Carroll, IA rainwater: 26,793 gallons off a roof

Rainwater from a roof in Carroll, IA

26,793gallons a year

That is a 1,500 sq ft asphalt roof under Carroll, IA's normal 33.7 inches of rain, about 795 gallons for every inch that falls.

In Carroll, IA 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 Carroll station (0.8 mi away). Carroll, IA ranks 38th wettest of the 41 cities covered in Iowa.

Month by month in Carroll

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	0.90	715
Feb	1.11	882
Mar	1.87	1,486
Apr	3.35	2,663	125
May	4.49	3,569
Jun	5.12	4,069
Jul	4.36	3,465
Aug	4.06	3,227	36
Sep	2.95	2,345	175
Oct	2.68	2,130	208
Nov	1.44	1,145
Dec	1.38	1,097

Jun is the wettest month (5.12 in, 4,069 gal off the roof); Jan the driest (0.90 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.

Barrel or cistern in Carroll?

Best fit: one 55-gallon barrel, which delivers about 444 gallons to the garden in a normal year (81.7% of its need).

If you set one up, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Carroll's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	444	25,404
110 gal	444	25,404
165 gal	444	25,404
275 gal	444	25,404
550 gal	444	25,404
1,000 gal	444	25,404
1,500 gal	444	25,404
2,500 gal	444	25,404
5,000 gal	444	25,404

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 Iowa?

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