While previous studies have shown that urban heat islands (UHI) tend to increase residential water use, they have not yet analyzed the feedbacks among vegetation intensity, diurnal temperature variation, water use, and characteristics of the built environment. This study examines these feedback relationships with the help of a path model applied to spatially disaggregated data from Phoenix, Arizona. The empirical evidence from the observations in Phoenix suggests the following: (1) impervious surfaces contribute to increased residential water use by exacerbating UHI; (2) larger lots containing pools and mesic vegetation increase water demand by reducing diurnal temperature difference; and (3) smart design of urban environments needs to go beyond simplistic water body- and vegetation-based solutions for mitigating uncomfortably high temperatures and consider interactions between surface materials, land use, UHI, and water use.

Problem: The prospect that urban heat island (UHI) effects and climate change may increase urban temperatures is a problem for cities that actively promote urban redevelopment and higher densities. One possible UHI mitigation strategy is to plant more trees and other irrigated vegetation to prevent daytime heat storage and facilitate nighttime cooling, but this requires water resources that are limited in a desert city like Phoenix.

Purpose: We investigated the tradeoffs between water use and nighttime cooling inherent in urban form and land use choices.

Methods: We used a Local-Scale Urban Meteorological Parameterization Scheme (LUMPS) model to examine the variation in temperature and evaporation in 10 census tracts in Phoenix's urban core. After validating results with estimates of outdoor water use based on tract-level city water records and satellite imagery, we used the model to simulate the temperature and water use consequences of implementing three different scenarios.

Results and conclusions: We found that increasing irrigated landscaping lowers nighttime temperatures, but this relationship is not linear; the greatest reductions occur in the least vegetated neighborhoods. A ratio of the change in water use to temperature impact reached a threshold beyond which increased outdoor water use did little to ameliorate UHI effects.

Takeaway for practice: There is no one design and landscape plan capable of addressing increasing UHI and climate effects everywhere. Any one strategy will have inconsistent results if applied across all urban landscape features and may lead to an inefficient allocation of scarce water resources.

Research Support: This work was supported by the National Science Foundation (NSF) under Grant SES-0345945 (Decision Center for a Desert City) and by the City of Phoenix Water Services Department. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF.

This study addresses a classic sustainability challenge—the tradeoff between water conservation and temperature amelioration in rapidly growing cities, using Phoenix, Arizona and Portland, Oregon as case studies. An urban energy balance model— LUMPS (Local-Scale Urban Meteorological Parameterization Scheme)—is used to represent the tradeoff between outdoor water use and nighttime cooling during hot, dry summer months. Tradeoffs were characterized under three scenarios of land use change and three climate-change assumptions. Decreasing vegetation density reduced outdoor water use but sacrificed nighttime cooling. Increasing vegetated surfaces accelerated nighttime cooling, but increased outdoor water use by ~20%. Replacing impervious surfaces with buildings achieved similar improvements in nighttime cooling with minimal increases in outdoor water use; it was the most water-efficient cooling strategy. The fact that nighttime cooling rates and outdoor water use were more sensitive to land use scenarios than climate-change simulations suggested that cities can adapt to a warmer climate by manipulating land use.

Creating a Healthier, More Livable and Prosperous Phoenix

Phoenix is poised to become the next great American City. The Tree and Shade Master Plan presents Phoenix’s leaders and residents a roadmap to creating a 21st Century desert city. The urban forest is a keystone to creating a sustainable city because it solves many problems with one single solution. By investing in trees and the urban forest, the city can reduce its carbon footprint, decrease energy costs, reduce storm water runoff, increase biodiversity, address the urban heat island effect, clean the air, and increase property values. In addition, trees can help to create walkable streets and vibrant pedestrian places. More trees will not solve all the problems, but it is known that for every dollar invested in the urban forest results in an impressive return of $2.23 in benefits.

Phoenix has a strong foundation on which to build the future. Phoenix residents value natural resources and have voted repeatedly to invest in the living infrastructure. For instance, the Phoenix Parks and Preserve Initiative was passed twice with over 75 percent voter approval. This modest sales tax has purchased land for the Sonoran Preserve, funded habitat restoration efforts along Rio Salado, built new parks and planted hundreds of new trees. These projects and others like it provide the base for a healthy urban forest. Trees and engineered shade have the potential to be one of the city’s greatest assets and the Tree and Shade Master Plan provides the framework for creating a healthier, more livable and prosperous Phoenix.

The Urban Forest – Trees for People

The urban forest is a critical component of the living infrastructure. It benefits and attracts residents and tourists alike to live, work, shop and play in the city. Phoenix’s urban forest is a diverse ecosystem of soils, vegetation, trees, associated organisms, air, water, wildlife and people. The urban forest is found not only in parks, mountain preserves and native desert areas, but also in neighborhoods, commercial corridors, industrial parks and along streets. The urban forest is made up of a rich mosaic of private and public property that surrounds the city and provides many environmental, economic, and social benefits.

In order for the urban forest to be a profitable investment, Phoenix must do more than just plant trees. The entire lifecycle of the tree must be addressed because the current planting, maintenance, and irrigation practices are preventing many trees from providing their maximum return on investment. The Tree and Shade Master Plan provides a detailed roadmap to address these issues, as well as many others, with realistic and incremental steps. To succeed, this plan requires a long-term investment from the residents and leaders of Phoenix.

