Urban Heat Island — Higher Temperatures and Increased Water Use

Another issue for water planners to ponder

by Joe Gelt

The urban heat island might be viewed as an unintended consequence of urban growth and development: UHI resulted when cityscapes were built-up and built-over. Unintended consequences then spawn their own set of consequences. A consequence of the UHI that is getting increased attention is its effect on the water resources of an area.

The workings of the UHI are generally understood. Urban areas are increasingly becoming “hot spots,” their impervious paved surfaces and reduced vegetation resulting in less of the sun’s incoming radiant energy reflected back into the atmosphere.

The unreflected heat is stored by asphalt, brick, concrete and other materials that make up much of the urban environment. These have greater thermal storage capacity than natural surfaces. Energy stored during the day and released after sunset results in higher nighttime temperatures; in other words, the UHI effect.
UHI effects are most pronounced at nighttime. In 1948, Phoenix’s average nighttime low temperature was 75 degrees; in 2003 it had increased to 86.7 degrees. Some researchers say that at some time in the future 100-degree nights will be the norm.

Compared to Phoenix the UHI is less of a problem in the Tucson area. One study showed that while Phoenix’s daytime temperature averaged 4.5 degrees higher than Tucson’s, its nighttime temperature averaged 10 degrees warmer. Partly accounting for the difference is Tucson’s smaller size; also the city contains less heat-absorbing materials.

Not as well understood as its causes are the UHI’s likely effects on the water resources of an area. Will higher UHI temperatures result in an increased consumption of water? What UHI-related factors could cause increased water use? This is an issue with public policy implications, of importance when reckoning water availability and consumption and devising efforts to achieve sustainability. Water lost due to the UHI is water unavailable for other uses.

Arizona State University researchers are attempting to answer such questions with studies of the Phoenix area.

Water Needed for Energy
One study is investigating the effects UHI has on energy and water sustainability in the Phoenix area. As is becoming increasingly recognized, energy and water are interrelated issues, with water needed to produce energy and energy needed to treat and distribute water. Researchers Jay Golden and Anthony Brazel, both with the ASU Global Institute of Sustainability, are examining the significance of the water/energy interrelationship in regards to the UHI.

Thermoelectric power generation accounts for more water withdrawal than any other water usage, including agricultural irrigation or municipal water consumption. The ASU researchers are considering the increased amounts of water needed to generate energy to power air conditioners. In response to the UHI, air conditioners will become necessary appliances, with more purchased, adding to the large number already in use.

In effect, greater amounts of water will be needed to generate needed power to run more air conditioners for longer periods of time to cope with the discomforts of higher UHI temperatures.
Examining historic climatic records enabled the researchers to determine that Phoenix is experiencing an increased number of cooling degree days and a reduced number of heating degree days. Cooling degree days and heating degree days, variances in the daily average temperature above or below 65 F respectively, are used to estimate the amount of energy needed to cool or heat buildings. Both are affected by urbanization.

During the 1950s the region averaged 42,264 heating degree hours per year while during the 1990s the region annually averaged 29,800 heating degree hours. Reversing the trend, cooling degree hours in the 1950s averaged 95,597 per year increasing to 112,551 cooling degree hours per year in the 1990s.
That calculation enabled the researchers to figure the net impact over time of overall increased energy consumption based on increased cooling degree hours and decreased heating degree hours of a single-family residence. It went from 7,888 kilowatt-hours in the 1950s to 8,706 kWh during the 1980s to currently 8,873 kWh from 1994 to 2004. The figures represent the electricity consumption requirements resulting from the thermal modification of urban environments.

These figures along with an analysis of power generation operations enabled the researchers to determine actual water consumption for residential electricity generation within the Phoenix region.

Increasing Evaporation
Sophisticated scientific enquiry is not needed to realize that additional UHI-related water consumption will result from increased evaporation. Higher UHI temperatures will cause larger volumes of water to evaporate from swimming pools, urban ponds and wetlands and surface-irrigated areas.

At the same time, however, evaporation cools, mitigating UHI effects. Some gain would seem to result from water lost. The problem suggests a solution that, if pursued, would worsen part of the problem.
Patricia Gober, co-director of ASU’s Decision Center for a Desert City, says research is needed to determine the amount of surface area covered with water in the Phoenix region, including urban lakes and pools. Knowing this would enable researchers to determine the amount of water evaporated. She says, “A pool replaces its volume of water every year through evaporation. People don’t know it; it is a silent thing.” She considers pools more UHI water-consumptive than landscaping in the Phoenix area.
UHI Effect on Vegetation

The UHI effect on vegetation is a complicated, unsettled matter. Some say since plants will be coping with higher nighttime temperatures they will become stressed, requiring more frequent watering to survive. Others are not so sure; they say photosynthesis occurs during daylight hours when temperatures are less affected by the UHI effect.

What is generally agreed upon, however, is that vegetation helps reduce the UHI effect. If impervious paved surfaces and reduced vegetation contribute to UHI, increased vegetation will counteract the effect. More vegetation — and increased watering — will help combat the UHI effect.

