Greening the desert city: The challenge of microclimatic moderation in arid urban areas

Transcription

1 Greening the desert city: The challenge of microclimatic moderation in arid urban areas David Pearlmutter Blaustein Institutes for Desert Research Ben-Gurion University of the Negev, Israel Green Infrastructures and Urban Forests for improving the environment and the quality of life, November 17 th 2014, Rome

2 Background: Arid cities and vegetation The population of arid regions is increasing Climatically harsh conditions exacerbate pedestrian thermal stress Urban green infrastructure is vital for climatic moderation but water scarcity highlights the need for efficient solutions

3 Background: Arid cities and vegetation Negev Desert, Israel Precipitation: Potential Evapotranspiration: Aridity Index: P ~ 80 mm/year PET ~ 2000 mm/year P/PET ~ 0.04

4 Modeling pedestrian thermal stress Index of Thermal Stress ITS = (R n + C + M ) / f [measured in watts] R n and C = radiant and convective energy exchanges between pedestrian body and urban environment M = metabolic heat f = sweat efficiency based on RH Pearlmutter, Berliner & Shaviv (2007) Net Radiation (W m -2 of body area): R n = (K dir + K dif + h K h + v K v )(1- s ) + L d +L h + L v - ε T s 4 Convection (W m -2 of body area): C = h C ΔT

5 Pedestrian thermal stress and thermal sensation ~ 300 W y=0.0032x +4.0 Pearlmutter, Jiao and Garb (2014)

6 Ground cover Grass Bare Vegetation & microclimate: A controlled outdoor experiment Overhead cover Trees Exposed Mesh Trees-Bare Exposed-Bare Mesh-Bare Trees-Grass Exposed-Grass Mesh-Grass Courtyard spaces used for comparing thermal stress conditions (Shashua-Bar, Pearlmutter & Erell 2009; 2011)

7 Experimental setup: Measurement tools & techniques Both courtyads: Elongated N-S axis H/W = 0.5 Prevailing NW wind Daily DBT o C DBT+WBT: Aspirated psychrometer Grass ET: Mini-lysimeter Tree transpiration: Sap-flow probes

8 Results: Thermal stress and thermal sensation Exposed-Bare Exposed-Grass

9 Results: Thermal stress and thermal sensation Mesh-Bare Trees-Bare Mesh-Grass Each landscape treatment made a clear contribution to pedestrian comfort, with the greatest reduction in mid-day thermal stress provided by a combination of shade trees and grass Trees-Grass

10 Results: Cooling and water use efficiency higher efficiency Cooling efficiency Trees-Grass Exposed-Grass Mesh-Bare [%] Daytime Water Cooling use: Efficiency: Cooling: Trees-Bare Mesh-Grass lower efficiency Cumulative Equivalent Ratio between latent reduction daytime heat* in of thermal evapotranspiration cooling (kwh) stress* and yielded from daily by grass water a landscape use trees, (kwh)*, strategy on a as daily a over (24 percentage hour) the daytime basis (%). hours (6:00-18:00), relative to the base case (Exposed- * Equivalent latent heat of evapotranspiration Bare) Highest cooling efficiency: shade trees alone * ΔITS Q E (kwh) (kwh) Exposed grass requires more water than grass and shade trees combined Vegetation reduces thermal stress despite negligible effect on air temp.

11 Vegetation: surface temperature, albedo & cooling efficiency The cooling effect of trees was more pronounced than artificial mesh, due to its elevated radiative surface temperature Likewise, grass reduces pedestrian exposure to longwave radiation due to its low surface temperature and uniquely among urban materials, planted surfaces also minimize short-wave reflection due to their low surface albedo However, grass requires intensive irrigation! Trees + Grass Mesh + Bare * Can alternative, water-efficient vegetation be utilized to create cool ground surfaces? 15.6

12 Vegetation: surface temperature, albedo & cooling efficiency The potential of succulent plants and other alternatives to grass in urban landscaping were examined in the Negev desert (Snir, Erell & Pearlmutter) Experimental were plots planted with six varieties of ground-cover vegetation, and compared with dry surfaces for measurement of radiant surface temperature and albedo Test plots with three types of succulent plants and three non-succulents including grass Dry surfaces: Artificial turf Concrete paving Results were used as input to calculate pedestrian thermal stress in hypothetical urban spaces using each landscape treatment Cooling efficiency was calculated from water requirements based on measured evapotranspiration Bare loess soil

13 Vegetation: Surface albedo succulents non-succulents Only minor differences in albedo were found between all plants, including grass Albedo values of all plants are significantly lower than concrete paving or bare soil, though higher than artificial turf Average mid-day albedo for planted and dry test surfaces, measured with paired upward and downward-facing pyranometers

14 Vegetation: Surface temperature Succulent plant varieties maintain somewhat higher surface temperatures than grass and other non-succulents succulents nonsucculents Compared to all plants, temperature of bare soil is elevated at all daytime hours - and artificial turf reaches 70 o C! Hourly surface temperatures of planted and dry ground surfaces

15 Vegetation: Reduction of pedestrian thermal stress All planted surfaces provide significant cooling (reduced ITS relative to dry surface) Differences between grass and alternatives are modest Hourly Index of Thermal Stress (ITS) calculated for pedestrians in urban spaces with planted and dry ground surfaces ITS calculation with L h and α h K h modified for temperature and albedo of each surface

