Rainwater

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.