Ottawa, IL rainwater: 31,856 gallons off a roof

Rainwater from a roof in Ottawa, IL

31,856gallons a year

That is a 1,500 sq ft asphalt roof under Ottawa, IL's normal 40.1 inches of rain, about 795 gallons for every inch that falls.

In Ottawa, IL 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 Ottawa 1.6 N station (1.6 mi away). Ottawa, IL ranks 111th wettest of the 225 cities covered in Illinois.

Month by month in Ottawa

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	2.22	1,764
Feb	2.04	1,621
Mar	2.65	2,106
Apr	3.76	2,988	74
May	5.22	4,149
Jun	4.37	3,473
Jul	3.77	2,996	72
Aug	4.26	3,386	11
Sep	3.36	2,671	123
Oct	3.23	2,567	140
Nov	2.92	2,321
Dec	2.28	1,812

May is the wettest month (5.22 in, 4,149 gal off the roof); Feb the driest (2.04 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 Ottawa

Best fit: one 55-gallon barrel, which delivers about 328 gallons to the garden in a normal year (78.1% 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 Ottawa's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	328	30,358
110 gal	328	30,358
165 gal	328	30,358
275 gal	328	30,358
550 gal	328	30,358
1,000 gal	328	30,358
1,500 gal	328	30,358
2,500 gal	328	30,358
5,000 gal	328	30,358

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 Illinois?

Local government can regulate rainwater harvesting per state plumbing law

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