Whoops: We actually have 4,799 square feet of metal roof that we will use for rainwater collection. I copied the wrong number; sorry about that. We plan to use this water on trees, bushes and plants--not much lawn. When I plant grass I plant Buffalo grass that dies back during a drought and then comes back strong after a rain. This system is not intended to provide potable water; although, lets be realistic. If our aquifer should disappear on us in the future we would be drinking and cooking with this water after running it through charcoal, ultraviolet, ozone and Heaven only knows what other kinds of filters. We have a neighbor a few miles up the road who doesn't use any well water whatsover (or so he claims). He collects water from the roof of his house (a large house because he's a stockbroker and has done well by himself the last few years) and saves the water in two, fiberglass, 10,000 gallon tanks. He has not done any irrigation up to this point and says that his tanks are a little over half full after a year's use by his wife and himself. From what I gather from others, if rainwater is the only source of water, a couple should think in terms of at least 40,000 gallons of water usage annually. As I said in an earlier post, I'm going to keep track of our rainfall during the next year or so and then we will make a decision on the size of our primary tank. I really don't expect our 4,000 gallons of water in our four catch-basins to take us much beyond the end of spring and the beginning of the dry summer season.