Knoxville, TN rainwater: 44,350 gallons off a roof
Knoxville, TN: gallons off the roof each year
44,350gallons a year
That is a 1,500 sq ft asphalt roof under Knoxville, TN's normal 55.8 inches of rain, about 795 gallons for every inch that falls.
In Knoxville, TN 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 Knoxville Exp Stn station (5.0 mi away). Knoxville, TN ranks 22nd wettest of the 65 cities covered in Tennessee.
Month by month in Knoxville
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 5.23 | 4,157 | |
| Feb | 5.12 | 4,069 | |
| Mar | 5.28 | 4,197 | |
| Apr | 5.25 | 4,173 | |
| May | 4.53 | 3,600 | |
| Jun | 3.96 | 3,147 | 49 |
| Jul | 5.82 | 4,626 | |
| Aug | 3.42 | 2,718 | 116 |
| Sep | 4.13 | 3,283 | 27 |
| Oct | 3.05 | 2,424 | 162 |
| Nov | 4.10 | 3,259 | |
| Dec | 5.91 | 4,697 |
Dec is the wettest month (5.91 in, 4,697 gal off the roof); Oct the driest (3.05 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 Knoxville
Best fit: one 55-gallon barrel, which delivers about 285 gallons to the garden in a normal year (80.5% 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 Knoxville's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 285 | 42,685 |
| 110 gal | 285 | 42,685 |
| 165 gal | 285 | 42,685 |
| 275 gal | 285 | 42,685 |
| 550 gal | 285 | 42,685 |
| 1,000 gal | 285 | 42,685 |
| 1,500 gal | 285 | 42,685 |
| 2,500 gal | 285 | 42,685 |
| 5,000 gal | 285 | 42,685 |
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 Tennessee?
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