Hermiston, OR rainwater: 6,843 gallons off a roof
Rainwater from a roof in Hermiston, OR
6,843gallons a year
That is a 1,500 sq ft asphalt roof under Hermiston, OR's normal 8.6 inches of rain, about 795 gallons for every inch that falls.
Hermiston, OR's rain arrives in a wet season and stops, so storage is what matters. It takes a cistern of 1,500 gallons to carry spring rain into the dry months.
Rainfall: NOAA 1991-2020 climate normals, the Hermiston Muni Ap station (1.2 mi away). Hermiston, OR ranks 57th wettest of the 57 cities covered in Oregon.
Month by month in Hermiston
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.14 | 906 | |
| Feb | 0.86 | 684 | |
| Mar | 0.77 | 612 | |
| Apr | 0.78 | 620 | 445 |
| May | 0.83 | 660 | 439 |
| Jun | 0.64 | 509 | 463 |
| Jul | 0.12 | 95 | 527 |
| Aug | 0.17 | 135 | 521 |
| Sep | 0.33 | 262 | 501 |
| Oct | 0.80 | 636 | 443 |
| Nov | 1.05 | 835 | |
| Dec | 1.12 | 890 |
Jan is the wettest month (1.14 in, 906 gal off the roof); Jul the driest (0.12 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 Hermiston
Best fit: a 1,500-gallon cistern, which delivers about 3,159 gallons to the garden in a normal year (94.6% of its need).
Before you buy, 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 | 715 | 5,598 |
| 110 gal | 1,385 | 4,928 |
| 165 gal | 1,797 | 4,516 |
| 275 gal | 2,106 | 4,207 |
| 550 gal | 2,381 | 3,932 |
| 1,000 gal | 2,831 | 3,482 |
| 1,500 gal | 3,159 | 3,153 |
| 2,500 gal | 3,159 | 3,153 |
| 5,000 gal | 3,159 | 1,307 |
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 Oregon?
Collecting and using rain from an artificial impervious surface (e.g. a rooftop) is an exempt use: no water right application, permit or certificate needed. Statute sets no storage cap in this section.
Checked against the current source on 2026-11-06. Source