317

The Impact of the PhoenixUrban Heat Island onResidential Water Use

One goal of the smart growth movementis a more compact urban form, intendedto reduce energy use and the cost ofmoving materials, products, and people.The benefits ot compactness are compro-mised, however, if higher densities andmore intense land use create urban heatislands, which increase water and energyuse. This study examines the effects ofPhoenix's urban heat island on water useby single-family residences, controllingfor relevant population and housingattributes. Our statistical analysisdemonstrates that increasing daily lowtemperatures by 1° Fahrenheit is asso-ciated with an average monthly increasein water use of 290 gallons for a typicalsingle-family unit. These results suggestthat planners should consider efFects onwater demand as well as other environ-mental consequences when they evaluategrowth strar^ies, and use incentives toencourage efficiency and sustainability.

Subhrajit Guhathakurta (subhro.guha@asu.edu) is a professor in the School ofPlanning and the Global Institute ofSustainability, Arizona State University,focusing on urban environmentalmodeling, simulating urban futures,urban economics, and regional planning.He directs the urban simulation andmodeling laboratory at ASU s Collegeof Design and is a lead investigator of theDigital Phoenix project. Patricia Gober(gober@asu.edu) is a professor of geog-raphy at Arizona State University. Herresearch centers on issues of migration,rctiremcni communities, and environ-mental change in metropolitan Phoenix.She co-directs the Decision Center tor aDesert City, which studies water man-agement decisions in the face of growingclimatic uncertainty in greater Phoenix.

Journal of the American Planning Associacion,

Vol. 73, No. 3. Summer 2007

©American Planning Associaiion. Chicagti. IL.

Subhrajit Guhathakurta and Patricia Gober

The smart growth and urban sustainability movements have drawnattention to the environmental consequences of urbanization. Both ofthese conceptualizations implicitly assume that more compact urban

designs lead to more efficient use of land, water, and energy. These benefits maybe less than previously thought, however, when the full impacts of the urban heatisland (UHI) ertect are considered.

The UHI effect, in which urban regions absorb a greater share of the sun'sradiant energy than natural landscapes would during the day, and release it atnight, is central to the field of urban climatology (Arnfield, 2003; Oke, 2006;Souch & Grimmond, 2006). The effect can occur throughout the year, is af-fected by local weather conditions, and is typically most intense in the urbancore and less severe on the urban periphery. The temperature variation causedby the UHI effect is greatest at night, when heat stored during the daytime isreleased into the atmosphere (Oke, 1981; Unger, 2004). Variation in the UHIeffect across an urban area is caused by the amount of sun-warmed urban surfaceexposed to the cold night sky, and, thus, related to variations in land use, buildingmaterials and heights, street geometry, and spacing between buildings, amongother natural and manmade factors (Eliasson, 2000; Unger, 2004). Thus, thepattern of urban development and its density are important to the severity of theUHI effect. A World Meteorological Organization guide {Oke, 2004, 2006)defined and ranked seven types of urban development zones' effects on climateat the local scale, showing higher density urban settings to result in more severeUHI effects per unit area.'

Growing awareness of the potential significance of UHIs in urban envi-ronments has led to empirical studies of their effects on human health (Harlan,Brazel, Prashad, Stefanov, &c Larsen, 2006), air quality {Cardelino &C Ghameides,1990; Stone, 2004), and energy use^ (Grutzen, 2004; Golden, Brazel, Salmond,& Laws, 2006; Rosenfeld, Akbari, Romm, & Pomerantz, 1998). However, weknow of no previous study of the effect of UHIs on residential water use.

Thus, this article examines whether Phoenix's large and intensifying UHIincreases the demand for residential water. We expect the increase in urbanovernight low temperatures to affect water use indirectly by increasing energy use,because thermoelectric power generation requires water for cooling (Golden et al.,2006). Higher temperatures should also affect water use directly by increasing

318 Journal of the American Planning Association, Summer 2007, Vol. 73, No. 3

outdoor demand for water for irrigating vegetation andfilling swimming pools, particularly in a desert city likePhoenix, where two-thirds of all residential water use is foroutdoor purposes, and, thus, sensitive to climate. Wedevelop a model to test whether the spatial variation intemperature affects residential water use at the scale of thecensus tract, controlling for other potentially contributingfactors. We conclude by describing the implications of ourfindings for planning research and practice.

