suffice for the lean months of February & March

10. Though roofs will have to be kept clean, rainwater will have to be filtered before storage and water collected tested, it can be safely said that properly collected rainwater is fit for drinking after boiling.

11. More work will be needed if one is looking at actually designing and implementing a system, but I hope this gives an idea of the fantastic potential of Rain Water harvesting in the islands

12. Just to give an idea of the likely costs, a RWH system for a 100 sq m roof in Bangalore with a 2500 litres storage capacity, inclusive of design and supervision charges, material, labour cost roughly 40-50000 rupees. This is life long investment and depending on cost of water money is recovered in a few years. After which money is made!!