[cybermuda]

Quick calculation:

if you have an annual rainfall of 48 inches and you have a 2,000 square foot roof, you have 50,000 gallons of rain falling on your roof each year.

Obviously you can't capture all of that - wind, evaporation etc. - but you can probably get 3,000 gallons a month on average.

As an earlier poster said, you probably need more than rain barrels for storage, but a large plastic tank used for heating oil might be an option. You might not want to put it below ground, as then you would have to pump the water back up to use it, or in your roof as it would weigh a lot when full, but you may have space in your garage or be able to hide it behind a tree. A 5' cube would hold around 800 gallons.