Phoenix and Its Urban Heat IslandA rapidly growing desen city such as Phoenix is an

ideal candidate for study of the UHI effect. Situated alongthe northern edge of the Sonoran Desert, metropolitanPhoenix has grown rapidly from a network of agriculturalsettlements containing just 330,000 people in 1950 to alarge metropolis approaching 4 million residents today. In1950, the city was surrounded by irrigated agriculture, whilethe desert itself was relatively far from the city's urban fringe.Today, the city encroaches on open desert landscapes, andthe urbanized area consists of a complicated patchwork ofcommercial, residential, remnant agricultural, urban fringe,and open desert lands. The average population density in2000 was 3,600 persons per square mile, spread across anurbanized area of almost 800 square miles (U.S. Census Bu-reau, 2006; U.S. EPA, 2004). Contrary to popular percep-tion, density within the Phoenix urbanized area is actuallyabout average for cities in its size class. New developmentoccurs at six to seven homes per acre, and the urban fringeis the site of many multi-family apartment and condo-minium developments. Gammage (1999) has argued thatPhoenix has a sharper, more defined urban boundary thanother cities becau.se most new development cannot proceedwithout connection to the existing water infrastructure.

In addition to rapid urbanization. Phoenix's warm,dry climate and large number of clear, calm days createsconditions that are conducive to UHI development (Brazel,Selover, Vose, & Heisler, 2000). The mean summer tem-perature at Sky Harbor Airport in the center of the metro-politan region is 87° F; the winter temperature averages 52°F (Stabler, Martin, & Brazel. 2005). From 1997 to 2000,the average daily low temperature was almost 10° F higherat the Sky Harbor weather station than at a companionstation 60 miles to the west, although the daily maximumtemperatures were nearly identical throughout the year(Baker etal., 2002).

Phoenix's UHI has been identified and quantified inseveral studies since the early 1980s (Balling & Brazel,1986a, 1986b, 1987; Brazel et al., 2007; Cayan & Doug-

las, 1984; Hawkins, Brazel, Stefanov, Bigler, & SafFell,2004). An examination of summertime low temperaturesfrom 1980 to 1985 shows a classic heat island formation,with the center of the city having the highest average values(Balling & Brazei, 1987, p. 77). In addition, the samestudy reports that temperature records from 12 stationsbetween 1949 and 1985 show rapid increases in low tem-peratures in the central portions of the city. A more recentstudy (Stabler et al., 2005) found agricultural and residen-tial land uses reduced summer low temperatures more thancommercial and industrial land uses, and vegetative cover,measured by the normalized differential vegetation index(NDVI), also correlated negatively with daily low temper-atures. The study also reported historical comparisons ofland use, NDVI, and microclimate data between 1976and 1999, showing increases in low temperatures to becorrelated with decreases in NDVI, an indicator of urbanexpansion.

Brazel et al. (2007) conducted a spatial and temporalanalysis of June low temperatures in Phoenix between1990 and 2004. Some summers were generally cooler andwetter than others, but overall, the intensification of landuse associated with urban growth over time contributed tohigher daily low temperatures. Variations in low tempera-tures across the metropolitan area were explained by thetype of urban development, ranging from the urban coreand infill sites to desert and agricultural fringe locations toexurban conditions, and by the pace of new home con-struction in the immediate vicinity of the weather stationsincluded in the study. The low temperatures around thosestations rose by almost 2° F for every 1,000 new homesbuilt nearby. Urban core locations were, on average, 4° Fwarmer than the surrounding rural countryside on a typicalJune night.

Phoenix's Water Supply and DemandWater is the key resource for growth in a desert city

such as Phoenix. Although the city itself receives just 8inches of rainfall annually, a significant share of its wateris from the Salt and Verde Rivers, both fed by more humidupstream watersheds. In addition, vast undergroundaquifers supplement surface water supplies during periodsof persistent and intensive drought. This combination ofsurface and groundwater supported first agriculture, and,after World War II, a large urban population. Between1973 and 1993 the Central Arizona Project, a 336-miieaqueduct, was constructed to deliver water from theColorado River to the desert cities of Phoenix and Tucson(Cober, 2006).

