Landscape Ecol (2016) 31:745–760 DOI 10.1007/s10980-015-0284-3 RESEARCH ARTICLE Micro-scale urban surface temperatures are related to land-cover features and residential heat related health impacts in Phoenix, AZ USA G. Darrel Jenerette . Sharon L. Harlan . Alexander Buyantuev . William L. Stefanov . Juan Declet-Barreto . Benjamin L. Ruddell . Soe Win Myint . Shai Kaplan . Xiaoxiao Li Received: 10 September 2014 / Accepted: 28 September 2015 / Published online: 19 October 2015 Ó Springer Science+Business Media Dordrecht 2015 Abstract Context With rapidly expanding urban regions, the effects of land cover changes on urban surface temperatures and the consequences of these changes for human health are becoming progressively larger problems. Objectives We investigated residential parcel and neighborhood scale variations in urban land surface temperature, land cover, and residents’ perceptions of landscapes and heat illnesses in the subtropical desert city of Phoenix, AZ USA. Methods We conducted an airborne imaging campaign that acquired high resolution urban land surface temperature data (7 m/pixel) during the day and night. We performed a geographic overlay of these data with high resolution land cover maps, parcel boundaries, neighborhood boundaries, and a household survey. Results Land cover composition, including percentages of vegetated, building, and road areas, and values for NDVI, and albedo, was correlated with residential parcel surface temperatures and the effects differed between day and night. Vegetation was more effective at cooling hotter neighborhoods. We found G. D. Jenerette (&) Department of Botany and Plant Sciences, University of California Riverside, Riverside, CA, USA e-mail: darrel.jenerette@ucr.edu B. L. Ruddell Fulton Schools of Engineering, Arizona State University, Tempe, AZ, USA S. L. Harlan  J. Declet-Barreto School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA A. Buyantuev Department of Geography and Planning, University at Albany, State University of New York, Albany, NY, USA W. L. Stefanov Astromaterials Research and Exploration Science Division, Exploration Integration and Science Directorate, NASA Lyndon B. Johnson Space Center, Houston, TX, USA S. W. Myint School of Geographical Sciences and Urban Planning, Arizona State University, Tempe, AZ, USA S. Kaplan The Jacob Blaustein Institutes for Desert Research, BenGurion University of the Negev, Beersheba, Israel X. Li Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, Tempe, AZ, USA J. Declet-Barreto Natural Resources Defense Council, Washington, DC, USA 123 746 consistencies between heat risk factors in neighborhood environments and residents’ perceptions of these factors. Symptoms of heat-related illness were correlated with parcel scale surface temperature patterns during the daytime but no corresponding relationship was observed with nighttime surface temperatures. Conclusions Residents’ experiences of heat vulnerability were related to the daytime land surface thermal environment, which is influenced by micro-scale variation in land cover composition. These results provide a first look at parcel-scale causes and consequences of urban surface temperature variation and provide a critically needed perspective on heat vulnerability assessment studies conducted at much coarser scales. Keywords Urban heat island  Parcel  MASTER  Land surface temperature  Social surveys  Vulnerability Introduction Urbanization is associated with increasing air temperatures in cities (Oke 1973), and it also leads to land surface warming and increasing land surface temperature (LST) heterogeneity (Voogt and Oke 2003; Jenerette et al. 2007; Cao et al. 2010; Imhoff et al. 2010; Peng et al. 2012; Zhou et al. 2014). Studies of urban LST have primarily evaluated midday spatial variation at neighborhood or coarser scales, such as the city, zip code, and census tract boundaries. However, extensive scale-dependent variation in LST is expected throughout any given day due to different speeds of warming within the complex spatial structure of urban environments (Li et al. 2011; Weng et al. 2011). Furthermore, the human consequences of urban LST warming are not well characterized at very fine spatial resolutions, such as individual residential parcels, although several studies have found LST variation affects spatial variability in the risk of heatrelated mortality at neighborhood scales (Johnson et al. 2009; Buscail et al. 2012; Hondula et al. 2012; Johnson et al. 2012; Harlan et al. 2013). LST spatial variation is influenced by material properties such as heat capacity, thermal conductivity, and thermal inertia (Elachi 1987), and several studies have proposed that local land cover composition is a primary determinant of LST patterns (Li et al. 2011; 123 Landscape Ecol (2016) 31:745–760 Connors et al. 2013; Zhou et al. 2014). Increasing vegetated land cover has been repeatedly shown to lower LST through radiation interception and shading and increases in latent heat fluxes associated with transpiration (Jenerette et al. 2007; Kalma et al. 2008; Kustas and Anderson 2009; Jenerette et al. 2011; Xiang et al. 2014; Zhou et al. 2014). In contrast, built surfaces, including buildings and roads, are generally warmer than natural surfaces (Buyantuyev and Wu 2010; Zhou et al. 2014). Several studies have shown that measurement scale affects the amount of observed variability in land cover—LST relationships (Liang and Weng 2008; Weng et al. 2011; Song et al. 2014), but relationships between urban land cover and LST have almost exclusively been evaluated using satellite-based thermal sensors on Landsat ETM? (60 m pixels) or ASTER (90 m pixels) instruments (e.g. Li et al. 2011; Myint et al. 2013; Song et al. 2014). At these resolutions, extensive mixing of land covers may obscure stronger or different relationships at finer resolutions where more urban land cover variability occurs (Zhou et al. 2011). An evaluation of urban LST variation at micro scales, such as \10 m, has been noted as an important research goal for improved understanding of the biophysical causes urban heat vulnerability (Small 2003; Deng and Wu 2013; Zhan et al. 2013; Song et al. 2014). In addition to improved