Duncan, OK rainwater: 29,519 gallons off a roof

How much rain a roof in Duncan, OK collects

29,519gallons a year

That is a 1,500 sq ft asphalt roof under Duncan, OK's normal 37.1 inches of rain, about 795 gallons for every inch that falls.

In Duncan, OK 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 Duncan station (1.3 mi away). Duncan, OK ranks 24th wettest of the 45 cities covered in Oklahoma.

Month by month in Duncan

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	1.52	1,208
Feb	1.84	1,462
Mar	2.76	2,194
Apr	3.45	2,742	112
May	5.58	4,435
Jun	4.49	3,569
Jul	2.58	2,051	221
Aug	2.90	2,305	181
Sep	3.79	3,012	70
Oct	3.86	3,068	61
Nov	2.33	1,852
Dec	2.04	1,621

May is the wettest month (5.58 in, 4,435 gal off the roof); Jan the driest (1.52 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 Duncan

Best fit: one 55-gallon barrel, which delivers about 553 gallons to the garden in a normal year (85.8% 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 Duncan's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	553	28,156
110 gal	553	28,156
165 gal	553	28,156
275 gal	553	28,156
550 gal	553	28,156
1,000 gal	553	28,156
1,500 gal	553	28,156
2,500 gal	553	28,156
5,000 gal	553	28,156

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 Oklahoma?

Encouraged through state legislation: Water for 2060 Act

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