Catchment Area for a Target Harvest
The inverse of the rainwater-yield tile: the roof footprint needed to harvest a target volume a year, area = target_gal / (annual_in x 0.6233 x efficiency). Harvesting 11,593 gal a year where 30 in of rain falls, at 0.62 efficiency, needs about 1,000 ft^2; a metal roof at 0.85 efficiency needs only 730 ft^2. Answers 'how much roof do I need' instead of the yield from one roof. The area is the horizontal footprint. A planning estimate; local rainfall, storage, and demand govern.
Formula and source
catchment_ft2 = target_annual_gal / (annual_in x 0.6233 x efficiency); the inverse of annual_gal = area x rainfall x 0.6233 x efficiency.
ARCSA (American Rainwater Catchment Systems Association) Rainwater Harvesting Manual by name; NOAA precipitation data.
Audience
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