spatial resolution, extending LST measurements to both daytime and nighttime allows for analysis of the differences in causes and consequences of spatial patterns of daily maximum and minimum urban LST (Buyantuyev and Wu 2010; Myint et al. 2013). Because vegetation and high albedo surfaces are cooler during the day and built surfaces are warmer, these trends may continue at night due to differences in stored heat (Buyantuyev and Wu 2010; Myint et al. 2013). Alternatively, trees may trap infrared radiation at night and lead to increased surface warming. Although the upward facing built materials are warmer during the day, they may more effectively radiate heat at night, leading to lower nighttime LST while still radiating heat horizontally. Albedo, as a property associated with an illuminated surface, may be unrelated to nighttime LST. Improved understanding of the biophysical drivers of urban LST will require reconciling the differences in day and night LST variation. Characterizing the biophysical basis of LST spatial and temporal variations and connecting LST with residents’ lived experiences should help us to better Landscape Ecol (2016) 31:745–760 understand the determinants of variability in heat vulnerability and implement better climate adaptation strategies for cities. In the large literature that examines air temperature and heat-related mortality, there is considerable discussion and debate about the relative importance of daily and lagged minimum, mean, and maximum temperature effects on health outcome (Kalkstein 1991; Basu and Samet 2002; Hondula and Barnett 2014; Petitti et al. 2015). Studies in different places and weather conditions have reached different conclusions. The important question about the effects of minimum and maximum temperatures has rarely been evaluated using urban LST because there have been many fewer studies that consider LST and most of them exclusively used either daytime (e.g. Harlan et al. 2013) or nighttime data (Uejio et al. 2011). One exception was Laaidi et al. (2012), who examined minimum, mean, and maximum surface temperatures, and found that minimum temperature was associated with heat deaths among the elderly during a heat wave in Paris. As these authors pointed out, variability in surface temperatures measured over the extent of the city more closely approximates the actual living conditions of urban residents than air temperature measured at weather stations that are not in close proximity to people (Laaidi et al. 2012). In this study, we present new results from high resolution (7 m) airborne thermal imagery acquired over the Phoenix, AZ USA metropolitan region. We link this high resolution thermal data with social survey data on perceptions of landscapes and heat illnesses that were collected during the same year. We use the geographic intersection of these data in conjunction with a sub-meter land cover classification and residential parcel boundaries to answer two questions: (1) How does landscape composition influence LST variability during the day and night at parcel and neighborhood scales? (2) Do parcel and neighborhood surface characteristics correspond to residents’ perceptions of their local landscapes and self-reported symptoms of heat-related illnesses? Answering these questions will improve understanding of how land-cover patterns influence LST in residential environments and the potential consequences of LST for residents. These are essential pieces of the puzzle for designing neighborhoods that reduce human vulnerability to increasing urban temperatures. 747 Methods Site description The Phoenix, AZ USA metropolitan region (population 4.3 million) is a well-studied model system that has been useful for learning about many aspects of urban ecology and climate (Grimm and Redman 2004; Chow et al. 2012). This metropolis has a hot, subtropical desert climate (Koppen classification BWh) and features substantial variation in vegetation and built surfaces associated with residential dwellings. Phoenix is an extreme endpoint of the urban heat spectrum within the United States (Stone 2012). The region has a pronounced nighttime urban heat island (UHI) (Chow et al. 2012), measured in air temperature and LST, and is home to one of the fastest growing UHIs within the United States (Stone 2012). Overall, and consistent with cities throughout the southwestern United States, the city has more vegetation than the outlying desert and higher-income neighborhoods (census tracts) have more vegetation than lowerincome neighborhoods (Jenerette et al. 2013). Vegetation, in turn, is correlated with LST and higherincome neighborhoods are cooler (Jenerette et al. 2007; Buyantuyev and Wu 2010; Jenerette et al. 2011). Microscale LST: MASTER flight and data processing We conducted an airborne LST data collection campaign over the Phoenix metropolitan area during a 5-day period in 2011: July 12–13 (daytime flights collected from 10 am to 1 pm local time; -7 from UTC) and July 15–16 (nighttime flights collected from 12:30–3 am local time). These times were selected to be representative of mid-day and mid-night periods outside the main transitions associated with sunrise or sunset. For this campaign the MODIS/ASTER Simulator, or MASTER, instrument was mounted on a Beechcraft B-200 aircraft and flown to obtain high spatial resolution data. The MASTER sensor acquires data over the visible through mid-infrared wavelengths (0.46–12.817 lm) in 50 spectral bands (Hook et al. 2001), which were used to derive LST, normalized difference vegetation index (NDVI), and albedo at approximately 7 m/pixel. These imagery data measure a combination of the top of urban canopy 123 748 and ground data, which may be distinct from temperature patterns only at the ground (Goldreich 2006). Although the original intent was to collect a true diurnal dataset, this was not possible due to the large area of data collection and pilot flight rules that constrain daily flying time and mandate stand-down periods for flight safety. Fortuitously, a stable weather pattern prevailed during the week of July 12 that enabled collection of an approximate diurnal dataset. During this period, mean daily maximum air temperature was 40.1 °C and