Trees are Solution Multipliers

Solution multipliers solve numerous problems simultaneously. Trees are a perfect example of a solution multiplier because when planted and maintained correctly, they can provide many economic, environmental, and social benefits. According to the US Forest Service, trees benefit the community by: providing a cooling effect that reduces energy costs; improving air quality; strengthening quality of place and the local economy; reducing storm water runoff; improving social connections; promoting smart growth and compact development; and creating walkable communities (US Forest Service and Urban & Community Forestry). Trees are high-yield assets; for example, the City of Chicago values its trees at $2.3 billion dollars. Trees have a documented return on investment (ROI) in Arizona of $2.23 for every $1 invested (US Department of Agriculture Forest Service). This demonstrates the important role that trees have within the city's economy. This is why it is critical to manage and invest in the urban forest; the health of the urban forest is closely linked to the economic health of the city.

Maintainable Infrastructure

Phoenix is a desert city that has a history of several decades of drought. In order to achieve a healthy urban forest we must use water wisely. Currently, 60 percent of Phoenix’s water is used outdoors, mainly for landscape irrigation. According to the City of Phoenix’s Water Services Department, Phoenix has an adequate sustainable water supply to meet the State of Arizona’s 100-year assured water supply standard. This includes growth in Phoenix’s system water demand over the next 20 years or more. Nonetheless, to achieve a maintainable urban forest, water must be used more efficiently. This is done with high-efficiency irrigation systems, use of drought-tolerant plant material, strategic placement of shade corridors and continued education. In order for a healthy urban forest to exist, it must be coupled with strong water management.

Implementation

The Urban Forest Infrastructure Team and the Parks and Recreation Department are charged with coordinating and maintaining the Tree and Shade Master Plan. Many City departments will implement the plan as they work to fulfill their own missions. The Tree and Shade Master Plan will not only provide a framework to achieve an average 25 percent tree canopy coverage by 2030 but will also help to achieve many goals and policies from the Green Phoenix Initiative and the voter ratified General Plan.

The plan proposes incremental steps to achieve the 2030 vision and canopy goal. The City of Phoenix is beginning to put a process in place to preserve, maintain, and redevelop the urban forest. This plan intends to increase the quality of life and economic vitality of the city by recommending ways to create a sustainable urban forest for future generations.

Background: Vulnerability mapping based on vulnerability indices is a pragmatic approach for highlighting the areas in a city where people are at the greatest risk of harm from heat, but the manner in which vulnerability is conceptualized influences the results.

Objectives: We tested a generic national heat-vulnerability index, based on a 10-variable indicator framework, using data on heat-related hospitalizations in Phoenix, Arizona. We also identified potential local risk factors not included in the generic indicators.

Methods: To evaluate the accuracy of the generic index in a city-specific context, we used factor scores, derived from a factor analysis using census tract–level characteristics, as independent variables, and heat hospitalizations (with census tracts categorized as zero-, moderate-, or highincidence) as dependent variables in a multinomial logistic regression model. We also compared the geographical differences between a vulnerability map derived from the generic index and one derived from actual heat-related hospitalizations at the census-tract scale.

Results: We found that the national-indicator framework correctly classified just over half (54%) of census tracts in Phoenix. Compared with all census tracts, high-vulnerability tracts that were misclassified by the index as zero-vulnerability tracts had higher average income and higher proportions of residents with a duration of residency < 5 years.

Conclusion: The generic indicators of vulnerability are useful, but they are sensitive to scale, measurement, and context. Decision makers need to consider the characteristics of their cities to determine how closely vulnerability maps based on generic indicators reflect actual risk of harm.

We investigated the spatial and temporal variation in June mean minimum temperatures for weather stations in and around metropolitan Phoenix, USA, for the period 1990 to 2004. Temperature was related to synoptic conditions, location in urban development zones (DZs), and the pace of housing construction in a 1 km buffer around fixed-point temperature stations. June is typically clear and calm, and dominated by a dry, tropical air mass with little change in minimum temperature from day to day. However, a dry, moderate weather type accounted for a large portion of the inter-annual variability in mean monthly minimum temperature. Significant temperature variation was explained by surface effects captured by the type of urban DZ, which ranged from urban core and infill sites, to desert and agricultural fringe locations, to exurban. An overall spatial urban effect, derived from the June monthly mean minimum temperature, is in the order of 2 to 4 K. The cumulative housing build-up around weather sites in the region was significant and resulted in average increases of 1.4 K per 1000 home completions, with a standard error of 0.4 K. Overall, minimum temperatures were spatially and temporally accounted for by variations in weather type, type of urban DZ (higher in core and infill), and the number of home completions over the period. Results compare favorably with the magnitude of heating by residential development cited by researchers using differing methodologies in other urban areas.

Preventing heat-associated morbidity and mortality is a public health priority in Maricopa County, Arizona (United States). The objective of this project was to evaluate Maricopa County cooling centers and gain insight into their capacity to provide relief for the public during extreme heat events. During the summer of 2014, 53 cooling centers were evaluated to assess facility and visitor characteristics. Maricopa County staff collected data by directly observing daily operations and by surveying managers and visitors. The cooling centers in Maricopa County were often housed within community, senior, or religious centers, which offered various services for at least 1500 individuals daily. Many visitors were unemployed and/or homeless. Many learned about a cooling center by word of mouth or by having seen the cooling center’s location. The cooling centers provide a valuable service and reach some of the region’s most vulnerable populations. This project is among the first to systematically evaluate cooling centers from a public health perspective and provides helpful insight to community leaders who are implementing or improving their own network of cooling centers.