Boise City, ID rainwater: 9,148 gallons off a roof
Boise City, ID: gallons off the roof each year
9,148gallons a year
That is a 1,500 sq ft asphalt roof under Boise City, ID's normal 11.5 inches of rain, about 795 gallons for every inch that falls.
Boise City, ID's rain arrives in a wet season and stops, so storage is what matters. It takes a cistern of 1,000 gallons to carry spring rain into the dry months.
Rainfall: NOAA 1991-2020 climate normals, the Boise Air Terminal station (2.2 mi away). Boise City, ID ranks 21st wettest of the 26 cities covered in Idaho.
Month by month in Boise City
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.41 | 1,121 | |
| Feb | 1.00 | 795 | |
| Mar | 1.33 | 1,057 | |
| Apr | 1.23 | 978 | 389 |
| May | 1.45 | 1,152 | 362 |
| Jun | 0.75 | 596 | 449 |
| Jul | 0.21 | 167 | 516 |
| Aug | 0.17 | 135 | 521 |
| Sep | 0.43 | 342 | 489 |
| Oct | 0.81 | 644 | 441 |
| Nov | 1.18 | 938 | |
| Dec | 1.54 | 1,224 |
Dec is the wettest month (1.54 in, 1,224 gal off the roof); Aug the driest (0.17 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 Boise City
Best fit: a 1,000-gallon cistern, which delivers about 2,758 gallons to the garden in a normal year (87.1% of its need).
If you set one up, ask about a rebate first: cistern-sized storage costs real money, and a buried or tall tank may need a permit. Place it where a full tank (about 8.3 lb a gallon) sits on firm, level ground.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 809 | 7,649 |
| 110 gal | 1,377 | 7,081 |
| 165 gal | 1,668 | 6,790 |
| 275 gal | 1,981 | 6,477 |
| 550 gal | 2,308 | 6,150 |
| 1,000 gal | 2,758 | 5,700 |
| 1,500 gal | 2,977 | 5,481 |
| 2,500 gal | 2,977 | 5,481 |
| 5,000 gal | 2,977 | 5,481 |
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 Idaho?
Allowed through a legal clarification made by the Office of the Attorney General
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