The average American household uses about 320 gallons of water per day, a third for irrigation and other outdoor uses. Collecting the water flowing down your downspouts in rainstorms so you can use it to irrigate in dry periods is often touted as a simple way to cut back. But setting up a functional rainwater irrigation system—beyond the ubiquitous 55-gallon barrels under the downspout, which won’t irrigate much more than a flower bed or two—is a fairly complicated DIY project.
If you live in a townhouse or apartment with a tiny yard, one of those store-bought barrels will probably do the trick. In that case, there’s not much to know—most come with instructions and all the hardware you need. But if you’re planning a more ambitious project consider the following pointers before getting started. Before you begin, make sure that rain harvesting is legal in your area.
Calculating Your Water Budget
To give you an idea of how much water you have at your disposal for irrigation, here is a simple equation for figuring out how many gallons your roof sheds when it rains:
roof size (in square feet) X annual rainfall (in inches) X .6
There are rain tank calculators online so you don’t have to do the math, but here is an example. If you lived in a modest 20′ by 30′ house (measure roof size as the length times the width of the house) in a modestly wet region with 40 inches of annual rainfall, that’s (20 X 30) X 40 X .6 = 14,400 gallons. Here’s a handy web tool to find out how much rainfall you get in your area each year.