Shawnee, OK rainwater: 32,023 gallons off a roof

Rainwater from a roof in Shawnee, OK

32,023gallons a year

That is a 1,500 sq ft asphalt roof under Shawnee, OK's normal 40.3 inches of rain, about 795 gallons for every inch that falls.

In Shawnee, 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 Shawnee station (2.3 mi away). Shawnee, OK ranks 18th wettest of the 45 cities covered in Oklahoma.

Month by month in Shawnee

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	1.56	1,240
Feb	1.84	1,462
Mar	2.93	2,329
Apr	3.82	3,036	66
May	5.73	4,554
Jun	5.30	4,212
Jul	3.66	2,909	86
Aug	3.40	2,702	118
Sep	3.68	2,925	84
Oct	3.82	3,036	66
Nov	2.37	1,884
Dec	2.18	1,733

May is the wettest month (5.73 in, 4,554 gal off the roof); Jan the driest (1.56 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 Shawnee?

Best fit: one 55-gallon barrel, which delivers about 354 gallons to the garden in a normal year (84.3% 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 Shawnee's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	354	30,813
110 gal	354	30,813
165 gal	354	30,813
275 gal	354	30,813
550 gal	354	30,813
1,000 gal	354	30,813
1,500 gal	354	30,813
2,500 gal	354	30,813
5,000 gal	354	30,813

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