Kennewick, WA rainwater: 7,082 gallons off a roof
Rainwater from a roof in Kennewick, WA
7,082gallons a year
That is a 1,500 sq ft asphalt roof under Kennewick, WA's normal 8.9 inches of rain, about 795 gallons for every inch that falls.
Kennewick, WA'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 Kennewick 0.4 Nne station (0.9 mi away). Kennewick, WA ranks 84th wettest of the 91 cities covered in Washington.
Month by month in Kennewick
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.19 | 946 | |
| Feb | 0.91 | 723 | |
| Mar | 0.70 | 556 | |
| Apr | 0.75 | 596 | 449 |
| May | 0.91 | 723 | 429 |
| Jun | 0.74 | 588 | 450 |
| Jul | 0.25 | 199 | 511 |
| Aug | 0.20 | 159 | 517 |
| Sep | 0.33 | 262 | 501 |
| Oct | 0.68 | 540 | 458 |
| Nov | 0.98 | 779 | |
| Dec | 1.27 | 1,009 |
Dec is the wettest month (1.27 in, 1,009 gal off the roof); Aug the driest (0.20 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 Kennewick
Best fit: a 1,000-gallon cistern, which delivers about 3,035 gallons to the garden in a normal year (91.6% 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 | 660 | 6,017 |
| 110 gal | 1,308 | 5,369 |
| 165 gal | 1,837 | 4,840 |
| 275 gal | 2,310 | 4,367 |
| 550 gal | 2,585 | 4,092 |
| 1,000 gal | 3,035 | 3,642 |
| 1,500 gal | 3,148 | 3,529 |
| 2,500 gal | 3,148 | 3,529 |
| 5,000 gal | 3,148 | 2,057 |
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 Washington?
Collecting and storing rooftop rainwater for use on site needs no water right permit, provided the roof is part of a fixed above-ground structure whose primary purpose is not rain collection. Ecology may set local restrictions on new systems if they harm instream values or existing water rights. No state storage cap in the policy.
Checked against the current source on 2026-11-06. Source