The demands of a rapidly growing population has resulted in a shift toward larger and more expansive urban areas [1
]. This has altered the surface energy and moisture balance of these urban areas and led to environmental issues such as the urban heat island (UHI) effect, human thermal discomfort, air quality degradation, and microclimate modification [2
]. To alleviate urban thermal stress, to promote urban ecosystem services, and to improve human and environmental health, various heat mitigation and energy saving strategies are applied, including employing reflective/white roof, adding photovoltaics to capture the solar energy, and using vegetation to create urban green infrastructure. Among all of these strategies, vegetation as one of the most important components of urban green infrastructure, is becoming an integral feature of urban designs [8
]. Commonly used urban green infrastructure includes residential landscaping, green corridors, green roofs and walls, and urban parks using a combination of trees, shrubbery, and turf grass [9
]. The question that remains is how to best integrate urban green infrastructure with the transportation, residential, commercial, and industrial infrastructure to maximize the environmental benefits that are offered by the green infrastructure.
The research presented here focuses specifically on how to effectively and efficiently incorporate shade trees in residential neighborhoods in a hot desert city. In hot desert areas, trees provide multiple microclimate benefits by reducing solar radiation penetration, blocking the exchange of long-wave (infrared) radiation inside urban canyons, and generating evapotranspiration [12
]. In addition, trees provide ecosystem services, including air quality improvement, storm water attenuation, carbon sequestration, and a vast range of economic, social, and health benefits [9
]. Use of tree shade requires a balanced and nuanced analysis of the tradeoffs between cooling by shade and the use of water, a scarce resource in semi-arid and arid regions [12
]. Since the tradeoffs between water and energy require efficiency in the number of trees to be planted on a given parcel, effective tree placement strategies are needed (e.g., tree location, arrangement, and spacing) [19
]. These strategies will help homeowners to maximize the overall benefits from trees with the fewest number of trees in an effort to simultaneously reduce both water consumption and energy use [23
The effect of shade trees in urban areas has been examined through real world in situ measurements and through numerical modeling, both confirming the conventional wisdom that tree shade reduces surface and air temperature [24
]. The in situ experiments apply meteorological instrumentation to measure environment accurately, but various real world conditions would influence the measurement results. The numerical modeling simulates physical environment through parameterization, but lacking of fieldwork validation makes the simulation results be less reliable. The differences among the in situ studies are the methods used, whether they measured surface or air temperature, and the impact of different types of shade, such as native, exotic, and artificial shade [26
]. Results show reduced temperatures between 1 Celsius degree (°C) and 9 °C, depending on these variables. The problem with the results is that a variety of in situ conditions influences the results, such as the geometry and material characteristics of trees (tree type, tree height, leaf area, etc.), building arrangements, and background meteorological conditions. Numerical simulation models offers the ability to manipulate tree placement, background materials, and analyze cooling from tree shade by simulating the microclimate and resulting human thermal comfort [29
]. Urban canopy models (UCMs) simulate tree foliage together with buildings to represent the emission and reflection of radiation, and mutual shading between buildings and trees, showing energy savings, and heat mitigation from shade trees. Computational fluid dynamics (CFD) modeling better represents the three-dimensional thermal environment than the UCMs and has been used to analyze air movement, pollution dispersal, and pedestrian wind tunnels [18
]. Like UCMs, CFD simulations consistently show that increased vegetation provides cooling effects under a variety of conditions [10
]. The challenge with modeling tree shade on buildings is that numerical simulations are unable to resolve the heat transfer of the wall (i.e., the buoyancy effect) [18
In contrast to in situ measurements and numerical simulations, physical scale models combine the experimental control of numerical simulation with the real world complexities that are related to the natural environment [36
]. There are comprehensive physical scale models that have been developed to measure urban albedo, aerodynamic drag, urban surface energy fluxes, thermal inertia, urban canopy microclimate, pedestrian energy exchange, convective heat transfer, thermal amelioration from water bodies, and evapotranspiration in urban canyons [37
]. They offer the ability to control many of the field parameters, such as street layouts, existence of vegetation, solar radiation, wind speed, and humidity, which provides enough flexibility for analyzing radiation, shading, and wind tunnel conditions. With the exception of Roberts in Arizona, USA [36
] and Pearlmutter et al. in Israel [45
], none represent the hot desert urban environment, and very few incorporate vegetation [49
]. This is because the morphology and materials of vegetation are much more complex than urban structures (such as cubes, blocks, or cylinders) in the physical scale modeling. Park et al. [51
] included vegetation (Gold Crest Wilma plants) in the Comprehensive Outdoor Scale Model (COSMO) in Japan to evaluate the thermal comfort of pedestrians, finding that trees along pedestrian walkways can reduce the wind speed by up to 51% and decrease the temperature up to 2.2 °C. Taleghani et al. [52
] also created a scale model site with vegetation to analyze roof configurations in courtyards. Their scale model experimental results showed that a green pavement with grass on a roof or courtyard could result in up to 4.7 °C air temperature cooling comparing to gravels and black materials.
