How to Maximize Rainfall in the Arizona Desert
A water harvesting system allows you and your neighbors to harness this cherished desert resource.
As desert natives, we’ve been taught to landscape with desert-appropriate plants. We know drip irrigation is the preferred method of watering in general, and we understand that organic fertilizers and pesticides are the way to go. But how many of us make use of a rainwater harvesting system to complement all of this effort?
Water harvesting involves simply capturing rainwater as it flows into your yard and off your roof, directing it through your landscape for immediate use or storing it in barrels or cisterns for future applications. This practice has many advantages. It is free, provides high-quality irrigation water, decreases erosion, cuts down on salt accumulation that could affect root growth and water uptake, and reduces flooding. Many landscapes actually repel rainwater, directing it to storm drains. Some cities, including Tempe, require parking lots and private properties to retain runoff in big catchment basins or by having concave front yards. This works (unless your yard is covered with gravel mulch atop plastic sheeting) but is not very aesthetically pleasing. In Phoenix, many public and residential landscapes are covered with hardscaping, such as sidewalks and driveways, that is impervious to rainfall.
A rainwater harvesting system has four components: collection, storage, distribution and maintenance. Collection refers to the surface across which the water flows, usually rooftops, hardscape and the ground. You then decide how the water is applied or saved. The easiest—and cheapest—way to harvest water is to simply let it soak into the soil. You can also store rainwater in a container, such as a homemade or store-bought rain barrel. Doing so involves more than leaving a bucket out in the rain; a proper collection setup requires a filter, pump and mosquito control—and it will need to be incorporated in your landscape design. Each option needs a method of distribution. This can be a simple gravity-fed system of gutters and downspouts, pitched hardscape and topographical design, or a more complex arrangement in which cisterns, pumps and a delivery system—which enables the water to get from the cistern out into the yard and to the plants—are utilized. Each system also requires regular upkeep, including debris removal, replacement of components, and leak inspection and repair.
When designing your harvesting system, observe how water flows through your landscape during a storm. Don’t build a single giant retention basin, but instead create a series of concave depressions, berms (horizontal barriers usually made of soil) and swales (basins located upslope from a berm). Created perpendicular to the slope of the yard, they slow down the flow of water, causing it to spread out and soak into a substantial area.
Gabions add another design element, and can be used to strategically direct water in a different direction. Because many summer monsoon storms dump a lot of water in a short amount of time, plan for an overflow route that also puts the water to use, for example guiding it toward a mulched or vegetated area.
If your community allows it, convert your driveway to a permeable surface, alternating concrete or pavers with grass or gravel, which will allow water to drain through. In the Southwest, rooftop catchment alone will generally not be enough to completely support a normal residential landscape, but it can certainly help reduce public-supply water use and contribute to conservation. Calculate your potential harvest by multiplying the amount of rain in inches by the square foot of your roof by 0.623 (quantity of water in gallons 1 inch deep in 1 square foot of space).
Besides berms and swales, other methods of water harvesting include terracing, French drains, mulching and ground cover. Terraces are nothing more than a shelf of soil built parallel to the contour of a slope to control erosion. A French drain is a trench filled with a porous material into which rainwater flows and percolates into root zones of the surrounding soil. Mulch spread over the soil reduces evaporation, runoff and soil erosion while increasing water infiltration. Vegetative ground covers also increase water infiltration and stabilize the soil. Which method to use is a matter of preference and aesthetics. A system could be put into place that incorporates several different methods mentioned above—it doesn’t have to be solely one or the other.
In the Sonoran Desert, rain should be cherished rather than treated as a nuisance. Creating a system to harvest rainwater does not need to break the bank and is something anyone can accomplish no matter the size of your landscape.