Madison, WI rainwater: 31,387 gallons off a roof
How much rain a roof in Madison, WI collects
31,387gallons a year
That is a 1,500 sq ft asphalt roof under Madison, WI's normal 39.5 inches of rain, about 795 gallons for every inch that falls.
In Madison, WI 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 Madison 4.0 W station (2.0 mi away). Madison, WI ranks 7th wettest of the 98 cities covered in Wisconsin.
Month by month in Madison
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.58 | 1,256 | |
| Feb | 1.66 | 1,319 | |
| Mar | 2.21 | 1,757 | |
| Apr | 4.08 | 3,243 | 34 |
| May | 4.64 | 3,688 | |
| Jun | 5.43 | 4,316 | |
| Jul | 4.54 | 3,608 | |
| Aug | 4.36 | 3,465 | |
| Sep | 3.71 | 2,949 | 80 |
| Oct | 3.06 | 2,432 | 161 |
| Nov | 2.32 | 1,844 | |
| Dec | 1.90 | 1,510 |
Jun is the wettest month (5.43 in, 4,316 gal off the roof); Jan the driest (1.58 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 Madison
Best fit: one 55-gallon barrel, which delivers about 218 gallons to the garden in a normal year (79.5% of its need).
Before you buy, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Madison's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 218 | 30,059 |
| 110 gal | 218 | 30,059 |
| 165 gal | 218 | 30,059 |
| 275 gal | 218 | 30,059 |
| 550 gal | 218 | 30,059 |
| 1,000 gal | 218 | 30,059 |
| 1,500 gal | 218 | 30,059 |
| 2,500 gal | 218 | 30,059 |
| 5,000 gal | 218 | 30,059 |
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 Wisconsin?
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