Also in: Disaster Response and Recovery
Catchment Area for a Target Harvest
The roof footprint needed to harvest a target volume a year.
Example
You enter
- Target annual harvest (gal) 15895
- Annual rainfall (in) 30
- Collection efficiency (0-1) 0.85
You get
- Catchment area needed 1000 ft^2
Checked against published worked examples from: Texas Water Development Board.
Details, formula, and sources
Scopearea = target gal / (annual in x 0.6233 x efficiency). Harvesting 11,593 gal a year where 30 in of rain falls needs about 730 ft^2 at the 0.85 collection efficiency installers assume (TWDB: 75-90%), and about 1,000 ft^2 if only 62% is captured. 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 or tablecatchment_ft2 = target_annual_gal / (annual_in x 0.6233 x efficiency); the inverse of annual_gal = area x rainfall x 0.6233 x efficiency.
Source and editionARCSA (American Rainwater Catchment Systems Association) Rainwater Harvesting Manual by name; NOAA precipitation data.
Free accessARCSA materials free at arcsa.org. NOAA precipitation data free at hdsc.nws.noaa.gov.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namestarget_annual_gal, annual_in, efficiency, catchment_ft2
- Conversion and efficiency 0.6233 gal per in-ft^2; collection efficiency 0.75 to 0.90 (TWDB), 0.85 defaultARCSA typical
- Horizontal footprint the area is the horizontal projected footprint, not the sloped roof surfaceARCSA method