Guhathakurta and Gober: The Impact of the Phoenix Urban Heat Island on Residenrial Water Use 319

Although the region currently has enough water tohandle immediate and medium-term future needs, it isconfronted with a number of risks to its longer-termsupply (City of Phoenix, 2005). First, water allocations inthe Colorado River system are based on early 20th centuryflow conditions that were about 2 million acre feet (14%)above the current levels. Second, California's claims onColorado River water come before those of Arizona, mean-ing Phoenix would likely have to give up some portion ofits share in case of shortage. Third, the Salt and Verde Rjverwatersheds, which supply the Salt River Project (SRP),another major source of surface water for the region, arethreatened by high rates of growth and uneven precipita-tion. Fourth, global climate models forecast considerableuncertainty in future climatic conditions in the Salt andVerde River watersheds, with most scenarios predictingmuch less runoff due to the projected rise in temperatureover the coming decades (Balling, Ellis & Cober, 2007).Phoenix has embarked on an aggressive program of waterconservation and reclamation because of these risks and thelikelihood of continued rapid growth, but UHI-inducedincreases in water consumption threaten to offset andcompromise legitimate conservation gains.

The demand for water in the City of Phoenix rangesfrom 140 million gallons per day in the winter months to430 million gallons per day in the peak summer months(City of Phoenix, 2005, p. 48). Of this total demand,residential uses account for two-thirds, of which three-quarters, or 50% of total demand, is used by single-familyunits. The majority of the water consumed by single-family units {about 66%) is for landscaping and otheroutdoor uses. In fact, more water is used for residentiallandscaping than for any other type of use, includingpools, golf courses, and indoor uses {Mayer & DeOreo,1999).

Per capita water consumption has been falling steadilyin Phoenix since the passage of the Arizona CroundwaterManagement Act in 1980. In 2004, the overall per capitawater demand was around 210 gallons per capita per day,which was more than 20% lower than the same figure in1980. This reduction in per capita water demand is due inpart to more water-efficient new housing and nonresidentialdevelopments, and to conservation programs implementedby the state and the city. These programs range fromproviding residential users free water-saving devices toimplementing educational programs and pricing strategies(Campbell, Johnson, & Larson, 2004).

Temperature and VViiter l^se Data

The objective of this study was to determine whetherresidential water use in Phoenix is affected by heat islandeffects, which result in higher nighttime temperatures andcompress the difference between daily temperature highsand lows. This required temperature data at a fairly de-tailed spatial scale in order to capture the potential effectsof variations in development patterns and densities. Noempirical record of actual nighttime low temperaturesexisted at a sufficiently detailed spatial scale, so we usedmodeled data representing air temperatures at 5 a.m., fromthe work of Grossman-Clarke, Zehnder, Stefenov, Liu, andZoldak (2005), whose simulation was performed for atypical day in June, 1998.^ Their modeled temperaturesagreed well with the actual temperatures measured at SkyHarbor Airport and a network of 15 weather stationslocated across a range of land uses, and while they are notperfect representations of real-world conditions, theyprovide good estimates of intra-urban variations in tem-perature resulting from land use and land cover conditions.We operationally defined daily low temperature as thesimulated temperature derived by Crossman-Clarke et al.(2005) at 5 a.m. on June 8. We used a geographic infor-mation system to spatially interpolate the output of thesimulation, available as 2-kilometer raster data, to thecensus tract level for our analysis. Figure 1 maps thesedaily lows for census tracts in the City of Phoenix. Usingthese data allowed us construct a model that used censuspopulation and housing parameters available for tracts.

The variation in summer low temperatures in Phoenixclearly shows higher low temperatures over the centralareas of Phoenix, including the Sky Harbor Airport, andlow temperatures that decline steadily toward both thenorth and the south. The mean of all of the tract lowtemperatures for June 8, 1998, was 70 °F, with the highestlow temperature {72.8 "F) recorded at the Sky HarborAirport. Our data show the lowest low temperatures thatday at the northern and northeastern edge of the city. 1 hedifference between the daytime high and nighttime lowtemperatures followed a slightly different pattern, withareas just northwest of the city center registering the leastdifference between highs and lows {approximately 17° F).However, we found the maximum difference occurred inthe same areas that had the lowest low temperatures in thecity (see Figure 2).

We obtained detailed breakdowns of water use fromthe City of Phoenix Water Services Department for June1998. Although the original dataset included many differ-ent categories of water users {single-family, several forms ofmulti-family units, various office types, industrial, public.

320 Journal of the American Planning Association, Summer 2007, Vol. 73, No. 3

Temperatures (°F)

64-65

66-67

68-69

70

71

72-73

Figure 1. Low temperatures in Phoenix census tracts, June 8, 1998.

