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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 Ft20"25"30"35"40"
1,00012K15K18K21K24K
1,50018K22K27K31K36K
2,00024K30K36K42K48K
2,50030K37K45K52K60K
3,00036K45K54K63K72K
3,50042K52K63K73K84K
4,00048K60K72K84K96K

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.

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