- Harvesting rainwater
I’m not an expert on irrigation, but the cool new rainwater collection system at our local Master Gardeners demonstration plot seems like a no-brainer to me. Worldwide consumption of water is rising fast — twice the rate of the population — but fresh water makes up less than 3 percent of all the planet’s water resources. When a scarce resource falls right out of the sky, it makes sense to harvest it. That’s exactly what the folks at the county demonstration garden are doing.
Fruit trees... demonstrating!
Rainwater can be collected from any relatively clean surface (rooftops and pool covers, for example) and then used for irrigation, flushing the toilet, washing the car, rinsing garden tools — just don’t drink it.
The system at the demonstration garden uses rain gutters on a small outbuilding to capture water:
These gutters capture 160 - 240 gallons per 1" rainfall
Even a tiny toolshed can yield a surprisingly large volume of fresh water. According to a formula from the Alabama Cooperative Extension System, a 10 x 10 foot surface collects approximately 50 gallons for every inch of rainfall. The main tank for the demonstration garden holds 1800 gallons, and it was full:
Next stop after the rooftop: the storage tank
The thick, vertical pipe visible at the corner of the red building is the first flush diverter. It’s a simple device that catches the first flush of water during each rain — the rain that rinses off any dirt, bird droppings, acorns or leaves that might have landed on the roof recently. Once the diverter is full, the remaining water passes over it and runs into the storage tank. See the plug at the bottom of the pipe? That’s where the first flush water can be drained, between rains.
Storage tanks can be made from all kinds of clean containers. .. however, the Cooperative Extension folks warn that you need to be very careful about what has been previously stored in them. New or never-previously-used fuel tanks, fiberglass containers or septic tanks are what they recommend for larger capacity. There are also polyethylene tanks manufactured for use in the sugar industry, which are cheap to buy and easy to rinse for repurposing as water storage.
Light colored tanks should be painted dark green or black to prevent light penetration. If you bury your storage tank, color doesn’t matter.
Distribution is by gravity or a small pump
(We’re down here in a subtropical climate zone, so temperature extremes are never a problem. But in colder climates, exposed storage tanks would need to be durable enough to tolerate water freezing and thawing during the winter. The recommendation is high-density polyethylene, and a domed top or overflow pipe to allow expansion.)
The demonstration garden slopes gently away from the water containment tank, so gravity alone was enough to provide pressure for drip irrigation. But it’s a big garden, so last week a small electric pump was installed at the base of the storage tank. Now the Master Gardener volunteers can sprinkle, mist and hose to their hearts’ content. If you don’t have electricity in the vicinity of your water storage, a gasoline pump will work.
I’m intrigued. I think a set of rain barrels and some spiffy new catchment gutters on the art studio might be a good fall project. After patiently answering my many questions, the County Extension agent gave me some sources of additional, more detailed information. I’ve listed them below. Happy harvesting!
Virginia Rainwater Harvesting Manual
The Texas Manual on Rainwater Harvesting
Harvesting Rainwater for Landscape Use