Source: Grossman-Clarke et al., 2005.

Guhathakurta and Goben The Impact of the Phoenix Urban Heat Island on Residential Water Use

in standard di-viations

321

Figure 2. Differences between high and low temperatures in Phoenix census tracts, June 8, 1998.

Source: Grossman-Clarke et al., 2005.

322 Journal of the American Planning Association, Summer 2007, Vol. 73, No. 3

and others), we extracted information on water use forsingle-family units only, which we then aggregated tocensus tracts.

According to the data provided by the Water ServicesDepartment, the consumption of water in June 1998 bysingle-family residences in Phoenix averaged approximately17,000 gallons per unit, with significant variation through-out the city. The top 10% of residential water consumersused at least 21,578 gallons that month, while the bottom10% used 11,515 gallons or fewer. Single-family residencesshowed a clear pattern of high demand just north andnortheast of the city center, near the oldest areas of the city(Figure 3). Average water usage by residents of single-family homes tended to decline further to the north, in theparts of the city developed more recently. Single-familyunits in the southern areas surrounding a large mountainpreserve also used less water than the city average. Althoughwater use corresponded roughly to age of development,with the oldest areas showing the highest average single-family water use, age of development also correlates withvegetative cover and land use type, other factors thatcontribute to water use.

Empirical Models of Water Usein Phoenix

We constructed two empirical models to determinethe specific and unique effect, if any, of Phoenix's heatisland on single-family water use. In our first model, weestimated the significance of cross-sectional variation indaily low temperatures in explaining variation in averagewater use in single-family units by tract across the Gityof Phoenix, after controUing for other sources of waterdemand. The second model used a different measure ofthe heat island effect to confirm results of the first model,testing whether the difference between daily high and lowtemperatures affected water use, using the same controlvariables as in the first model.

The log-linear form provided the best fit in bothmodels, and yielded intuitive results. Other researchershave used this functional form in water demand models,obtaining reasonably robust results (Mansur & Olmstead,2006). The generalized forms of the models tested arespecified as follows:

SFWDf= or

USFWD>) = K,*yai.

LniSFWD,) = K- +2;^,1=1

Where:

n ti n

(=1

Vi + c,

Model 1

II n n

yb.D, +y r;A, +y. diSVi + e,

Model 2

or

SFWD = tract average single-family water consumption/' = census tract

/ / = a vector of tract housing characteristics£) = a vector of tract population characteristicsT= tract low temperature (temperature at 5 a.m.) on

June 8, 1998SV= adjusted NDVl (a measure of vegetative cover)A = difference between tract high and low temperaturesK= constant terme = error term.

After eliminating census tracts with missing data, weestimated the model for 287 census tracts in the City ofPhoenix.

In constructing our models we included the variablesmeasuring UHI effects and housing and demographicvariables to control for other factors that may contribute cosingle-family residential water use. We chose explanatoryvariables because of their hypothesized relationship withwater use in single-family homes, as tested in other studies(Aitken, Duncan, McMahon, 1991; Dube & van derZaag. 2003; Mayer & DeOreo, 1999; Wentz & Gober,2007). We explain these hypothesized relationships below.Table 1 lists the independent variables in the empiricalmodel, providing both descriptive statistics for each andwhere the data were obtained.

Tract-Level Measures of the Urban HeatIsland Effect

Daily Low Tempera lure. The UHI phenomenon ismanifested when radiated heat from terrestrial surfaces(both manmade and natural) increases the daily low tem-perature compared to low temperatures in nonurbanizedareas nearby. We hypothesize that increased minimumtemperatures will lead to increases in water use in single-family households due to a range of factors includinghigher rates of evaporation, transpiration, and cooling andconsumption needs.

Difference between Daily Hi/̂ h and Low TemperaliireH.Areas affected by UHI effects are also characterized bysmaller differences between daily high and low temperatures

Guhathakurta and Gober: The Impact of the Phoenix Urban Heat Island on Residential Water Use

Gallons

323

Figure 3. Mean water use per single-family residential unir in Phoenix census tracts, June, 1998 (gallons).

Source: City of Phoenix Water Resources Department.

324 Journal of the American Planning Association, Summer 2007, Vol. 73, No. 3

Table 1. Descriptive statistics for City of Phoenix census tracts.

