It’s no secret that more trees and more shade would make Phoenix a happier and healthier place. But do trees just use too much water that we don’t have enough of to begin with? The answer is, NO. The water it would take for Phoenix to significantly increase its tree canopy is a drop in the proverbial bucket when compared to the city’s overall water usage.
In 2017, University of Arizona researchers published the results of a tree irrigation study conducted just outside of Phoenix in Maricopa, Arizona–an area that gets about the same amount of annual rainfall as Phoenix. The researchers used several varieties of trees commonly used in desert landscaping, starting with trees about 6 to 8 feet tall in standard nursery containers. To set up the study, the trees were planted in an empty field with no improvement other than creating shallow basins in the dirt around each tree to hold irrigation water. The researchers then watered the trees well for 18 months to establish them–about 20 gallons of water per week per tree during the summer.
Then, the researchers irrigated the trees at three different levels of irrigation for five years to study growth and health. Although all trees got about the same amount of water each time they were irrigated, some were irrigated more frequently than others. In total, the “wet” group got about 940 gallons of irrigation per year, while the “medium” group got only about 644 and the “dry” group only 518.
In general, the trees grew and thrived, but the researchers found that many common species did just as well with “dry” irrigation as they did with “medium” or “wet” irrigation. Species including velvet mesquite, palo verde, Red Push pistache, desert willow, and Texas ebony grew about as large with the least irrigation. Indeed, the researchers concluded that these species should be irrigated at the “dry” rate.
At the end of the five years in March 2014, the researchers discontinued watering to simulate a drought, but the mesquite, palo verde, pistache, and desert willow “were not detrimentally affected.” Despite enduring a typical hot and dry spring and early summer with average high temperatures soaring to near 110 degrees and zero rainfall, the trees remained healthy and leafed out when the monsoon rains eventually came. The researchers therefore concluded that these species may thrive with even less irrigation after they have been established.
Based on the study, Phoenix could plant a million new trees with minimal effect on overall water use. Over the past decade or so, Phoenix’s total water usage has remained fairly constant at about 300,000 acre-feet per year, or about 100 billion gallons. Irrigating a million trees with 518 million gallons of water annually would only amount to about 0.5% of Phoenix’s current annual direct water usage.
Moreover, direct water usage is a small fraction of overall consumption: 72% of the 2.2 trillion gallons used annually in Arizona is used for agriculture. Phoenix indirectly consumes agricultural water by consuming agricultural products, including the water-intensive products produced in Arizona like beef and cotton. It takes more water to produce a single hamburger and four times more to produce a pair of cotton jeans than it does to water a tree for a year. If Arizona’s total water usage were represented by a three-gallon bucket full of water, irrigating a million new trees would take about half a teaspoon–almost literally a drop in the bucket.
Big picture: desert trees are pretty frugal when it comes to water use!
And 518 gallons per tree per year may even be an overestimate of the water needed. The estimate assumes that trees would continue to be irrigated at the same rate even after they are fully established, which may not be necessary. Nor does it account for landscaping improvements to increase runoff capture or ground covers that reduce soil evaporation, which may reduce irrigation needs. Similarly, trees planted where buildings or other structures shade the ground may need less irrigation. And finally, the estimate assumes that none of the trees will be planted in places that are already irrigated.
So even though there may be challenges to increasing Phoenix’s tree canopy–money probably being the biggest–the good news is that water usage really shouldn’t be a huge barrier. And a million new trees would be well worth the water invested. It wouldn’t get the city all the way to its stated goal of a 25% tree canopy to mitigate the city’s urban heat island effect, but it would make a substantial dent in that goal, increasing the city’s tree canopy by several percentage points. Trees will help improve air quality, reduce overall heat, and provide badly needed shade, which will save many lives while helping ensure Phoenix’s long-term future as a happy, healthy, and prosperous place.