Catchment Area for a Target Harvest
The roof footprint needed to harvest a target volume a year.
Example
You enter
- Target annual harvest (gal) 11593
- Annual rainfall (in) 30
- Collection efficiency (0-1) 0.62
You get
- Catchment area needed 1000 ft^2
Details, formula, and sources
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.
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.
ARCSA materials free at arcsa.org. NOAA precipitation data free at hdsc.nws.noaa.gov.
Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
Field names used by the API: target_annual_gal, annual_in, efficiency, catchment_ft2
- Conversion and efficiency 0.6233 gal per in-ft^2; collection efficiency about 0.62 sloped shingle to 0.85 metalARCSA typical
- Horizontal footprint the area is the horizontal projected footprint, not the sloped roof surfaceARCSA method