Minimum Maximum MeanS(d.

deviation Source

Dependent variatiieMean water use per single-family

residential unit, June, 1998 (gallons) 7,481 80,415 17,025 6,712 Water Resources Department, City of Phoenix

Indepi'iiflcnl variiibiesMedian household income ($1999)Median persons per unitMean lot size (square feet)Mean age of single-family units (years)% of single-family units with poolMean pool surface area (square feet)% of single family units with evaporative

coolersVegetation index (NDVI)

% housing units ou'ner occupiedWater supply from SRP (0 = no, 1 = yes)Mean land parcel price, single-family

residences [$)Daily low temperature (°F)Difference between daily high and low

temperatures ("F)

9,6772.005,259

100

0.44

00

7.37364.57

174,8408.40

83,04458

92%832

iOO%.62

100%1

217,094

72.77

44,7924.99

10,42826

24%400

26%.50

63%.44

24.77970.09

225971.23

6,93412

21%

133

28%.03

25%.50

20,5161.87

17.08 22.37 18.39 1.21

U.S. Census Bureau, 2000 Summary File 3U.S. Census Bureau. 2000 Summary File 3U.S. Census Bureau, 2000 Summary File 3Maricopa County Assessor's OfficeMaricopa County Assessor's OfficeMaricopa C^ounty Assessor's Office

Maricopa County Assessor's OfficeAuthors' calculations, from Stefanov,Ramsey, & Christensen (2001)U.S. Census Bureau, 2000 Summary File 3Salt River Project

Maricopa County Assessor's OfficeDerived from Grossman-Clarke et al. (2005)

Derived from Grossman-Clarke et al. (2005)

compared to nearby nonurbanized areas, due to radiatedheat at night.

TriU't Population and Housing AttributesMedian Household income. The median income of the

households in a census tract is potentially an importantindicator of water use, especially if water is priced appro-priately according to usage. If water is a normal good itshould exhibit a positive income elasticity of demand,meaning water use should rise with income, leading us ropredict a positive coefficient on income. However, in thecase of Phoenix, it is difficult to predict the effect of incomebecause of the nonmarket pricing of water and becausesome of the effects of income may be captured indirectlythrough variables such as pool availability and lot size.

Median I'ersons per liiiit. Personal use of water forconsumption and hygiene contributes to the total demandfor water in single-family units. The hypothesis here isstraightforward: Increasing the median number of personsper household in single-family units in a tract results inhigher water demand, all else being constant.

Mean Lol Size. Upward of two-thirds of water con-sumption in single-family units is for outdoor uses. A

significant part of this water is used to maintain lawns,plants, and other vegetation. A larger lot would likelyinclude more vegetative cover, and, hence, is expected tohave a positive correlation with water use.

Mean Ape of Singio-Famiiy I'nits. The age of a housingunit often determines the vintage of its appliances andfixtures that use water. Newer appliances and fixtures aretechnologically more sophisticated and resource efficient.Also, newer equipment exhibits less wear and tear, result-ing in less loss of water through leakage. Hence, olderhouses can be expected to have higher water demand, allelse being the same.

Percentage ofSingie-Family Units wilh POOIK. Outdoorpools are common in Phoenix and require significantamounts of water to compensate for water lost throughevaporation. It is reasonable to expect a positive relationshipbetween water demand and the percent of single-familyunits with pools in a census tract.

Mean i'ooi Surface \rv». Besides the percentage of unitswith pools, larger pools would also contribute positively towater use. We hypothesize that pools with larger surfiiceareas exposed to hot summer temperatures will requiremore water to compensate For evaporative water loss.

Guhathakurta and Gobct: The Impact of the Phoenix Urban Heat Island on Residential Water Use 325

orSin^lo-i'aiiiily I'liits with l''va|i(tra(ive

s. Evaporative coolers are an efficient and less expen-sive alternative to air conditioning. The devices work whenthe dew point is below 50° F, which is usually the case inPhoenix in June, the month represented in our models.The cooling effect is achieved through evaporation of water.Hence, the use of evaporative coolers is hypothesized to bepositively related to water demand.

Vcftdalion Index.' The amount of vegetation as re-flected in lawn surfaces and density of plants and treesdirectly affects water demand in single-family units. Weexpected a higher vegetation index to be correlated withhigher levels of water use, since leafy plants require morewater than xeriscaping.

