Honolulu, HI rainwater: 24,822 gallons off a roof
How much rain a roof in Honolulu, HI collects
24,822gallons a year
That is a 1,500 sq ft asphalt roof under Honolulu, HI's normal 31.2 inches of rain, about 795 gallons for every inch that falls.
In Honolulu, HI a single barrel does most of the work. Storms come often enough that a small tank refills before it runs dry, and a bigger one would mostly sit full.
Rainfall: NOAA 1991-2020 climate normals, the Punchbowl Crater 709 station (1.0 mi away).
Month by month in Honolulu
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 2.98 | 2,369 | |
| Feb | 2.79 | 2,218 | |
| Mar | 3.97 | 3,155 | |
| Apr | 2.35 | 1,868 | 249 |
| May | 1.70 | 1,351 | 330 |
| Jun | 1.55 | 1,232 | 349 |
| Jul | 1.69 | 1,343 | 332 |
| Aug | 1.72 | 1,367 | 328 |
| Sep | 2.16 | 1,717 | 273 |
| Oct | 2.83 | 2,249 | 190 |
| Nov | 3.69 | 2,933 | |
| Dec | 3.80 | 3,020 |
Mar is the wettest month (3.97 in, 3,155 gal off the roof); Jun the driest (1.55 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Barrel or cistern in Honolulu?
Best fit: one 55-gallon barrel, which delivers about 1,650 gallons to the garden in a normal year (80.5% of its need).
If you set one up, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Honolulu's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,650 | 22,092 |
| 110 gal | 1,684 | 22,058 |
| 165 gal | 1,684 | 22,058 |
| 275 gal | 1,684 | 22,058 |
| 550 gal | 1,684 | 22,058 |
| 1,000 gal | 1,684 | 22,058 |
| 1,500 gal | 1,684 | 22,058 |
| 2,500 gal | 1,684 | 22,058 |
| 5,000 gal | 1,684 | 22,058 |
Simulated storm by storm through a normal year using the station's count of days with 0.1 in or more of rain each month.
Is rainwater collection legal in Hawaii?
Regulated through Department of Health and Safety
From the 2015 (Pacific Northwest National Laboratory report 24347); not yet re-checked against current law, so check with your state water agency before building anything large. Source