RAINWATER HARVESTING

Now, this is touchy, especially considering how ridiculous California is. Before you start doing this, you need to check into the legality in your state, county and neighborhood. That's my disclaimer.

Rainwater harvesting varies for what you want to use it for. Some people harvest rainwater just to water their gardens, so they keep it in a separate barrel outside. Some people tie it into their home grid and use it for flushing toilets and washing cars. Some actually purify it and use it as their their main water source... though they're usually off-gridders.

Check out this link if you're interesting in learning about rainwater harvesting:
An Introduction to Rainwater Harvesting

And purifying rainwater:
Slow Sand Filter
How To Pedia: How to Filter Water With a Sand Filter

And, this might just be me, but once I'd filtered the water with a slow sand filter, I'd probably use something like a Berkey Water Filter (which we use at Zombie Con) which is then drinkable. Course, me being me, I would then completely over do it be putting that through a Brita Water Pitcher Filter.