Some things to consider:

Would having some barrels of rain water be useful to you in an emergency?

One thing to find out is where the rain coming off your roof goes now. If it is mixed into the sewer system, there there are needless extra costs in processing it.

Simply switching to Composting Toilets can save up to 40% of household water.

What are some tips for building composting toilets?

You can calculate how much rain you could capture, and compare that to your water usage.

Brian Fey's answer to What is the formula for how much rain water hits a roof based on the area of the roof?

Brian Fey's answer to What is the best roofing material to use if you plan on collecting rainwater?

See these related answers:

Brian Fey's answer to What are the best ways to treat rainwater for drinking?

Risposta di Brian Fey a Com'è la qualità dell'acqua (di rubinetto) dove vivi?

Catturare l'acqua piovana è abbastanza facile, è lo stoccaggio che è costoso.

Ho circa 100.000 litri d'acqua, ma una parte di questa è in cisterne di pietra o cemento, quindi un terremoto potrebbe portare via la mia acqua. Preferisco quelle di plastica, soprattutto se sono all'interno dell'edificio che fornisce massa termica, e protette dai raggi del sole.

Guarda questo video che ho fatto:

Questo libro è fantastico:

Water Storage: Tanks, Cisterns, Aquifers, and Ponds for Domestic Supply, Fire and Emergency Use--Includes How to Make Ferrocement Water Tanks: Art Ludwig: 9780964343368: Amazon.com: Books

Wikipedia: Rainwater harvesting

An interesting TED talk: The ancient ingenuity of water harvesting