Shawnee, KS rainwater: 32,412 gallons off a roof

Rainwater from a roof in Shawnee, KS

32,412gallons a year

That is a 1,500 sq ft asphalt roof under Shawnee, KS's normal 40.8 inches of rain, about 795 gallons for every inch that falls.

In Shawnee, KS 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 Lenexa 2.0 Ne station (3.2 mi away). Shawnee, KS ranks 5th wettest of the 38 cities covered in Kansas.

Month by month in Shawnee

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	1.29	1,025
Feb	1.66	1,319
Mar	2.53	2,011
Apr	4.33	3,442	2
May	5.64	4,483
Jun	5.43	4,316
Jul	4.24	3,370	14
Aug	4.37	3,473
Sep	4.16	3,306	24
Oct	3.29	2,615	132
Nov	2.21	1,757
Dec	1.63	1,296

May is the wettest month (5.64 in, 4,483 gal off the roof); Jan the driest (1.29 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 143 gallons to the garden in a normal year (83.1% 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	143	31,294
110 gal	143	31,294
165 gal	143	31,294
275 gal	143	31,294
550 gal	143	31,294
1,000 gal	143	31,294
1,500 gal	143	31,294
2,500 gal	143	31,294
5,000 gal	143	31,294

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 Kansas?

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