Rainwater harvesting is an ancient technique that has been practiced for thousands of years in desert regions. The Hohokam canals were one of the first known local rainwater collection systems in the low desert of Arizona.
Water is a precious resource in the Valley of the Sun where our annual rainfall is typically 9 inches or less. Harvesting rainwater involves capturing and directing precipitation, which is then used outdoors in landscaped areas. (Ref 1). In Arizona, it is legal to collect any rainwater that falls on your property, which is not true in all U.S. states.
There are two types of rainwater harvesting, passive and active. Passive water harvesting works by shaping the earth to slow the velocity of runoff, infiltrate it into the soil, and direct it to basins and other areas where it can be beneficially used by vegetation. Active water harvesting, in contrast, uses gutters, downspouts and containers to store water for later use. The stored water can be used outdoors to irrigate vegetation or indoors for non-potable uses (toilet flushing, laundry washing). (Ref 2). Harvested water should not be used for consumption without appropriate filtration.
Before I get too far into this article, I want to mention that both active and passive methods discussed below require resources. Passive harvesting requires labor, to shape the earth and make basins to direct the water. Active rainwater harvesting requires rainwater diversion and storage equipment. However, there are easy and inexpensive ways to capture water on a small scale. These include setting buckets at the drip-line of the roof during rainfall and catching the water from your AC unit condensate line. During the heat of summer, water coming out of the AC condensate line can be 2 – 5 gallons per day. Water from the condensate line is extracted from the air. Like rainwater, it is close to pH neutral, low in Total Dissolved Solids (TDS), and contains none of the additives that we find in our tap water (chlorine, fluoride).
Your plants will love rainwater, and so will your pocketbook, as capturing rainwater will decrease your water bill. Garden, landscape, and turf irrigation use more water than we use typically use inside our homes. Water rates are continuously increasing as water becomes scarce due to our long-term drought. Most water companies impose a tiered pricing structure that has imposed – essentially – a penalty for higher water use. The more water you use, the higher the rate you pay for ALL the water you use. (Ref 3)
Active rainwater harvesting
Active harvesting systems use gutters or scuppers and downspouts to direct rooftop runoff to collection tanks. If you have a roof of 1,500 square feet that receives 9 inches of rain a year (typical annual rainfall for the Phoenix area), you can collect over 8,410 gallons of “free” water per year!
To calculate the rainwater potential of your home, multiply the home’s square feet x amount of rainfall per year in inches x 0.623. This calculation assumes you have an efficient system that captures most of the rainwater that falls on the roof and a cistern large enough to contain the water. While our rain does not fall at once, it does fall evenly throughout the year except for April through June.
Active collection systems do not need to be expensive or large enough to hold thousands of gallons. At Tenth Generation Farm, we have several poly 55-gallon barrels near each downspout and capture enough rainwater for our use in the vegetable garden and on shrubs. Rain is wonderful for plants that don’t appreciate alkalinity.
A common rain collection container is the 55-gallon poly barrel. They are inexpensive ($25-30). Multiple barrels can be connected by a tube or hose to increase capacity. Another option is to use Intermediate Bulk Container (IBC} totes. If the containers are raised off the ground, a spigot can be installed low on the side to attach a hose for watering. There are also hand pumps available for barrels.
The Texas A&M Extension has several videos on building a DIY rain barrel on their website: https://agrilifeextension.tamu.edu/solutions/rainwater-harvesting/
Water storage systems will attract mosquitoes, so openings should be screened or closed during mosquito season.
Passive Rainwater Harvesting at the Extension’s Living Classroom
Our goal at the Maricopa County Cooperative Extension (MCCE) is to make a walk in our landscape a beautiful demonstration of best practices that complements University of Arizona publications on rainwater harvesting and encourages more sustainable landscapes.
The landscape is designed to use passive water collection to capture and concentrate water at the root zones of trees. We are following nature's way of shedding water and leaves to the edge of tree canopies (the “drip line”). There are several advantages to this type of water harvesting: it is Inexpensive; simple to build; low maintenance; and it turns your land into a sponge. (Ref 6)
To implement passive water harvesting, we first sculp the earth to create basins near the drip line of trees and shrubs. Then we add organic material to the surface of the depressions (mulching). The benefits multiply from retained moisture, cooler soil, weed suppression and because organic matter is food, a robust community of beneficial soil microbes. Rather than disposing of fallen pine needles and leaves, we rake or blow them into the basins, resulting in free mulch.
