Ashland Wisconsin averages 4.02 inches of rain in 11 rainfalls for the month of July. Over the month, a 33-gal rain barrel can capture about 363 gallons (which is only about one fourth of the flow off an average roof of 20×30 ft. in July). The excess water overflows which can sometimes be a problem if it’s near a building, plus you are losing free rain water that could be directed to your garden, trees or yard.
If you want to capture more rainwater, you can link barrels to increase your rainwater harvest. I built this 5-barrel system to capture water from one half of my garage roof (approximately 850 sq. ft.). The total cost was $64. My construction notes and photos are here.
A 1/2″ rainfall on my roof produces about 240 gallons of water. By linking five 55-gallon barrels I avoid overflow and can capture up to 273 gallons of water. I am really happy with this system. The site is uphill from my garden and the water pressure in the hose is terrific! I like this system because you can utilize the whole barrel by turning it upside down, so when the barrel drains, it drains all the way out since the outlet is on the bottom of the barrel. This system require less drilling of holes into the barrel; you only drill the 3/4″ knock-out in the bung for each barrel, one 6″ hole in the end barrel to capture the downspout water, and a tiny air vent hole at the top of each barrel to relieve the airlock in the system.
Here is how you calculate how much water come off your roof and to size your rain harvest system to maximize your harvest.
|Rain barrel volume can be determined by calculating the roof top water yield for any given rainfall, using the following general equation: 1|
|Equation 1. V =A2 x R x 0.90 x 7.5 gals./ ft.3 where:|