differed less than 1.6 °C between sampling days. We performed atmospheric correction to obtain apparent surface reflectance, temperature, and emissivity, which allows for comparison of the flight line data collected on four different days. The ENVI/IDL image processing environment was used to perform all processing of the MASTER data. Atmospheric correction of the mid-infrared wavelength data was accomplished using an in-scene atmospheric compensation technique (Johnson and Young 1998). We used an emissivity normalization approach (Kealy and Hook 1993) to obtain LST from the MASTER data—an emissivity value of 0.98 was used to calculate the temperature in each band of the MASTER thermal IR data for each pixel, and then the highest temperature obtained was used to create the LST layer. That temperature was then used to invert the Planck function to calculate emissivity in each of the bands. Atmospheric correction of the visible through shortwave infrared wavelength data was accomplished using the quick atmospheric correction (QUAC) algorithm (Bernstein et al. 2005) within ENVI/IDL. Reasonableness of both LST and apparent reflectance results were assessed qualitatively by random examination of returned pixel values (surface temperature) and image spectra (vegetation and soil pixel spectra were compared to laboratory spectra of similar materials). In addition to LST we also processed MASTER data to obtain the NDVI, a measure of vegetation (Tucker 1979), and an estimate of surface albedo. NDVI was calculated as follows: NDVI ¼ ðBand9  Band5Þ=ðBand9 þ Band5Þ where Band9 and Band5 correspond to MASTER spectral channels 9 (0.87 lm) and 5 (0.66 lm). Albedo, the ratio of upwelling to downwelling radiative flux at the surface, was estimated by converting 123 Landscape Ecol (2016) 31:745–760 narrowband to broadband albedo using an empirical formula previously developed for the MODIS sensor (Liang 2001). Specifically, we derived the total visible broadband albedo (avis) as follows avis ¼ 0:331a5 þ 0:424a1 þ 0:246a3 where a1, a3, and a5 are reflectances in MASTER spectral channels 1 (0.46 lm), 3 (0.54 lm), and 5 (0.66 lm), respectively. Phoenix Area Social Survey The 2011 Phoenix Area Social Survey (PASS) was the second wave of a household survey conducted every 5 years in the metropolitan area to study humanenvironment interactions in an urban setting. It is part of the long-term monitoring conducted by the Central Arizona—Phoenix Long-Term Ecological Research (CAP LTER) project (http://caplter.asu.edu/research/ long-term-monitoring/). A two-stage sampling design was used for PASS. First, the sample of PASS neighborhoods was drawn mostly from long-term ecological monitoring sites that are part of the CAP LTER project (Grimm and Redman 2004). U.S. Census Bureau block groups (i.e., relatively homogeneous populations living within an area of approximately 0.65 km2) were used to define boundaries of PASS neighborhoods. We selected a sample of 45 PASS neighborhoods (39 from long-term monitoring sites and six other CAP LTER research sites) stratified by location (urban, suburban, fringe) and median household income. Percent retired and percent minority residents were also considered in the selection to ensure that different types of residential situations were represented (Table 1). Second, the sampling frame for households within each neighborhood was a list of eligible residential addresses created from maps and the tax assessor’s parcel numbers (APN), which included single family and multi-family houses, apartment buildings, and mobile home parks. A random sample of 40 households was selected within each neighborhood. Respondents were offered a small financial incentive for participation and they could complete the survey online, by telephone, or in person with an interviewer. All materials were available in English and Spanish and repeated follow-up efforts with non-respondents were on-going through the study period. A total of 806 Landscape Ecol (2016) 31:745–760 749 Table 1 Stratification of the 2011 Phoenix Area Social Survey neighborhoods and survey respondents Table 2 Responses of participants to questions in the 2011 Phoenix Area Social Survey (n = 806) Neighborhood classification Survey question Neighborhoods (n) Respondents (n) Low income, urban core 6 97 Low income, suburban 6 110 Low income, fringe 2 34 Subtotal low income 14 241 Middle income, urban core 6 105 Middle income, suburban 6 111 Middle income, fringe 7 136 Subtotal middle income 19 352 High income, urban core 1 20 High income, suburban 5 94 High income, fringe Subtotal high income 6 12 99 213 Grand total 45 806 Median annual household income for each neighborhood was obtained from 2000 U.S. Census block group data and classified as follows: low income (\$35,000); middle income ($35,000–$70,000); high income ([$70,000). Urban core neighborhoods were within 8 km of downtown Phoenix or within 2.4 km of the 7 other large-city downtowns. Fringe areas had a moderate amount of undeveloped land within 1.6 km of the neighborhood in 2005. All other neighborhoods were classified as suburban. At least 30 % of residents in five neighborhoods were 65 years or older in 2010 (retirement communities) and at least 50 % of residents in ten neighborhoods were non-Hispanic white (Hispanic, African American, Native American, or Asian/Pacific Islander) in 2010 respondents in 45 neighborhoods, ages 18 or older (one per household), completed the survey for a minimum response rate of 43.4 % using the standard definitions of the American Association for Public Opinion Research (2008). All surveys were completed between May 26 and December 31, 2011. The survey asked respondents’ opinions about quality of life and condition of the natural environment in their neighborhoods and in the region. Questions about land use and climate were two areas of emphasis. We selected three questions from PASS as indicators of residents’ perceptions of environmental conditions in their neighborhoods and one question on self-reported heat illnesses (Table 2). The question about experiencing heat-related symptoms was asked in the survey context of 10 other questions about heat and the symptoms we included in the question (Table 2) were excerpted from the medical conditions directly attributable to excessive heat exposure Response % n During the summer of 2010, do you think your neighborhood was (_) than most other neighborhoods in the Valley? A lot cooler 0.9 7 A little cooler 14 117 About the same temperature 22 174 A little hotter 9.4 76 A lot hotter 45 366 Are you (_) with trees that provide shade in your neighborhood? Very satisfied 27 215 Somewhat satisfied 36 292 Somewhat dissatisfied 22 178 Very dissatisfied 13 106 Please indicate whether having too many paved surfaces such as roads or parking lots is a (_) in your neighborhood Big problem 5.8 47 Little problem 18 142 Not a problem at all 74 594 During last summer, did anyone in your household have symptoms related to heat or high temperatures such as leg cramps, dry mouth, dizziness, fatigue, fainting, rapid heartbeat or hallucinations? Yes 30 245 No 60 486 Do any of the following prevent you from using your airconditioning when the weather is hot? Please select all that apply Cost of electricity 36 291 Unit doesn’t work 5.3 43 Cost of repairs 4.1 33 Noise 1.0 8 Do not have an air conditioner 3.6 29 Other 1.9 15 Summary: had at least one impediment 45.3 365 Missing and ‘‘don’t know’’ answers were omitted from calculations on an item-by-item basis and 100 % was based on number of valid answers for each question, or in the case of the question on impediments to using air conditioning, valid answers for each item. (For the question on use of air conditioning, respondents could select multiple answers). Responses to the question on neighborhood temperature were coded so that higher scores indicate hotter neighborhood relative to others. Responses to the question on satisfaction with trees were coded so that higher scores indicate higher satisfaction. Responses to the question on too much pavement were coded so that higher scores indicate a smaller problem (higher satisfaction). Subsamples of respondents in neighborhoods that correspond to useable imagery from the daytime and nighttime MASTER land cover dataset (n = 640 and 695, respectively) were used in the analyses (see ‘‘Geoprocessing: MASTER georectification and data overlays’’ section) 123 750 published by the United States Environmental Protection Agency (2006, p. 11). Self-rated health questions are widely used in the health sciences and other fields because, over decades of research, they have been found to be strong correlates of clinical conditions (Lee 2015) and have been used in assessments of heat vulnerability (Belanger et al. 2015). In evaluating the effect of LST on respondents’ reported experiences with heat illness, we used a question about economic impediments to home air conditioning. Regulating indoor temperatures with air conditioning is an important method of coping with extreme outdoor temperatures in Phoenix and inability to access this coping mechanism could increase the likelihood of heat illness. We tested for a potentially confounding effect of ambient outdoor temperature on respondents’ answers to the heat illness question by evaluating the influence of maximum air temperature (obtained from Sky Harbor Airport) on the survey response date. The Arizona State University Institutional Review Board approved the use of PASS data for this study. Geoprocessing: MASTER georectification and data overlays For this study, MASTER data were extracted for 41 (out of the total 45) PASS neighborhoods within the flight boundary. The atmospherically corrected MASTER data were processed to at-sensor radiance but did not include geolocation data. We georeferenced each neighborhood to align with the corresponding National Agriculture Imagery Program (NAIP) data. This process included visually locating and matching feature points in the MASTER image and the base NAIP image. These feature points were used to create a polynomial geometric transformation model that aligned the MASTER image to the same coordinate system of the base image. In addition we incorporated a nearest neighbor resampling so that corresponding pixels represent the same objects. For each neighborhood, a different number of matching points was used depending on its size and level of distortion in the MASTER data. All neighborhoods were georeferenced to a target registration error of no more than 0.5 pixels. In cases where severe distortions of the original MASTER data did not allow registration to this level of accuracy, registration to the lowest possible error was performed. In four neighborhoods the daytime 123 Landscape Ecol (2016) 31:745–760 thermal images were too distorted for acceptable georeferencing; these highly distorted images were not included in further analyses. Registration of the higher resolution visible reflectance data precluded the use of two more neighborhoods for NDVI or albedo data. The final processed MASTER dataset included high spatial resolution LST, NDVI, albedo, and direct estimates of emissivity that were atmospherically corrected, and georeferencing (Fig. 1). We linked the MASTER and PASS data with a high resolution (1 m) land cover classification for the Phoenix metropolitan region, which had a classification accuracy of 92 % and a kappa statistic of 0.91 (Li et al. 2014). This classification was conducted using NAIP aerial photography and following an objectoriented expert rule algorithm (Baatz et al. 2008), which is becoming increasingly common for urban regions (Myint et al. 2011; Grove et al. 2014). Preliminary analyses showed that 2.8 % of parcels had no observed built land cover. The presence of trees above the built surface may have obscured buildings and there may also have been errors in the NAIP data or land cover classification. We overlaid the land cover classification and MASTER data with parcel boundaries obtained from the Maricopa County Tax Assessors office and extracted all residential plots (Fig. 2). A complete geographic overlay of land cover, LST, and parcel boundaries for 37 PASS neighborhoods during the day (n parcels = 21,589 and n survey respondents = 640) and 41 neighborhoods at night (n parcels = 25,011 and n survey respondents = 695) was available for our analyses. Image acquisition occurred over approximately