Too often, residential water use is viewed from a supply-side perspective. When you consider reducing demand, especially in regard to exterior landscaping, many more options are available to meet conservation goals.
Given the small size of most new residential lots (in most cases less than 8000 square feet) there is no reason that rainwater could not be the primary or secondary landscape water source for many residents:
Practiced for centuries by smart desert dwellers, rainwater harvesting is an obvious yet seldom used strategy in the modern southwest. However, with water prices on the rise in the arid west, capturing rainwater to water landscape plants is receiving some overdue attention.
ZonaGardens advocates complete rainwater harvesting design solutions that incorporate desert-adapted plants into designs that store rainwater in the ground and/or in above-ground storage tanks. Designing gardens with xeric plants is an integral part of designing a water harvesting system. Native plants, particularly those that are adapted to our two seasonal rainfall patterns, are ideal candidates for such a garden. Using native plants provides the added benefit of creating gardens with regional sense of place.
There are two methods of rainwater harvesting: simple and complex, both of which are discussed below. The most successful rainwater harvesting systems use either simple or complex methods or a combination of both, based on the realities of the lot.
Simple methods of rainwater harvesting include sloping a patio, sidewalk, or driveway toward a planting area or tree. The goal of these simple methods is to slow water down allowing it to percolate into the soil. Swales (berms), French drains, and gabions (low rock dams) are other simple devices used to slow water and direct it to planting areas. Swales can be created at the time of the final grade at very little additional cost.
Complex rainwater harvesting methods involve capturing rainwater in above ground cisterns. ZonaGardens has used above ground cisterns extensively. Although these systems are called “complex,” the cisterns do not employ pumps or other moving parts relying instead on gravity to transfer water to planting beds. All of the current cisterns in the community are above ground cisterns that hold between 422 and 580 gallons of water. Each cistern costs between $600-1000 installed.
Scott Calhoun, ZonaGardens principal, has designed and installed over 30 rainwater collection systems in Tucson and has used rainwater as the primary irrigation source for his own garden for over four years.
Scott Calhoun’s latest quest, in collaboration with horticulturist Greg Corman, is to design beautiful and functional gardens that will thrive after establishment on rainwater alone.
Scott’s Tips for Making a No Irrigation Garden
1. In what situations do cisterns work best?
Cisterns are great in small spaces that would make creating swales impractical. They also are effective in very heavy soils where swales would create standing water for an unacceptable amount of time. Cisterns allow the gardener to delay the use of the rainwater. Cisterns also allow the water to be applied to plants at a slower rate via a drip system or other means. Small gardens are usually ideal candidates for cisterns.
2. Can I hook up a drip system to a cistern?
Yes, but not with a traditional automatic timer and valves. Drip irrigation connected to a rainwater cistern is by nature a manual system that relys on the gardener to manage the water. A drip system will run off of the pressure in the cistern but it needs to be designed for low pressure. Most cisterns produce less than 10 psi. Experience has shown that Rainbird 2 gallon per hour xeribug emitters or 4 gallon per hour flag-type emitters work best.
3. How much rainfall can I collect?
This depends when the rain comes. In general, during southern Arizona’s two rainy seasons, your cisterns will overflow. It is usually impractical to install enough cisterns to capture every drop. Your roof will almost always shed much more water that you can collect. The formula for figuring out how much water your roof will generate is the roof catchment area (sq. ft) times the average yearly rainfall (inches) times 550 gallons divided by 1000. If you can capture half of this number, you will be doing well.
4. How can I camouflage my cistern?
The cisterns ZonaGardens recommends are upended galvanized culverts. Most folks either love them or hate them. They can be painted. The most elegant way to hide you cistern is to put a trellis around it and grow vines up it.
5. How do I know how much water is in my cistern?
Besides going out in rain in your underwear and climbing up a ladder to look inside, you can rig up a simple gauge on the side of your cistern.
6. What are cistern rainwater capacities, by size?
7. Will I get mosquitoes inside my tank?
If you keep the tank covered with a floating lid (Styrofoam) you should not have mosquito problems.
The two best rainwater harvesting links are:
Harvesting Rainwater for Landscape Use
Texas Guide to Rainwater