What dimensions would a rainfall cistern need to be to (a) hold all the rain that falls on the roof over the average year, and (b) satisfy all the indoor water use of a family of four for an average year? I ended up estimating that a tank capable of storing about 50,000 gallons would hold all the rain for an above-average year, and if it filled up over time, would cover the entire family’s water use for a year.
Here are my calculations:
First, I measured our roof area, ignoring the slant. The main roof and first floor roof lengths and widths yields approximately 31 feet wide by 50 feet long, which totals 1,550 square feet. That’s 372 x 600 inches, or 223,200 square inches.
In our region, the average rainfall is 45.45 inches. If that much rain falls on a 223,200 square inch area, the volume of water running off will be 223,200 x 45.45 = 10,144,440 cubic inches. There are 0.004329 gallons per cubic inch, so that’s 43,915 gallons per average year of rainfall hitting our roof. (And, suppose it’s a very wet year — with 20% more rain, the volume could rise to 52,698 gallons.)
So, that’s how much water we could harvest, but how much are we likely to use? According to The Integral Urban House, typical indoor water use per US person is 67 gallons per day. The book also estimates a conservationist’s indoor water use to be as low as 20 gallons per person per day. So, for a family of four, indoor water use falls within a range of 97,820 to 29,200 gallons per year (e.g. for typical: 67 x 4 x 365 = 97,820).
That means there’s more than enough rain in the average year to cover all of our indoor water needs if we conserve, but not enough if our usage is “typical” of the patterns cited in The Integral Urban House.
How big would our cistern need to be to hold all the rain that falls in an above-average year? Suppose we figure that some years there will be more water than we can use, and we want to store that leftover water for a future year (or just to water the garden)… A cylindrical cistern 10 feet deep with a 15 foot radius would hold 7,068 cubic feet (radius2 * pi * depth). There are 7.4805 gallons per cubic feet, so this tank holds 7,068 x 7.4805 = 52,875 gallons.
In the context of the house, that’s an enormous tank, as shown here:
Maybe a series of smaller cisterns located around the house would be better than one? But then would we need multiple pumps? Need to think about it some more!