a 3-h flight period and therefore we evaluated the potential for LST to increase during daytime sampling and decrease during nighttime sampling. We compared means across all parcels in maximum, minimum, and mean LST. During the day we only observed increasing LST in the minimum parcel temperatures and, therefore, no correction was needed. At night, we observed a significant cooling in the maximum, minimum, and mean parcel temperatures over the flight periods. We used linear regression to identify the rate of cooling and then corrected all parcel temperatures to the median sampling time (approximately 1 am local time). Subsequent analyses conducted using both time-corrected and uncorrected data had consistent results for correlation analyses and significance tests. Landscape Ecol (2016) 31:745–760 751 Fig. 1 Representative MASTER LST measurements for a highly vegetated neighborhood (upper) and minimally vegetated neighborhood (lower). MASTER bands 9, 5, and 3 were used for RGB display shown as false-color composites on the left Analysis We used a suite of standard statistical tools including correlations, linear regressions, and generalized liner models (GLM) for data analysis; all statistical analyses were conducted in Matlab 2013 (The Mathworks, Natick, MA). For each individual residential parcel we determined the means of LST, NDVI, and albedo and proportions of each land cover type. We analyzed the relationships between LST and the proportions of tree, grass, built, and road area at the parcel scale using a non-spatial linear correlation analysis. Because there are many thousands of parcels, all results at the parcel scale were statistically significant, consistent with most ‘‘big-data’’ analyses and, therefore, we focused on effect sizes in these analyses (Nuzzo 2014). We also conducted analyses that compared parcel variation in LST between different neighborhoods. We computed regressions between mean parcel LST and NDVI within each single neighborhood in order to compare differences in vegetated cooling between neighborhoods. We compared expected bare surface temperatures (i.e. the intercepts of the linear regressions) with the LST-cooling effectiveness of vegetation (i.e. the slopes of the NDVI-LST regressions). This analysis showed the relative sensitivity of LST to vegetation abundance across neighborhoods. We examined survey responses at the parcel scale and compared mean parcel LST for the group of respondents who reported symptoms of heat illness with the group reporting no symptoms. To account for differences in capacity to cope with heat, we restricted some analyses to the subsample of respondents who reported an impediment to using home air conditioning. These analyses were conducted using GLM for binomial distributions. At the neighborhood scale we used regression to evaluate the effects of mean neighborhood LST and land cover characteristics on aggregate measures (means) of respondents’ perceptions of the local landscape and experiences of heat illness symptoms. Results Parcel land cover and LST relationships Mean LST of individual residential parcels varied tremendously: daytime parcels ranged between 35.1 123 752 Landscape Ecol (2016) 31:745–760 Fig. 2 Representative overlay of land cover data, parcel boundaries, and MASTER LST measurements for a highly vegetated neighborhood. The upper left and upper right show day and night time LST (°C) and parcel boundaries. The lower left shows land-cover classification and parcel boundaries. The lower right shows a detail subset of the nighttime LST and parcel delineations and 67.4 °C and nighttime parcels ranged between 15.1 and 32.1 °C. At the parcel scale, some measures of residential land cover were correlated with LST. During the daytime (Fig. 3), parcel-scale mean LST was inversely correlated with vegetation density. This finding was supported across three different measures of vegetation density: mean NDVI and the proportions of tree or grass cover. Tree cover had the highest correlation with LST (r = -0.44) and grass cover had the lowest correlation (r = -0.27). Built surface area, including roads, and albedo were weakly positively associated with mean daytime parcel LST. At night (Fig. 4) many of the relationships were reversed. Vegetation, quantified as NDVI, tree area, or grass area, was weakly positively associated with nighttime LST. As in the daytime, tree area had the highest of these correlations at night, although the relationship 123 Landscape Ecol (2016) 31:745–760 753 65 65 35 r = -0.34 55 55 45 45 35 r = 0.05 r = -0.09 25 0.4 0.8 35 0 0.3 0.6 Albedo NDVI 65 65 r = -0.27 r = -0.44 55 55 45 45 35 0 50 100 35 0 50 100 Grass Area (%) Tree Area (%) 65 65 55 45 45 15 0 0.4 0.8 r = 0.21 50 Built Area (%) 100 35 0 50 0.3 35 35 r = 0.07 r = 0.20 25 25 15 0 0.6 Albedo 50 100 15 0 50 100 Grass Area (%) Tree Area (%) 35 r = 0.06 r = -0.36 25 25 35 0 15 0 NDVI 35 r = 0.10 55 Night Land Surface Temperature (C) Day Land Surface Temperature (C) 25 35 0 r = -0.23 100 Road (%) Fig. 3 Parcel scale relationships between land cover characteristics and daytime land surface temperature. Pearson correlation coefficients are shown for all relationships with LST was much weaker at night than in the day (r = 0.20). Larger areas of built surfaces and higher albedo were associated with cooler residential parcel LST at night (r = -0.36 and -0.23, respectively). Roads were minimally positively associated with nighttime LST. For all of these daytime and nighttime relationships there were large amounts of unexplained variation in mean LST at the parcel scale. Regressions of parcel daytime LST on NDVI calculated independently for each neighborhood revealed that vegetation had a significant cooling effect within all neighborhoods. The magnitude of these effects, however, varied tremendously between neighborhoods. The slope magnitude between LST and NDVI in Fig. 5 is a quantification of the cooling effectiveness for differences in NDVI between the neighborhoods. Notably, the slope was strongly related to the regression intercept for each neighborhood, such that vegetation was more effective at cooling hotter neighborhoods (p \ 0.001; R2 = 0.54). 