16 Grass Succulent Cooling and water use efficiency: Alternative ground-cover non- succulents succulents Cooling efficiency of landscape treatments in urban spaces with different ground-cover vegetation (water requirements based on measured ET) * Despite negligible differences in albedo, surface temperature or cooling effects, alternative ground cover plants are substantially more water-efficient than grass Before > After 10 days without irrigation

17 Conclusions of initial studies The effective use of urban vegetation for reducing thermal stress in hot-arid cities requires a holistic planning approach, which considers: the geometry of the built urban fabric: compact open spaces the combination of shade trees with vegetative ground cover the integration of drought-resistant plant species AVOID LARGE EXPANSES OF EXPOSED GRASS!

18 Introduction: Coastal urban parks & environmental challenges Coastal urban locations are attractive for parks but pose severe environmental challenges in warm regions with limited fresh water The sea breeze provides cool air, but ground surfaces and people are vulnerable to solar exposure and overheating Given local and global warming trends, and the imperative of achieving more sustainable urban design, this study aims to: quantify the level of thermal stress in an urban coastal park with a prominent sea breeze and expanses of unshaded grass compare the levels of physical thermal stress with the subjective thermal perception of park users (Saaroni, Pearlmutter & Hatuka)

19 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa The park is located along the eastern Mediterranean coast in the city of Tel Aviv- Jaffa, with Jaffa port to the north and the Ajami residential neighborhood to the east. Tel Aviv- Jaffa

20 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa Former construction debris waste site, transformed in 2010 into a 200-dunam (50- acre) urban park with largescale recycling and public participation. Part of municipality s vision to create a continuous waterfront boardwalk for pedestrians and cyclists along the metropolitan area. Three pedestrian axes connect the neighborhood with the sea shore, crossing grasscovered hills and a concrete plaza.

21 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa 40% of total area (80 dunam) covered with grass lawns, irrigated with fresh water Expanses of grass and concrete are exposed to the sky, with only small areas shaded by built landscape elements or trees Question: How does the park s design respond to the environmental stresses that are prominent in the Tel-Aviv urban area? Approach: Thermal comfort conditions in summer analyzed through surveys of 300 pedestrians, with responses compare to calculated indices of thermal stress

22 Evaluating pedestrian thermal stress and thermal sensation Physical measurements Radiation: Global and diffuse radiation Ground surface temperature and albedo Convection/evaporation: Air temperature Wind speed (average and upper gust) Relative humidity Survey of user satisfaction 300 questionnaires divided evenly in: two locations (grass and concrete plaza) three daily periods (13:00-15:00, 17:00-19:00 and 20:00-22:00)

23 Results: Ground surface temperature comparison GRASS (albedo = 0.26) CONCRETE PAVING (albedo = 0.42) Ground temperature as measured on a typical summer day, and as modeled with a modified sol-air equation

24 Results: Thermal Stress vs. Thermal sensation ~300 watts Relation between Index of Thermal Stress (ITS) and comfort vote (1 = very cold, 2 = cold, 3 = cool, 4 = comfortable, 5 = warm, 6 = hot, 7 = very hot)

25 Results: Thermal Stress vs. Thermal sensation 17:00-22:00 13:00-15:00 Relation between Index of Thermal Stress (ITS) and comfort vote (1 = very cold, 2 = cold, 3 = cool, 4 = comfortable, 5 = warm, 6 = hot, 7 = very hot)

26 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa Some conclusions and food for thought The degree of aesthetic and visual satisfaction appears to reinforce the perception of comfort in the park, but does not change its basic relationship with physical climatic stress. 72% of respondents opposed the use of drinking water for irrigation, but more than two-thirds of these said they would not give up the large lawn areas to save water. Integration of trees and alternative ground cover species was thwarted by practical considerations and environmental conditions.

27 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa Some conclusions and food for thought Regional warming and drying trends would indicate that open space design should focus on increasing comfort and minimizing use of fresh water. How sustainable is this approach to the design of urban parks?...

28 Jaffa Slope Park: Urban coastal park in Tel Aviv-Jaffa Thank you! With special acknowledgement and appreciation to the students and staff of Tel Aviv University and Ben-Gurion University for their contributions to this study

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30 Results: Cooling and water use efficiency higher efficiency Cooling efficiency Trees-Grass Exposed-Grass Mesh-Bare [%] Daytime Water Cooling use: Efficiency: Cooling: Trees-Bare Mesh-Grass lower efficiency Cumulative Equivalent Ratio between latent reduction daytime heat* in of thermal evapotranspiration cooling (kwh) stress* and yielded from daily by grass water a landscape use trees, (kwh)*, strategy on a as daily a over (24 percentage hour) the daytime basis (%). hours (6:00-18:00), relative to the base case (Exposed- * Equivalent latent heat of evapotranspiration Bare) Highest cooling efficiency: shade trees alone * ΔITS Q E (kwh) (kwh) Exposed grass requires more water than grass and shade trees combined Vegetation reduces thermal stress despite negligible effect on air temp.

31 Results: Air temperature above different surfaces GRASS (albedo = 0.26) CONCRETE PAVING (albedo = 0.42) Air temperature as measured on a typical summer day