Baltimore, MD rainwater: 34,757 gallons off a roof
Rainwater from a roof in Baltimore, MD
34,757gallons a year
That is a 1,500 sq ft asphalt roof under Baltimore, MD's normal 43.7 inches of rain, about 795 gallons for every inch that falls.
In Baltimore, MD 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 Md Sci Ctr Baltimore station (1.3 mi away). Baltimore, MD ranks 19th wettest of the 23 cities covered in Maryland.
Month by month in Baltimore
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.07 | 2,440 | |
| Feb | 2.75 | 2,186 | |
| Mar | 3.93 | 3,124 | |
| Apr | 3.55 | 2,822 | 100 |
| May | 3.39 | 2,694 | 120 |
| Jun | 3.36 | 2,671 | 123 |
| Jul | 4.71 | 3,744 | |
| Aug | 4.35 | 3,457 | |
| Sep | 4.49 | 3,569 | |
| Oct | 3.49 | 2,774 | 107 |
| Nov | 2.98 | 2,369 | |
| Dec | 3.66 | 2,909 |
Jul is the wettest month (4.71 in, 3,744 gal off the roof); Feb the driest (2.75 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 Baltimore?
Best fit: one 55-gallon barrel, which delivers about 361 gallons to the garden in a normal year (80.2% 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 Baltimore's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 361 | 33,301 |
| 110 gal | 361 | 33,301 |
| 165 gal | 361 | 33,301 |
| 275 gal | 361 | 33,301 |
| 550 gal | 361 | 33,301 |
| 1,000 gal | 361 | 33,301 |
| 1,500 gal | 361 | 33,301 |
| 2,500 gal | 361 | 33,301 |
| 5,000 gal | 361 | 33,301 |
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 Maryland?
No statewide rainwater harvesting regulations; check county for local incentives
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