By David Knapp
For every inch of rain you will collect 600 gallons of water for every 1,000 square feet of horizontal collection surface. Example: Central Texas - Average rainfall of 31 inches per year. To be safe, let's assume 25 inches per year.
Yearly collection: 25 inches Roof size: 2,000 sq ft. 25 inches x 600 x 2 (2,000 sq. ft. roof) = 30,000 gallons
That allows for an average monthly use of 2,500 gallons. You will want several months worth of storage to account for the dry season. This is certainly not enough for an average home, but more than enough for an Earthship with adequate storage.
Average Rainfall Yield
Roof ---------------------Inches of Rainfall----------------------- Sq Ft 20" 25" 30" 35" 40" 1,000 12K 15K 18K 21K 24K 1,500 18K 22K 27K 31K 36K 2,000 24K 30K 36K 42K 48K 2,500 30K 37K 45K 52K 60K 3,000 36K 45K 54K 63K 72K 3,500 42K 52K 63K 73K 84K 4,000 48K 60K 72K 84K 96K
K = Gallons in Thousands
The above information was extracted from Rainwater Collection Systems booklet, distributed by Morris Media Associates, Inc., Austin, Texas. There is additional information on tanks, pumps, plumbing, insulation, etc. There is also a companion video which I purchased through Real Goods a few years back.