Regional Gardening Reports :: National Gardening Association

In the Garden:
Southwestern Deserts
August, 2011
Regional Report

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This chain directs rain into an underground PVC pipe that carries water away from the house foundation to root zones in the yard.

Rainwater Harvesting with Rain Chains

An ironic component of typical home building is that properties are designed to remove rainwater in a speedy fashion, as if it is a waste product, directing it to flow away to retention basins and storm drains, always being shunted off somewhere else. Then we homeowners pay to have water brought back to the property!

Rainwater is the healthiest drink we can provide for plants. It contains nutrients and is less salty than typical municipal water sources. Most Southwestern gardeners have seen plants suffering from salt burn caused by high levels of salts in our soils and water supplies, combined with ineffective shallow irrigation.

At first glance, it may seem that harvesting rainwater in the arid Southwest would not be worth the effort. However, a surprisingly large volume of water is available for harvest. A study conducted by the University of Arizona Environmental Research Laboratory found that the amount of rainfall on a square mile in Tucson was equal to 74 percent of the water delivered by the municipal utility to that area. The researchers also found that rainwater harvested for use on the landscape could reduce residential water use from utilities by 30 to 40 percent.

Use this formula to calculate the volume of rainwater that runs off your roof:

(roof area in square feet) X (annual rainfall in feet) X (conversion factor 7.48) X (runoff coefficient .90) = potential gallons of annual rainwater.

Roof area. Length times width.
Rainfall in feet. Divide your regional average annual rainfall in inches by 12.
Conversion factor. Changes water from feet to gallons.
Coefficient factor. It is impossible to collect 100 percent of the rain that falls. Ninety percent takes into account loss from splashing and evaporation.

As an example, assume you have 1000 square feet of rooftop and you live in Phoenix, which averages 10.8 inches of rain annually. 10.8/12 = .9

1000 X .9 X 7.48 X .90 = 6058 gallons of water per year!

Many rainwater harvesting options are available. If you are building a home from scratch, it pays to design harvesting features into the home and landscape from the start, or look for a developer that is doing so. If you are in an existing home, something as simple as a rain chain can help you send water to your plants.

Rain chains may be decorative copper works of art (often pricey), or you can use inexpensive linked chains bought by the foot at a hardware store. Keep in mind that, like a rain gutter and downspout, the chain should help prevent puddles of water from pooling next to your foundation. Consider having the chain direct water into a rain garden, some type of collection device or cistern, or an underground PVC pipe that carries water to a planting area that benefits from an extra deep soak. I will cover more rainwater harvesting in an upcoming report.

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