Rapid City, SD rainwater: 15,912 gallons off a roof
Rapid City, SD: gallons off the roof each year
15,912gallons a year
That is a 1,500 sq ft asphalt roof under Rapid City, SD's normal 20.0 inches of rain, about 795 gallons for every inch that falls.
In Rapid City, SD 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 Rapid City Wfo station (0.4 mi away). Rapid City, SD ranks 13th wettest of the 14 cities covered in South Dakota.
Month by month in Rapid City
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 0.31 | 246 | |
| Feb | 0.53 | 421 | |
| Mar | 0.94 | 747 | |
| Apr | 2.26 | 1,796 | 261 |
| May | 4.12 | 3,275 | 29 |
| Jun | 3.33 | 2,647 | 127 |
| Jul | 2.62 | 2,082 | 216 |
| Aug | 2.17 | 1,725 | 272 |
| Sep | 1.40 | 1,113 | 368 |
| Oct | 1.49 | 1,184 | 357 |
| Nov | 0.48 | 382 | |
| Dec | 0.37 | 294 |
May is the wettest month (4.12 in, 3,275 gal off the roof); Jan the driest (0.31 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Which tank size is worth it in Rapid City
Best fit: 2 linked 55-gallon barrels, which delivers about 1,379 gallons to the garden in a normal year (84.7% of its need).
If you buy one, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Rapid City's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,132 | 14,090 |
| 110 gal | 1,379 | 13,843 |
| 165 gal | 1,379 | 13,843 |
| 275 gal | 1,379 | 13,843 |
| 550 gal | 1,379 | 13,843 |
| 1,000 gal | 1,379 | 13,843 |
| 1,500 gal | 1,379 | 13,843 |
| 2,500 gal | 1,379 | 13,843 |
| 5,000 gal | 1,379 | 13,843 |
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 South Dakota?
No rainwater harvesting regulations
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