Cambridge, MD rainwater: 37,189 gallons off a roof
Cambridge, MD: gallons off the roof each year
37,189gallons a year
That is a 1,500 sq ft asphalt roof under Cambridge, MD's normal 46.8 inches of rain, about 795 gallons for every inch that falls.
In Cambridge, 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 Cambridge 1.4 Wnw station (1.7 mi away). Cambridge, MD ranks 10th wettest of the 23 cities covered in Maryland.
Month by month in Cambridge
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.15 | 2,504 | |
| Feb | 2.73 | 2,170 | |
| Mar | 4.12 | 3,275 | |
| Apr | 3.48 | 2,766 | 108 |
| May | 4.26 | 3,386 | 11 |
| Jun | 4.00 | 3,179 | 44 |
| Jul | 4.72 | 3,751 | |
| Aug | 4.53 | 3,600 | |
| Sep | 4.59 | 3,648 | |
| Oct | 4.12 | 3,275 | 29 |
| Nov | 3.22 | 2,559 | |
| Dec | 3.87 | 3,076 |
Jul is the wettest month (4.72 in, 3,751 gal off the roof); Feb the driest (2.73 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Which tank size is worth it in Cambridge
Best fit: one 55-gallon barrel, which delivers about 148 gallons to the garden in a normal year (77.1% of its need).
Before you buy, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Cambridge's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 148 | 35,856 |
| 110 gal | 148 | 35,856 |
| 165 gal | 148 | 35,856 |
| 275 gal | 148 | 35,856 |
| 550 gal | 148 | 35,856 |
| 1,000 gal | 148 | 35,856 |
| 1,500 gal | 148 | 35,856 |
| 2,500 gal | 148 | 35,856 |
| 5,000 gal | 148 | 35,856 |
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