Urging increased vegetation might undercut a central message of most outdoor water conservation programs, that xeriscape principles be applied. Xeriscape means planting low-water use vegetation in a water-scarce environment. A reader’s response to a recent Arizona Republic editorial demonstrates that some people, depending upon how they perceive the message, believe they are being advised to discard xeriscape in favor of more water-consuming vegetation to relieve UHI discomforts.

The July 23 editorial noted a study supported by ASU’s Decision Center for a Desert City that the growth and the intensity of the UHI is exceeding researchers’ expectations. It warned that it won’t be long before overnight lows will likely reach 100.

To counteract the UHI effect the editorial exhorts citizens to “Plant more vegetation. Shade from trees and bushes reduces the heat absorbed by walls and pavement. Leaves, meanwhile act as nature’s air-conditioners, cooling the surrounding air when water evaporates through their pores.”

A reader’s take on the message is evident in her July 25 response: “For 30-some years Phoenicians have been exhorted to save water by covering their lawns with sand and gravel and a few desert plants. Now, according to Sunday’s paper, we’re told we need more grass and shade trees to fight the heat island we’ve created.

“Forty years ago we had a desert lawn in the front yard, but only for two years until we measured the temperature and found a 15-degree difference between the front yard and the grassy backyard. We quickly shoveled up the sand and planted grass out front, cooling the whole house down.”
And at the same time greatly increasing their water use.

Some call it a trade-off, determining the amount of water that can be used to grow vegetation to deter the UHI before its use becomes excessive, threatening valuable supplies needed for other applications and for achieving sustainability. In other words, how much water can we spare or trade-off in efforts to cope with the UHI.

Others don’t see so much a trade-off as a bind, that in the rush to grow and develop urban areas we have created various problems; that to resolve the UHI problem we consider exacerbating another human-engendered problem: water scarcity. One problem is worsened to mitigate the effects of another.
There is another aspect to the UHI-vegetation issue to ponder. The significantly higher temperatures extending longer into the evening will modify the ecosystem. With winter freezes in Phoenix likely becoming events of the past, citrus and other high water-use vegetation will have a better survival rate and be more often grown. This will increase water use.

Residential Water Use
What effect might the UHI have on residential water use? Some believe its effect is already evident, if not recognized in water use records. For example, Gober says, “Residential water use in Phoenix has declined over time on a per capita basis. But I think it would have declined faster than it did due to conservation practices if the UHI had not been around.”

Subhro Guhathakurta of ASU’s School of Planning and the Global Institute of Sustainability and colleagues are studying the effect the UHI has on Phoenix residential water use. They want to determine if a higher UHI temperatures will result in increased residential water use.

Despite UHI’s overall impact and its general increase in temperature over a broad area, a spatial variation is evident, with different nighttime temperatures occurring in localized areas. By focusing on the spatial variations in summer nighttime temperatures Guhathakurta wanted to determine what effect varied nighttime temperatures have on household water use.

He looked at average water demand for single-family residential units by census track and correlated it with certain water demand variables, including size of house and lot, size of pool and amount of vegetation. Demographic variables also were included in the analysis, along with nighttime temperature variables.
Guhathakurta says, “We discovered that the nighttime temperatures were a fairly significant predictor of water demand.” A cross-sectional analysis of a detailed dataset of water use in June 1998 showed that water demand for single-family residences increases two percent for each percent rise in nighttime temperatures.

He described the work as an exploratory study to determine whether the UHI significantly affects residential water use. He plans follow-up studies to better understand the cause-and-effect relationships that account for the increased residential water use.

An Anomaly
With UHI raising concerns about increased water use, one study stands out by suggesting that the UHI might actually cause an increase in precipitation in a certain situation. J. Marshall Shepherd, a climatologist at the University of Georgia, relied on a 108-year-old data record as well as data from NASA’s Tropical Rainfall Measuring Mission satellite to study rainfall patterns in Phoenix. His records showed a 12 to 14 percent increase in monsoon rainfall in the northeast suburbs of Phoenix from the pre-urban (1895-1949) to the post urban (1950-2003) periods. Other areas in the region did not share this rainfall increase.
Shepherd said, “There was something strange and interesting about that lower Verde River basin area to the northeast of downtown Phoenix.” He wanted to account for the anomaly.

He hypothesizes that UHI conditions are a likely factor to account for the increased rainfall. He says, “During the monsoon season thunderstorms form in the mountains east of Phoenix. Many of those storms produce outflow boundaries that move west toward the city of Phoenix. We hypothesize that the outflow from those mountain storms are interacting somehow with the urban heat island circulation in the northeastern part of the metro area; that is why it is a preferred area for enhanced rainfall.”
He says, “The results showed us just how sensitive the water cycle can be to human-induced changes, even under arid or drought conditions.”

Reducing the UHI effect would be good news, although its reduction also might result in more water being used. A major concern about UHI is that its higher temperatures will discourage growth and development. Reducing the UHI then might ensure continued growth which in turn might mean more people using more water.

When it comes to conserving water, reducing the UHI effect could be a win-some, lose-some situation.