Project Statement

Extreme heat in Southern Arizona is not a new phenomenon, but recent trends reiterate the need to proactively plan for increasingly hot days on a nearly year-round basis. 2023 was the hottest year on record, with Phoenix and Tucson metro areas reporting significant increases in heat-related morbidity and mortality. Tree planting is a cost-effective way of mitigating urban heat; yet, in desert urban environments, scarce water resources limit the provisioning of adequate water for tree growth. Groundwater has been depleted in previous decades by extensive (in space) and intensive (in rate) pumping while declining Colorado River flows and decreased monsoon activity exacerbate drought. This study addresses these challenges with smart watering.

Project Narrative

Research Objectives

Identify scalable smart watering strategies for tree establishment using minimal water

Identify Southern Arizona urban tree species that positively respond to smart watering strategies

Observe tree phenology to understand whether smart watering strategies limit growth or increase susceptibility to disease

Analyze microclimate data to quantify urban heat reduction

Research Design

Watering treatments were selected for comparative study against a control (conventional drip irrigation). The selected treatments are 1) rainwater harvesting basin, 2) basin with organic mulch, and 3) basin with plastic-based hydrogel or 4) cellulose-based hydrogel. Experimental treatments received standardized irrigation from bubbler emitters, and five 1-hour drip emitters are used for conventional irrigation. All trees received a total of 5 gallons per irrigation event. The planting treatments represent a range of intensiveness and installation costs, reinforcing the need for scalable strategies applied throughout Southern Arizona.

Species were selected for their presence across Southern Arizona urban sites. We used a combination of native, desert-adapted, and non-native trees with a range of characteristics. Ironwoods grow slowly but are highly drought resistant; Joan Lionetti live oaks require more water but are hardy to 0°F and provide ample shade. Both canyon hackberries and desert willows grow quickly during drought, but the latter blooms productively from a young age, making desert willows a pollinator favorite.

We conducted extensive data collection using above- and below-ground sensors to measure multiple variables: temperature, humidity, soil moisture content, and soil electrical conductivity. Additionally, monthly phenological surveys of each tree were conducted to monitor seasonal changes in appearance.

Results and Discussion

Our project goals were two-fold. First, we identified drought-adapted tree species to test minimal water consumption during establishment. We found that water usage can be reduced by 30-50% during the first year of tree establishment while maintaining positive moderate growth in all species but the Joan Lionetti live oak. Actively working with stakeholders and horticulturalists increased the overall survival rate of trees and resulted in annual savings of 1,700 gallons of water per tree. In addition to correct plant selection for reduced water use, we tested slow water practices to quantify impacts on soil health and tree growth. Our observations show that experimental treatments maintained 22-37% higher soil moisture content compared to conventional drip irrigation planting. While Joan Lionetti live oak had the slowest growth during the 2023 summer growing season, the average growth of the canyon hackberry was 53 cm, which was the most significant change in height of all species. Our results empirically demonstrate that smart watering treatments provide water-saving benefits and mitigate heat within Southern Arizona’s urban environment.

Implications for Further Study

Over the next year, we will conduct additional soil analyses to determine the impact of smart watering treatments on soil organic carbon and water-holding capacity to further optimize growth during the establishment period. Observations will continue as scheduled to further validate the effectiveness of the experimental treatments. Additionally, specimens will be unearthed to allow for the study of root structure and development.