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Adding to Neil's answer above, rainwater harvesting has advantages if:

• there is a suitable pattern of rainfall for collection
• the system is affordable to build and maintain
• the community want to have a rainwater harvesting system and are willing to maintain it

Typically it is used where other sources of water are insufficiently available (perhaps seasonal) and unreliable, inconveniently located (such as too far away), or polluted (as in the case of arsenic contamination in parts of Asia and SEAsia).

The disadvantages are:

• Installation of a collection and storage system can be surprisingly costly, depending on the location, and is likely to be the highest cost water supply available
• The system does need some maintenance, particularly in terms of cleaning
• Clean, stored water can become polluted by leaf debris, birds and small animals (as in any water storage system). Particular care should be taken to avoid creating breeding sites for mosquitos.

If you do a search you will find a lot of references for information on rainwater harvesting systems. A good source is Chapter 7 in the book by Smet and van Wijk (2002).

Harriette