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Garden Math

Articles: Garden Math

In many cases seasonal storms represent an untapped resource just waiting to be harvested. Rainfall is free water.

Do the math!

To convert inches of rainfall into gallons of harvested water multiply the square footage of your catchment area x inches of rain x .62. So an inch of rain falling on a 1,000 square foot roof amounts to around 620 gallons or, factoring in a 10 percent loss due to evaporation and/or absorption, a potential 558 gallons of harvested water.

Depending on the size of your catchment area it looks like this:

As you can see, the volume adds up quickly. Just 1 inch of rain captured off a small 10- by 10-foot patio roof will fill a 55-gallon rain barrel. For catchment areas larger than 100 square feet we recommend you consider a larger tank or cistern to handle the volume.

In what was previously referred to as an “average” year Los Angeles could expect about 15 inches of rain; lately it’s been closer to 6 inches. Seattle precipitation averages about 38 inches, Portland gets around 42 inches, and San Francisco receives about 24 inches. Again, if you do the math you can see we’re talking about a significant volume of water. Then start multiplying those f...


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