Catching Rain
November 1, 2017
How much roof area do I need to catch my own water?
Annapolis average rainfall for the months March through November inclusive is 37.52 inches, with an average of 4.17±0.25 per month with a 95% CI. Note however that May of 2015 was about 1.5 inches, June 2015 was over 12.5 inches of rain.
Formula
1 inch of rain = 1/12 foot of rain
1 inch of rain on one square foot of surface = 1/12 * 1 * 1 cubic feet of water
1 cubic foot = 7.48049 gallons
1 inch of rain = 1/12 * 1 * 1 * 7.48049 gallons
1 inch of rain on 1 square foot = 0.62337 gallons
gallons captured = rainfall (inches) * roof area (sq ft) * 0.62337
A household will use about 125 gallons per dayThe range is is large, up to 644 g/day, with median of about 125. with no deliberate conservation. That equates to about 45,625 gallons per year. That means for this part of Maryland, a catchment area of about 2000 sqft will be approximately sufficient for a household assuming sufficient storage to buffer use over time. With reasonable conservation practices less roof area could suffice.
| Rainfall Amount (inches) | |||||||
|---|---|---|---|---|---|---|---|
| Area (sq ft) | 0.25 | 1.0 | 2.0 | 4.2 | 8.0 | 10.0 | 37.5 |
| 1 | 0.2 | 0.6 | 1.2 | 2.6 | 5.0 | 6.2 | 23.4 |
| 10 | 1.6 | 6.2 | 12.5 | 26.0 | 49.9 | 62.3 | 233.9 |
| 100 | 15.6 | 62.3 | 124.7 | 259.9 | 498.7 | 623.3 | 2338.7 |
| 200 | 31.2 | 124.7 | 249.3 | 519.9 | 997.3 | 1246.7 | 4677.5 |
| 400 | 62.3 | 249.3 | 498.7 | 1039.7 | 1994.7 | 2493.3 | 9355.0 |
| 800 | 124.7 | 498.7 | 997.3 | 2079.4 | 3989.3 | 4986.7 | 18710.0 |
| 1000 | 155.8 | 623.3 | 1246.7 | 2599.3 | 4986.7 | 6233.3 | 23387.5 |
| 2000 | 311.7 | 1246.7 | 2493.3 | 5198.6 | 9973.3 | 12466.7 | 46774.9 |