15 0 50 Built Area (%) 100 15 0 50 100 Road Area(%) Fig. 4 Parcel scale relationships between land cover characteristics and nighttime land surface temperature. Pearson correlation coefficients are shown for all relationships Neighborhood land cover, LST and household income Consistent with previous findings in the Phoenix region, more impoverished neighborhoods were hotter (r = 0.47, p \ 0.01; proportion of neighborhood households living below the federal poverty line and mean residential parcel daytime LST) and had less vegetation (r = -0.45; proportion of households in poverty and proportional cover of both grass and trees in parcels). Mean neighborhood daytime LST was inversely correlated with vegetation (r = -0.37, p \ 0.01; proportion of neighborhood tree and grass area with mean daytime LST). Micro-scale environments, residents’ perceptions, and symptoms of heat illness Mean daytime LST in the PASS neighborhoods ranged from 40 to 55 °C, tree area varied from 1.2 to 26.8 %, and road area varied from 3.7 to 53 %. Survey 123 754 Landscape Ecol (2016) 31:745–760 Vegetation Cooling Effect (C) 5 -5 -15 -25 -35 46 50 54 58 62 Bare Temperature (C) Fig. 5 Neighborhood variation in average parcel-scale daytime vegetative cooling (quantified as the regression slope between NDVI and LST) effect in relation to projected bare surface temperature (regression p \ 0.001, R2 = 0.54). Each datum represents a single neighborhood respondents also varied widely in their perceptions of neighborhood environmental conditions and they used the full spectrum of available answers for the survey questions (Table 2). Residents’ perceptions of temperature and land cover correlated with empirical observations of these characteristics (Fig. 6). Residents’ mean perception of neighborhood temperature relative to other neighborhoods on a five-point scale (higher values indicate a hotter neighborhood) was higher in neighborhoods with higher daytime LST (p = 0.002, R2 = 0.23). No relationship was detected between perception of relative neighborhood temperature and nighttime LST (p [ 0.3). Perceptions of drivers of LST were also consistent with observed data. Residents’ mean satisfaction with shade trees in their neighborhood on a four-point scale (higher values indicate more satisfaction) was positively related to percentage of neighborhood area covered by trees (p = 0.006, R2 = 0.18). Residents’ mean perception of the magnitude of paved surfaces as a neighborhood problem on a three-point scale (higher values indicate smaller problem, thus higher satisfaction) was negatively related to road area (p = 0.002, R2 = 0.23). The frequency of self-reported heat- related symptoms was correlated with daytime LST at both the parcel and neighborhood scales. The mean parcel daytime LST was significantly higher (0.93 °C, p \ 0.001) for respondents who reported a heat- 123 Fig. 6 PASS respondents’ perceptions of their neighborhoods and neighborhood conditions observed by MASTER. Upper panel shows mean perception of relative neighborhood temperature and observed daytime neighborhood mean temperature (p = 0.002; R2 = 0.23). Middle panel shows mean satisfaction with neighborhood trees and neighborhood tree area (p \ 0.006; R2 = 0.18). Lower panel shows mean satisfaction with amount of roads and paved area (p = 0.002; R2 = 0.23). For all panels the scale of y-axis represents range of possible survey answers. Each datum represents a single neighborhood related illness in their household than for those reporting no heat related illnesses (Table 3). Furthermore, within the subgroup of respondents who had impediments to using home air conditioning, the parcel daytime LST difference between those who reported heat illness and those who did not was more than 50 % higher (1.52 °C, p \ 0.001) than in the whole sample. This means the effect of parcel daytime surface temperature on heat illness was greater for people with reduced access to home air conditioning. The relationship between parcel nighttime LST and reported heat illness was not statistically significant for the whole sample or for the subgroup with restricted access to air conditioning. Daytime maximum air temperature on the day of survey completion did not have a statistically significant effect on the likelihood of reporting heat symptoms, indicating that it did not affect the results of our study. At the neighborhood scale, the incidence of experiencing heat illness varied between 5 and 53 % of residents. An exponential increase in the rate of Landscape Ecol (2016) 31:745–760 755 Table 3 Differences in parcel land surface and air temperature (°C) between groups of PASS respondents who reported yes or no for experiencing household heat-related illnesses in the summer of 2010 Temperature metric (°C) All respondents Reduced access to air conditioning No heat illness symptoms Heat illness symptoms GLM p value Daytime LST mean 49.05 (3.71) 49.98 (3.83) p \ 0.001 Nighttime LST mean 23.95 (1.66) 23.84 (1.71) p = 0.107 Maximum Tair day of survey 35.98 (9.23) 35.12 (9.85) p = 0.258 Daytime LST mean 48.60 (3.67) 50.12 (4.15) p \ 0.001 Nighttime LST mean 23.92 (1.77) 23.92 (1.67) p = 0.197 Maximum Tair day of survey 35.75 (8.91) 37.28 (8.25) p = 0.491 Values in cells represent means (standard deviations) of temperature for the individual parcels of respondents during daytime, means (standard deviations) of temperature for the individual parcels of respondents during the nighttime, and the maximum air temperature at Sky Harbor Airport on the day of survey response. The upper panel represents the 2011 PASS sample. The lower panel represents PASS respondents who reported an impediment to using air conditioning in their home (too expensive to operate, broken unit, etc.). Statistical significance was assessed through a GLM for binomial distribution evaluation of land cover composition and LST variation, our results showed large differences in day and night LST accompanied by moderate couplings of LST with some land cover characteristics that differed between day and night. There was consistency between residents’ perceptions of their neighborhood environments and empirically measured temperature and land covers. Using self-reported symptoms of heat illnesses, we found heat-related