Great Falls, MT rainwater: 12,264 gallons off a roof
Rainwater from a roof in Great Falls, MT
12,264gallons a year
That is a 1,500 sq ft asphalt roof under Great Falls, MT's normal 15.4 inches of rain, about 795 gallons for every inch that falls.
In Great Falls, MT 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 Great Falls 16st station (1.4 mi away). Great Falls, MT ranks 4th wettest of the 8 cities covered in Montana.
Month by month in Great Falls
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 0.61 | 485 | |
| Feb | 0.58 | 461 | |
| Mar | 0.63 | 501 | |
| Apr | 1.79 | 1,423 | 319 |
| May | 2.66 | 2,114 | 211 |
| Jun | 2.74 | 2,178 | 201 |
| Jul | 1.34 | 1,065 | 375 |
| Aug | 1.30 | 1,033 | 380 |
| Sep | 1.39 | 1,105 | 369 |
| Oct | 1.13 | 898 | 401 |
| Nov | 0.70 | 556 | |
| Dec | 0.56 | 445 |
Jun is the wettest month (2.74 in, 2,178 gal off the roof); Dec the driest (0.56 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
What size rain barrel for Great Falls
Best fit: 2 linked 55-gallon barrels, which delivers about 1,848 gallons to the garden in a normal year (81.9% 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 Great Falls's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,294 | 10,325 |
| 110 gal | 1,848 | 9,771 |
| 165 gal | 1,953 | 9,666 |
| 275 gal | 1,969 | 9,649 |
| 550 gal | 1,969 | 9,649 |
| 1,000 gal | 1,969 | 9,649 |
| 1,500 gal | 1,969 | 9,649 |
| 2,500 gal | 1,969 | 9,649 |
| 5,000 gal | 1,969 | 9,649 |
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 Montana?
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