Note that artificial trees were used in our outdoor scale model field measurement. As a result, the biophysical functions of real trees, e.g., transpiration by stomatal control, root uptake, foliage dynamics, and diurnal/seasonal variabilities, were not represented. However, artificial trees effectively capture the radiative shading mechanism of real trees. This is particularly true for xeric trees in an arid or semi-arid environment, such as Phoenix, where evapotranspiration is largely inhibited by excessive heat as well as relatively sparse foliage [53
The goal of this research is to build an outdoor urban physical scale model to measure and understand the shading effect of different tree densities, locations, and arrangements in a typical residential area in a hot desert city. The physical scale model makes it possible to create and investigate a wide variety of tree locations and arrangements scenarios that is not practical to test in situ. We conducted our experiment in Tempe, Arizona, a municipality in the greater Phoenix metropolitan area in Arizona, USA. We designed this study based upon Park et al. [51
] and Taleghani et al. [52
] who demonstrated how vegetation can be an asset in physical scale models. Existing research has not yet explored the shading benefits of trees under different locations and arrangements in a physical scale model experiment. This is an obvious research gap in the outdoor urban physical scale modeling literature that we intend to fill and will be a crucial step in designing green infrastructure for the long-term sustainability of urban areas.
Our experimental results demonstrate that tree density and arrangements influence hourly facade cooling benefits on the order of 0.5–6.6 °C, which is consistent with existing research [66
]. The first contribution of this research is to provide a quantitative measurement of how tree density influences the facade cooling benefits. A single shade tree can induce a maximum hourly cooling of 2.3 °C of the facade temperature, and two trees can decrease the hourly facade temperature by up to 6.6 °C in the scale model experiment (Table 5
). These findings confirm that a higher tree density can substantially enhance the shading benefits on the building facade. Second, the physical scale model provides the capability to examine various tree locations and arrangements scenarios that are not practical to test in situ. When locating one or two trees in the mock urban canyon, the field experimental results consistently show that tree shade cooling benefits are greater when locating trees at the central and east side of the building south front yard (Table 5
). Although conventional wisdom suggests that residents plant their shade trees at the southwest corner of their house, the research results demonstrate the effectiveness of planting trees in the southeast of the building structures when considering facade cooling. When the central area of the south front yard space is limited or occupied (etc. driveway), we suggest home owners to plant a new shade tree at the southeast corner of the residential properties to provide extra shading and cooling benefits to the building facade. Planting trees in the southeast will offer ample shade in the morning for the west facade, and provide substantial cooling benefits to the east side of building facade in the afternoon. It will be valuable to examine the effectiveness of locating trees at the southeast corner of the properties in the future research.
By comparing the shading benefits from the cluster and disperse arrangement, the results show that a disperse arrangement is not necessarily worse than the cluster arrangement. A cluster arrangement with better afternoon shading provides the most cooling benefits in this particular urban layout, but the disperse arrangement also offers a good level of cooling benefits to the whole building facade. All of the results confirm the importance of the benefits from tree shade coverage in relation to the residential buildings. In this compact urban setting, nearby surrounding buildings also receive evident tree shade cooling benefits (around 2 °C, Figure 9
), especially when planting trees at the edge of the residential parcels.
