92,160 square inches of roof + 46.83 inches of rain = I'm gonna need a bigger rain garden!

Male fern, Dryopteris filix-mas, in the mini rain garden

This past week I was invited to present my Slow the Flow native garden project and storm run-off solutions to a Boston University class on urban water issues. And, as college students usually do, they asked me a lot of really good questions. The one that stumped me and made me head straight home to find a calculator was, "How many gallons of rainwater runs off your roof and into your mini rain garden?" 

Wow. I had no idea, all I know is in a heavy rain it fills up the rain garden.

There actually is a very simple calculation to figure this out. It's so easy it didn't even make me cringe in my usual manner when I have to tackle a math word problem. You can even try this a home!

My rear roof is about 32' x 20'. The gutter slopes towards the rain garden side of the house, so I can assume almost all the water goes down that one down spout. After the massive downpour on August 10th that caused my mini rain garden to flood in this picture (right) my rain gauge had 2" of rain in it. Now comes the math to figure out how many gallons actually did fill that rain garden:

• Take the dimensions of the roof and convert them to inches. So my 32' x 20' roof is 384" x 240" or 92,160 square inches. 
• Multiply the roof dimensions by the amount of rain fall in inches. For the August 10th storm that's 92,160 x 2" = 184,320.
• Divide by 231 (1 gallon = 231 cubic inches) = 798 gallons fell on my back roof and flowed into the mini rain garden on August 10th (give or take a bit, my rain gauge probably isn't that accurate).

According to NOAA's climate data website Massachusetts received 46.83 inches of rain in the 12 months since we purchased our house. As anyone living in Ipswich knows we often vary greatly from the rest of the state but I will use this number as an estimate. If I redo the math for the last 12 months a whopping 18,683 gallons of water flowed down that one corner downspout and, until I built the rain garden in August, straight into the Ipswich river and out to sea not recharging the watershed at all. It was basically wasted water.

I'll have to add up the whole roof and find out how much rain water could be captured in rain gardens, or even better, a cistern for later use in the garden. With this math in hand you can imagine how productive a single rain barrel can be when placed properly. My two rain barrels only ran dry once this summer. It really doesn't take much rain to fill them.

As cute as the mini rain garden is it clearly needs to be much larger to capture the heavy rain we often get. Since I built the mini rain garden in August we've had two storms heavy enough to fill it, and it's only been two months.

Our long-term plan is to reposition the back gutter to slant towards the still unlandscaped other side of the backyard (left). You can see that the downspout extension hose drains to a very large sloped area. This is just begging to be terraced into a series of rain gardens.

But that is a project for next summer. I think it's time I put away my gardening gloves for the season and enjoy the view of the river.