Building the basins is simple; it requires a shovel and some effort to sculpt soil with depressed basins or low spots that catch rainwater. Add organic matter to create spongy cool areas. When rainfall is low during dry periods, the low areas become handy watering basins located right where the “feeder” or absorbing roots have congregated.
We have an extended roof eave on the north side of the MCCE building that sheds significant rainwater. To ensure the drainage from the roof is diverted where it is most beneficial, we trenched and used planting pots cut in half to direct the flow to our basins.
.In some areas, we needed to go underneath a pathway to further direct the flow of water to another area. We installed 3” corrugated drain pipe under the path to enable the water to flow below the surface and avoid creating a muddy walkway during rain.
At the ends of the pipe, we added grates to avoid debris clogging the pipes.
Next time you are at the Extension, please look at the passive rainwater harvesting system. The areas that have improved thus far are on the East side, between the parking lot and the building, and along the north side of the property.
Gray Water Harvesting
Residential gray water is all wastewater from a residence excluding wastewater from the toilet, dishwasher or kitchen sink. Homeowners can recycle their gray water after its initial home use (laundry, showering, bathtubs and bathroom sinks) with no pretreatment. Black water should never be reused. Black water is from toilets, kitchen sinks, and dishwashers. (Ref 4 – 5) At our farm, we have a washing machine that discharges outdoors to tree basins. It is not recommended to use gray water for edible vegetables but acceptable for fruit and nut trees. Although you don’t need a formal permit for permission to use gray water, you must abide by the 13 best management practices (BMPs) listed in Arizona Department of Environmental Quality (ADEQ) brochure, Using Gray Water at Home. (Ref 5). Finally, keep in mind that you may need to adjust your soap and laundry additives (e.g., bleach) to ensure you are not dispensing harmful or harsh chemicals to your trees and shrubs.
Give water harvesting a try! If you are already doing so, please comment on this article and let me know how it is working for you. To contact me personally, use the "Contact" form on the website. Constructive comments and personal experiences are always welcomed and will be incorporated into this guide.
Rebecca Senior is a University of Arizona Assistant in Extension, Ornamental Horticulture and an ISA Certified Arborist. Rebecca is turning the landscape at the MCCE into a living classroom. Rebecca's goal is to create a place for citizens of Maricopa County to be inspired, informed, and supported so they may create more sustainable and livable homes and communities in the low desert.
Laura Ward is a Tenth Generation Farmer, an Arizona Master Gardener, and an Arizona-licensed Home and Termite Inspector (at least one job must pay the bills!). She has too many animals and poultry to mention them all, but is especially proud of her two Clydesdale horses, for the good work they do on the farm and their help teaching adults and children about heritage draft breeds and how gentle they are.
1. Waters, S., and Haley, P. AZ1566: Using Rainwater in Urban Landscapes: Quick Guide for Maricopa County, University of Arizona Cooperative Extension (2012)
2. UA Water Resources Research Center, Quick Resource: Active Water Harvesting (2013) Retrieved from https://wrrc.arizona.edu/sites/wrrc.arizona.edu/files/Quick%20Resouce-%20Active%20WH%20%28final%29.pdf
3. Waters, S., Klawitter, R., Haley, P., and Hamilton, J. AZ1614: Saving Water and Money at Home in Arizona: Step 1. Understanding Water Rates, University of Arizona Cooperative Extension. (2014)
4. Ref D. Artiola, J.F., Hix, G., Gerba, C., and Riley, J.J. AZ1610: An Arizona Guide to Water Quality and Uses, University of Arizona Cooperative Extension. (2014)
5. Arizona Department of Environmental Quality, ADEQ Publication No. C 07-01, Using Gray Water at Home (n.d.). Retrieved from: https://legacy.azdeq.gov/environ/water/permits/download/graybro.pdf
6. Daily, C., and Wilkins, C. AZ1564: Passive Water Harvesting, University of Arizona Cooperative Extension. (2012)
The Monthly Rainfall Table reference: Waterfall, P. AZ1344: Harvesting Rainwater for Landscape Use. University of Arizona Cooperative Extension. (2006)