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Gardens, harvesters make best use of rainfall

By Sharon Omahen
University of Georgia

When summer arrives, gardeners will wish they could have saved some of the rain that ran down their driveways this spring. A University of Georgia scientist is studying ways people can do just that.

"The greatest demand for outdoor water use in Georgia occurs during our state's hottest, driest months," said Rose Mary Seymour, a UGA agricultural engineer.

Irrigation water in high demand

"Outdoor use is a major component of the total water demand for urban areas of our state," she said. "But in times of drought and water restrictions, landscape irrigation will most likely be a low priority for potable water supplies."

Seymour, a researcher in the UGA College of Agricultural and Environmental Sciences, has found that using rainwater can reduce the amount of drinkable water used for irrigation.

"Captured rainwater is of suitable quality to be used for irrigation if the rain falls on relatively clean, impermeable areas," she said. "Collecting rainwater in a retention area also helps remove nutrients and other pollutants and recharges the groundwater."

Seymour's studies highlight two rainwater collection methods.

One, often called rain harvesting, collects rainwater in a cistern, tank or pond. The other involves installing a rain garden or bioretention area in your landscape.

Save for future use

Rain-harvesting systems most often collect rainwater in a storage tank, either above or below the ground. A pumping system can supply an automated irrigation system, but the water must be filtered. Or the rainwater can be siphoned from the tank and applied by hand.

Unlike gray water, or wastewater from clothes washers, showers, etc., collecting and applying rainwater doesn't require special plumbing codes, Seymour said. It doesn't contain the detergents and chemicals found in gray water.

The catch is that rain harvesting systems are best when they're designed and installed in a new building, Seymour said. "Usually, retrofit rain harvesting systems are more expensive and may not fit well with the existing overall site design," she said.

Rain-harvesting systems add some costs over using municipal water alone for irrigation. But Seymour said landscape designers need to be forward-thinking.

"If municipal water costs continue to rise and water utilities set up conservation fee structures, the payback period for rainwater harvesting could be shortened," she said.

Route rain into garden

Rain gardens and bioretention areas are other options.

"These are both intentional low areas where runoff water from impervious surfaces is diverted and contained so the runoff will infiltrate the soil," Seymour said.

Rain gardens are most often used in home or other small-scale landscapes, she said. Using plants that fare well in wet and dry extremes, these gardens create a more natural flow. They keep rainwater in the landscape, rather than letting it run into streets and storm drains.

"A rain garden catches the runoff water from a particular impervious area such as a rooftop, patio, driveway or parking area," Seymour said.

Ideally, water shouldn't stand in a rain garden more than 48 hours after the rain stops. Since it isn't standing water, mosquito breeding isn't a problem.

Don't install rain gardens over the drain field of septic systems or next to building foundations.

Bioretention areas serve a similar function, but tend to be part of large, commercial landscapes. They collect rainwater from large roofs and parking lots.

Can't pick? Use both

Large businesses can combine rain harvesting tanks, bioretention areas and rain gardens, Seymour said.

"Creating a rain garden downstream from the overflow of a rain harvesting system can be an even better on-site storm-water- reduction system than either independent method," she said.

"No matter which system you choose," she said, "using rainwater to reduce the use of potable water on landscapes is a win-win strategy for both commercial and residential landscapes."

(Sharon Omahen is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)

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