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Funny that this thread came up at this time. As an unemployed landscape architect - unemployment is near 40% in our business in my part of the country - I've been researching starting a rainwater harvesting system business. In Arizona, not only is it encouraged, but there's a 25% tax credit (up to $1000) for installing a "water conservation system." It used to only be for graywater collection for landscape irrigation, but has been amended to include rainwater harvesting systems. I think Tucson's new Rainwater Harvesting Ordinance is a great idea; one that I hope sets a precedent for more western cities, counties, and states.

Rainwater is not only "free" in most places, but it's higher in nitrogen than municipal or well water; it contains no salts; and is slightly acidic, which makes it a nutrient for plants, and it facilitates plants' uptake of other nutrients. This also makes it a good supplemental source for pools, whose owners in arid regions are always fighting a battle to lower the pH because of hard water. It can also leach existing salts below the root zones of plants, and can be an emergency source of water in wildfire-prone areas.

The problem here, as stated in the quoted article, is that when you can collect the most water - you don't need it, because it rains so hard in so short a time. Here in Phoenix, we get about 8 inches of rain per year; half of it between July1 and September 15. Storage tanks need to be a compromise between price and practicality. Even though a relatively small, 1,200 SF roof can capture up to 3,000 gallons during our 10-week monsoon, it's impractical to store that amount on a small residential lot (At 7.48 gal/cu ft, 3000 gallons would require a 400 cubic foot tank) . But as long as mosquitoes and light are kept from entering the tank, it can be stored until needed. Personally, I think a tank between 600-800 gallons is a good compromise. Total cost for gutters, tank, and a pump to integrate it with an existing drip irrigation system would be around $2500-$3000 for an average, 1500 SF house. And let's not forget that (in AZ) would come with a $625-$750 tax credit. You're not going to get a quick return on your investment as long as water is kept artificially inexpensive - as it is here. Most people's motivation will be the same as for buying hybrids or installing solar panels - because it's the right thing to do for our planet and our children's future.

However, the time it takes for ROI could change rapidly - water could easily quadruple in price in the next 10-20 years, and if water bans occur - it could save your landscape. I think this is going to be a very big green industry in the years to come, if states like Colorado would get their heads out of their asses - and it looks like that may happen.

ps - just in case anyone wants to consider it for their property, here's the math:
You can collect 0.6 gallons per inch of rain per square foot of roof space.

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