Percentage of Single-Family Units Ooeupied by Owners.The tenure of households has been used as a predictor ofhousing condition in many hedonic models. If we assumenonowners have less motivation to protect the physicalcondition of their housing units than do owners, for whomhousing is an investment, they would be less likely toundertake regular maintenance. Poor maintenance maylead to inefficient use of water. Further, landlords mayhave no incentive to install water-efficient appliances ifrenters are responsible for water bills. Therefore we hy-pothesize that owner-occupied housing will likely use lesswater, all else held constant.

VVaier Supply from K|{|*. Some older neighborhoods inPhoenix receive recycled water for outdoor use, whichwould replace water demand from municipal sources.Hence, it may be expected that a binary variable indicatingwhether the tract is located in SRP-supplied areas wouldhave a negative relationship with water demand frommunicipal sources (the source of our data on water use).

Mean Land Value. In a desert environment like Phoe-nix, people value watered, green landscapes. Thus, weexpect higher land values to be associated with higher ratesof water use.

KisiiUs

Table 2 presents the results for the first and secondmodels. The most important result is the significant andunique contribution of measures of the heat island effect towater use by single-family units in Phoenix, after control-ling for most known factors affecting water demand. Themodels explain more than 60% of the variation in wateruse, and the plot of residuals exhibited no significant bias.The collinearity statistics are also within normal limits(tolerance < 1 and variance inflation factor < 10), suggest-ing no significant mukicollinearity problems.

The two n:iodels each contain the same control varia-bles. The most important explanatory variables in the firstmodel are mean lot size, household income, and mean ageof housing. The most important explanatory variables inthe second model are lot size, the percentage of units withevaporative coolers, and mean pool surface area. As ex-pected, average lot size has the greatest impact on wateruse. Controlling for all other variables, each 1,000 squarefoot increase in average lot size increases water use byabout 1.8%. For example, if the average household livingin a single-family unit on a 10,428-square-foot lot andconsuming 17,025 gallons of water per month increasesthe lot by 1,000 square feet, the average water demand forthe unit will increase by approximately 307 gallons for themonth.

Overall, the variables related to housing were the mostimportant predictors of residential water uses. Houses withevaporative coolers use significantly more water than thosewithout such cooling systems. In the first model, each 1%increase in the percentage of single-family units withevaporative coolers adds 16% to that census tract's averagesingle-family water demand for the month. This increase is28% for the second model. Less notably, a 1% increase insingle-family units with pools in a census tract leads to anincrease of 0.2% (0.3% in the second model) in averagewater demand for all single-family units in the tract. Inaddition, if the average pool surface area increases by 1square foot, the average water demand grows by 0.1%, allelse remaining constant. Also as expected, older neighbor-hoods use more water than newer ones, and this result isstatistically significant.

The amount of vegetation does not exhibit a statisti-cally significant correlation with water use after accountingfor all other water demand factors. There are several factorslikely to have contributed to this result. First, we used avegetation index for the entire cetisus tract to impute thevegetative cover of single-family lots, which make up onlya portion of each tract. Second, NDVI index values rangefrom —1 to +1, although when vegetation is present (asopposed to other forms of land cover) the values rangefrom 0.1 to 0.7, with higher values indicating denservegetative cover. Phoenix data ranged only between 0.444and 0.623, with a standard deviation of 0.03. Third,studies have also shown that many Phoenicians over-watereven desert landscaping treatments, meaning that the typeof landscaping has little impact on residential water use(Martin, 2001). These three factors may partly explain whythe vegetation index did not have a statistically significanteffect on water demand in our model.

Population characteristics played only a minor role indetermining water use. Although household income was

326 Journal of the American Planning Association, Summer 2007, Vol. 73, No. 3

Table 2. Results of regression models prediaing the natural log of tract mean water use by single-family units, June 1998 (gallons).