From Figure 6
, Figure 7
and Figure 8
, the temperature of exposed facade increases in the afternoon. The potential explanation is that artificial tree serves as a heat source in the afternoon and radiates heat to the nearby building facade. Although artificial tree provides various benefits and convenience in the experiment (Section 2.1
), this is an unavoidable issue due to lack of evapotranspiration in artificial trees. This phenomenon diminishes and underestimates the cooling benefits of tree shade in the scale model experiment. However, this is further validated the importance of tree shade coverage for the building facade. When comparing to artificial turf with similar materials to the artificial tree, the existing research show that artificial turf increases the surface temperature and raises health issues on the sports playground [67
]. However, the shading from artificial tree is still found to be valuable and reduces the facade temperature. The finding here emphasizes the contribution of shading to the facade surface temperature and corresponds with the finding in the existing literature that natural shading and artificial shading provide similar thermal comfort in the hot dry desert climates [24
Several limitations exist in this scale model experiment. First, we used iButton loggers to measure the near surface air temperature and approximately represented the building surface temperature with the validation of thermal images. Even though some existing research used iButton loggers to measure surface temperature [71
], flat surface thermistors or thermocouples may provide better surface temperature measurements with more experimental efforts. Second, this research did not account for the building’s open structures, such as windows, doors, and ventilation. Adding these important building components into the physical scale mode is expected to improve the accuracy of the quantitative study. Further, we used concrete blocks to represent the building structure in the physical scale model experiment. Some houses may not be constructed by concrete structure, but with wood or stucco structures, and would have different physical properties (albedo, emissivity, etc.). Future study can be conducted to include this aspect of building properties. Last, this research particularly focuses on a hot arid urban environment that radiative shading from trees is the predominant factor to cool the outdoor environment. The best tree locations and arrangements results might vary under different geographical locations and climate conditions [75
]. Other factors, such as evapotranspiration and wind speed, need to be considered.
This research represents a case study that attempt to assess the shading benefits under different tree locations and arrangements in a residential neighborhood by an outdoor urban physical scale model. A number of improvements can be made to improve this work in subsequent studies. For example, different tree species, alternative leaf area index/canopy density, crown size, and tree heights are all important options for flora [76
]. All of these factors can be added and evaluated in the physical scale model. The comparison between artificial tree and real tree will also be important to understand how evapotranspiration and retention heat issue influence surface temperature in the built environment. In addition, the specific compact urban arrangement that we simulate limits the orientation of buildings and location of trees in the experiment [78
]. Different building arrangements and orientations can be adopted to this outdoor urban physical scale model in future studies. Furthermore, trees cool down the building structures in the daytime, but they also trap the long wave radiation during the night because of the low sky view factor under tree canopy [79
]. This outdoor urban physical scale model can be used to explore the overall advantages and disadvantages of trees for mitigating UHI effects in both daytime and nighttime and serve as a field experiment site for the validation of numerical modeling of urban areas. The research results in the scale model experiment can also be used as an input scenario in a larger area numerical simulation modeling to explore how tree arrangement influences the neighborhood climate environment.
The research finding from this scale model experiment can translate into important landscape architecture design implication and policy recommendation to the compact residential neighborhood in the desert city. Because of the space limitation in the compact residential areas, green infrastructure in these areas is normally not adequate [80
]. Thus, the policy encouragement of expanding urban green infrastructure from the city government and/or the homeowner association is important and necessary. According to our research findings, city residents or single-family homeowners should plant their first tree to shade the east side of the south facade, and allow enough space between multiple trees to maximize the overall shading benefits. In addition, trees located at the edge of residential parcel will be valuable through neighboring shading. They will provide ample shading to multiple houses and improve the overall living environment in the neighborhood. In Wentz et al.’s recent research [19
], they mentioned that the homeowner association only provided a minimum landscaping guideline in Goodyear, AZ. Our research results will be useful to providing landscaping suggestions in the homeowner association guidelines for arranging trees wisely in the residential neighborhood. Although this research focuses on the tree shade benefits at building and neighborhood scales in the compact desert residential settings, the research contribution will be beneficial for understanding the landscaping deployment of urban green infrastructure, such as urban parks and street trees in the entire urban area [81
]. We anticipate to raise the policy attention from city mayors, policy makers, and homeowner association to understand the importance of urban green infrastructure, emphasize the use of urban green infrastructure in the future city developmental plan [85
], and wisely design tree locations and arrangements in the existing tree and shade program [86
]. The overall efforts will help improve the urban thermal environment and mitigate UHI effects under hot dry desert climates [87