health impacts in the Phoenix region were positively associated with daytime LST of the respondent’s residential parcel and with neighborhood daytime LST. Fig. 7 Proportion of PASS respondents reporting someone in their household had suffered from symptoms related to heat illness during summer and corresponding neighborhood mean daytime LST (exponential regression; p = 0.011; R2 = 0.42). Each datum represents a single neighborhood reported heat-related illness was associated with neighborhood daytime LST (Fig. 7, p = 0.011; R2 = 0.42). Variation in nighttime LST was not correlated with reported symptoms of heat-related illness at the neighborhood scale (p [ 0.1). Discussion Fine resolution LST data coupled with comprehensive land cover maps, household surveys, and residential parcel delineations provide a robust description of the biophysical environment of homes and experiences with heat of the occupants. In this first parcel scale Improving understanding of parcel and neighborhood LST variation Both daytime and nighttime surface temperatures were influenced by characteristics of the vegetated and built land covers at the parcel scale, which is consistent with previous findings at neighborhood scales (Weng et al. 2011; Myint et al. 2013). Of importance for designing heat mitigation strategies, the magnitude and direction of the relationships differed between day and night. Strategies to reduce day and night LST may differ in their effectiveness or even be in opposition. The hypothesis that vegetationbased LST cooling during the day is dominated by evaporative energy partitioning was supported by consistent results for all the greenness metrics—NDVI mean and percentages of tree cover or grass cover. Among these different vegetation variables, tree cover 123 756 was the strongest correlate, which supports previous findings that grass is less effective than tree canopy for LST cooling (Myint et al. 2013), likely because trees with deeper roots have more consistent access to water and thereby maintain more cooling capacity. Our finding of a small positive nighttime parcel vegetationLST relationship contrasts with previous results from Phoenix conducted at much larger (90 m pixels) scales (Buyantuyev and Wu 2010; Myint et al. 2013), which showed negative relationships between vegetation and LST during both the day and night. Also in contrast to regional UHI studies in this hot semi-arid environment (Myint et al. 2015), parcel scale built land covers were negatively related to nighttime LST. These reversals of daytime patterns at night suggests that at the very local scale, vegetation traps heat and building roofs more rapidly lose heat at night. Resolving these scale differences are an important research need. The negative relationship between albedo and nighttime LST and a smaller cooling effect of albedo throughout the day is consistent with an explanation that higher albedo surfaces have reduced energy storage and are therefore cooler early in the night. For parcel-scale land surface cooling, our results support both the use of vegetation and high albedo (e.g. white roofing) building materials (Georgescu et al. 2014). Because of generally stronger land cover—LST relationships during the day, peak temperatures may be more easily managed than nighttime minimum temperatures through landscape modification. Notably, the effectiveness of vegetation (measured either as NDVI or vegetated area) on moderating temperatures varied substantially across neighborhoods. Although prior work has shown a cooling capacity for vegetation, it is generally believed that this capacity is regulated primarily by the area of vegetation (e.g. Jenerette et al. 2011; Li et al. 2011). Our findings suggest that, in addition to the area of vegetation, other factors influence the cooling capacity of vegetation, which we quantified as slope of LSTNDVI relationship within a neighborhood. We showed that vegetation cooling effectiveness at the parcel scale was correlated with expected bare surface temperature, which is consistent with prior neighborhood analyses examining patterns across seasons (Jenerette et al. 2011; Zhou et al. 2014). These findings suggest increased transpiration rates associated with hotter environments may lead to enhanced cooling. However, the effectiveness of vegetation may 123 Landscape Ecol (2016) 31:745–760 also be regulated by other factors we did not examine, including irrigation (Jenerette et al. 2011), landscape configuration (Connors et al. 2013; Zheng et al. 2014; Zhou et al. 2014), and plant species characteristics (Lundholm et al. 2010; McCarthy et al. 2011). Strategies for reducing LST through increased planting should be targeted toward areas with the highest temperatures because in these locations mitigation is likely to be more effective than planting in areas with lower LST. Connecting environment with heat vulnerability Daytime LST at neighborhood and parcel scales were correlated with residents’ perceptions of the heatrelated risk factors and heat-related symptoms of illness. Residents recognized regional variation in temperature and were similarly aware of important drivers of local LST including tree and road cover area in their surroundings. Such consistencies between perceptions and environment were previously suggested by comparisons with modeled air temperature data (Ruddell et al. 2012), although we present the first data-based comparison of perception-environment linkages for LST urban microclimates. The consistency between perceptions of factors associated with heatrelated health risks and observed patterns of these drivers suggest there is value in consulting residents about their views of effective heat mitigation strategies. Critical for understanding residents’ heat vulnerability, we showed that both parcel and neighborhood daytime LSTs were correlated with self-reported symptoms of heat-related illnesses. Symptoms are markers of specific heat-health effects that provide more information about the broader health impacts of heat than can be gleaned from retrospective analysis of mortality and morbidity records. The incidence of selfreported heat illness symptoms in our sample greatly exceeded the frequencies of