Model 1 Model 2

B

7.348*.00000384*.026.0000179*.009*.002*.001*.159*.203.049.013.000000268.017*

Beta

.297

.110

.425

.377

.148

.171

.153

.018

.041

.021

.020

.110

/

11.182.80.98

6.546.372.173.722.480.410.560.460.222.07

R

8.958*,00000154.048*.0000140*.001*.003*.001*.339*.810.143.049"̂.000001420

Bela

.119

.203

.331

.219

.180

.228

.326

.073

.121

.083

.100

28.2421.0581.6684.5493.4932.3974.3465.1861.4981.5271.6651.013

ConstantMedian bousehoid income ($)Median persons per unitMean lot size (square feet)Mean age of single-family units (years)% of single-family units with poolMean pool surface area (square feet)% of single-family units with evaporative coolersVegetation Index (NDVI)% housing units owner occupiedWater supply from SRP (0 - no, 1 = yes)Mean land parcel price, single-family residences (S)Daily low temperature ("F)Difference between high and low temperatures (°F) -.040* -.166 -3.901

0.68287

0.62287

7<.1O ><.O5

significant in the first model, its coefficient was small,showing that demand for water is relatively inelastic withrespect to income. Household size was insignificant,showing the relative unimponance of interior uses ofwater compared to outdoor uses in single-family homes inPhoenix.

Finally, the hypothesis that the UHI affects water useis sustained. Mean low temperature, the first measure ofUHI effects, has the hypothesized positive sign, and ishighly significant and fairly large. A 1° F increase in a tract'slow temperature increases average water use in single-familyunits by 1.7%, or 290 gallons for the typical single familyunit for the month, holding all else constant. The differ-ence between daily high and low temperatures, the secondmeasure of UHI, resulted in greater changes in water use.If the difference between high and low temperature de-clines by 1° F, reflecting warmer nighttime temperatures,average water use in single-family units increases by 681gallons.

Planninjr ImplicationsThe literature provides convincing evidence of rural-

urban differences in temperature variation as a function ofpopulation and infrastructure growth and land cover

changes (Brazel et al., 2000; EUasson, 2000; Landsberg,1981; Stone, 2004). Temperature differences of about 11°F have been reported between parks and built-up areas ofthe city (Spronken-Smith & Oke, 1998). In addition,street geometry and the sky view-factor (SVF) have beenshown to be highly correlated to spatial variations in urbanheat islands in different cities (Oke, 1981). Another im-portant contributor to heat island intensity is urban con-struction materials' capacities to store heat (Oke. Johnson,Steyn, & Watson, 1991; Stone, 2004). Other manmadesources of urban heat include waste heat released fron:automobile emissions, smokestacks, and air conditioningsystems.

Our findings suggest a previously unidentified tradeoffbetween higher and lower urban densities. The results ofour empirical analysis indicate chat four factors whichmight be affected by planning and design decisions sig-nificantly influence household water use: (1) lot size, (2)presence and size of pools, (3) use of evaporative coolers, and(4) urban heat island effects. Thus, for example, zonitigregulations might consider the intensity of the heat islandeffect in specifying lot size for a partictilar area. Similarly,outdoor pools might be regulated in areas impacted byheat island effects. Such measures would complementother strategies to mitigate heat island effects. Althoughour analysis did not account for multi-family housing or

Guhathakurta and Gober; The Impact of the Phoenix Urban Heat Island on Residential Water Use 327

mixed uses, we expect heat island effects to affect water usein these environments as well.

The environmental consequences of a warmer environ-ment for water demand are not trivial. Based on the resultsof this study, that a 1" F increase in temperature results ina 290-gallon increase in water use per single-family house-hold in the month of June, the city's 322,828 single-familyhousing units in 2005 (U.S. Census Bureau, 2007) used anadditional 94 million gallons of water in the month ofJune as a result of the UHI. This was about 0.8% of thecity's overall June water use in 2005- An increase of 5° F inlow temperatures would raise water usage by 9% or 1,532gallons in one month for the average single-family home.Brazel et al. (2007) showed that adding only about 2500homes (at a density of 3 or more units per acre) will raisedaily low temperatures locally by 5° F-

This study suggests that the consequences of morecompact development are more complex than generallyassumed, at least in a desert city like Phoenix. Efforts tomodel the pros and cons of compact development mustconsider both the water savings from smaller lots and morevertical development, and the increased water consumptionresulting from the UHI.

So what can be done to plan cities that are energy,water, and thermally efficient? We know that complexrelationships among the various aspects of sustainable citydesign sometimes work at cross purposes. As noted earlier,increasing densities may make travel more energy efficient,but reduce thermal efficiency and increase water use. Thetradeoffs between water use and energy efficiency are alsohighlighted by the high water intensity of evaporativecoolers that are otherwise energy efficient. We need moreexperimentation and empirical analyses to obtain reliable,environmentally efficient, yet highly urbanized, city design.Such experimental research is critical given the increasingpace of urbanization around the world.