deaths and hospitalizations attributed to heat in the Phoenix region (Petitti et al. 2015). Heat-related illnesses are associated with a suite of individual, social, and environmental factors that are themselves strongly interrelated (Reid et al. 2009; Hondula et al. 2012; Harlan et al. 2013; Petitti et al. 2015). The causal mechanisms leading to heat illnesses and deaths are not fully understood, although research is making progress in explaining linkages between spatial variations in living conditions and Landscape Ecol (2016) 31:745–760 individual risks. For example, poverty is associated with chronic health problems that are exacerbated by heat (Balbus and Malina 2009), reduced coping capacities that include access to air conditioning (Reid et al. 2009), and elevated risks from working in higher temperature conditions (Petitti et al. 2013). At present the correlations between LST and heat symptoms do not imply direct causation of heat illness but our study clearly shows that daytime LST can be one spatial indicator of heat risk. This was borne out in our analysis showing positive relationships between reported heat symptoms and daytime parcel and neighborhood LST, as well as a much stronger positive relationship between daytime parcel LST and symptoms of heat illness in the subsample of residents without consistent access to air conditioning (the most vulnerable subpopulation) compared to the entire population. Perhaps surprisingly, no similar correlations were observed between heat stress and nighttime LST in this setting. Comparisons between LST and survey respondents’ lived experiences were consistent with several of the few existing neighborhood scale studies, including Harlan et al. (2013) analysis of Phoenix, which showed an association between spatial variability in daytime LST and heat-related deaths. The present study also examined, but did not find, a relationship in this setting between mean parcel nighttime LST in mid-summer and the likelihood that survey respondents reported experiences with symptoms of heat illness. Our finding differs from a coarserscale (1 km LST data) study in Paris, which found nighttime (minimum) LST was significantly associated with elderly heat deaths but there was no relationship between deaths and daytime (maximum) LST (Laaidi et al. 2012). More comparative research on spatial and temporal variability in surface temperatures and heat-related health outcomes in urban areas is needed. Resolving the influence of daytime and nighttime temperature variations on health impacts at appropriate spatial and temporal scales is important for identifying effective landscape interventions that reduce health vulnerability. Yet a limitation of all temperature-related health studies, including our own, is the inability to make direct mechanistic links between the temperature metric, the time, and the site of where a heat-related health incident occurred. Some of the symptoms reported in our study undoubtedly 757 occurred outside the home and neighborhood. Future work directed to individual microclimate monitoring (Kuras et al. 2015) will greatly improve the ability to make such temporal and spatial linkages. Looking towards more heat resilient cities Our findings highlight the potential value for microscale landscape interventions to reduce the heat exposure risk and vulnerability of urban residents. To meet current and future challenges of reducing urban heat vulnerability, cities are using four main strategies to manage heat risks: trees and vegetation, green roofs, cool roofs, and cool pavement (United States Environmental Protection Agency 2008). Georgescu et al. (2014) concluded from their model of the regional climate in Phoenix that more cool roofing materials would have a greater impact than more vegetation on decreasing regional air temperatures. However, for reducing health risks associated with higher LST, mitigating daytime LST in residential areas may be more effective than lowering nighttime air temperature in hot climates like Phoenix. At microscales, increasing greening in pedestrian pathways and outdoor play spaces may be especially useful (Vanos 2015). In comparing the relative cooling effectiveness of vegetation and albedo, our findings support increasing urban greening, especially by planting trees around homes with a large fraction of human-built land surfaces. Because the cooling effectiveness of increasing albedo was observed primarily at night, this strategy may not be as valuable for reducing heat-related illnesses. However, as trees in hot arid cities require irrigation, balancing the cooling benefits with water use is an essential sustainability challenge (Gober et al. 2010; Jenerette et al. 2011; Pataki et al. 2011). Increasing albedo does not have an associated water consumption trade-off and could more readily be implemented anywhere. Reconciling the effectiveness of alternate mitigation strategies is an important challenge for reducing urban vulnerability to climate changes (Liang and Weng 2008; Georgescu et al. 2014). Current global demographic trends projecting 2.5 billion more urban residents by 2050, a 64 % increase from current distributions (United Nations Department of Economic and Social Affairs Population Division 2014), will place more people’s health at risk from extreme heat due to combinations of global warming 123 758 associated with greenhouse gas emissions and regional warming associated with UHIs (Intergovernmental Panel on Climate Change 2014). Interdisciplinary approaches such as this one, which geographically overlay data describing high resolution environmental and human variation at the scale of individual homes, provide a powerful lens for understanding and designing the coupled natural and human systems that contribute to reducing heat vulnerability. Acknowledgments This work was supported by National Science Foundation Grants GEO-0816168, GEO-0814692, BCS-1026865, EF-1049251, EF-1049224, and DEB-0919006. We thank Anthony Brazel and Chris Martin for their advice on the MASTER data collection effort and David Hondoula for insightful discussions. All data are available from CAP-LTER (caplter.asu.edu). 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