Despite our incomplete knowledge, some aspects ofplanning offer promise of sustainable city design. Theseinclude:

1. city form and structure, including aspects of road-way alignment, vertical to horizontal ratios forbuildings, building setbacks, and the design ofspaces between buildings;

2. use of materials, especially with respect to life cyclecosts, thermal efficiency, reflectivity, and porosity ofmaterials used in the built environment;

3. land cover. Including the type and amount ofnatural cover in relation to the built forms and othernatural endowments (such as water availability);and

4. geographic and cultural context of place, in order tocapitalize on local natural and cultural endowments(such as alternative energy sources) and to conserverelatively scarce resources (like fresh water in thesouthwestern United States).

A detailed set of guidelines for design of urban struc-tures and urban spaces could stunt creative solutions, andmake urban environments banal. Instead, we recommendadopting performance-based criteria, and offering incen-tives for designs that achieve higher levels of energy, ther-mal, water, and overall lifecycle efficiencies, and are suitedto the local geographic and cultural context. Such criteriashould offer standardized measures to developers andbuilders for assessing building performance. The Leader-ship in Energy and Environmental Design (LEED) GreenBuilding Rating System offers a good starting point fordeveloping performance criteria that could be adjusted tothe local context. More importantly, such performancecriteria should be incorporated into the plan approvalprocess by offering higher incentives for achieving highermeasures of environmental performance. Adoptingsuch incentives should go a long way toward fosteringsustainable urban environments.

Conclusions

This study demonstrates that the elevated low tempera-tures resulting from Phoenix's UHI contribute significantlyto water use in single-family homes, with economic andlong-term sustainability consequences. Mitigating the heatisland impacts in Phoenix would potentially allow thecity to accommodate the water needs of thousands morehouseholds than otherwise.

Planning research and practice should address ways tocombat waste heat in dense urban areas and consider thefeedbacks between climate, water use. and the built envi-ronment. The strategies for mitigating heat island effectshave been known for many years, but are little used, sinceair is a common resource, subject to "the tragedy of thecommons" (Hardin, 1968). If heat were a regulated pollu-tant, as advised by some scholars, including Stone (2004),it would be within the jurisdiction of air quality manage-ment districts. As we have shown, this would help conservewater, which is extremely important in arid regions such asthe Phoenix metropolitan area.

328 Journal of the American Planning Association, Summet 2007, Vol. 73, No. 3

AcknowledgmentsThis research was supported by the National Science Foundation underGrant No. SES-0345945 Decision Center for a Desert City (DCDC).All opinions, findings, conclusions, and recommendations expressed inthis article are those of the authors and do not necessarily reflect theviews of the National Science Foundation.

Notes1. By contrast, the empirical work of Stone and Rogers (2001) foundthat lower density urban residential development in Atlanta producedmore radiant heat energy per household than higher-density designs.2. Recognizing the possible impacts ot heat islands on energy and airquality in cities, the Environmental Protection Agency (EPA) estab-lished a Heat Island Reduction Initiative to quantify benefits of heatisland reduction (HIR) strategies. As part of this effort, the Heat IslandGroup at Lawrence Berkeley National Laboratory calculated potentialannual energy savings, peak power avoidance, and annual carbon-dioxidereduction resulting trom heat island reduction strategies in three initialcities: Baton Rouge, Sacramento, and Salt Lake City (Konopacki &Akbari, 2000). The results of the study show that potential annualenergy savings from direct and indirect HIR strategies would he aboutSi 5 million in Baton Rouge, $26 million in Sacramento, and $4million in Salt Lake City.

3. The simulation produced values for air temperature two meters abovethe surface hased on a modified fifi:h-generation Pennsylvania StateUniversity/National Center for Atmospheric Research MesoscaleModel. The researchers further modified the model, adding a new landcover classification derived from 1998 Landsat Thematic Mappersatellite images and from a detailed ground surface survey. They alsoimproved results significantly by adding radiation trapping, heat storage,and manmade sources of heating to che model. They performed their72-hoiir simulation during June 1998, producing results at the scale ofcells in a 2-km hy 2-km grid.

4. We created an adjusted NDVI as a tract-level measure of vegetativecover based on spectral signatures in Landsat imagery. The tract-levelmeasure was hased on the classification of land cover originally under-taken by Stefanov, Ramsey, and Christensen (2001).

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