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Vegetation Configuration as Microclimate Control Strategy In Hot Humid Tropic Urban Open Space

Jono Wardoyo1), Eko Budiharjo2), Muh Nur3) , Edy Prianto4)

Corresponding Author : mas_joon@yahoo.com

*)Paper presented in International Seminar on Sustainable Environment and Architecture (SENVAR) 2008 KualaLumpur Malaysia

Abstract

Minimizing direct solar radiation and optimizing wind velocity are factors in designing urban open space in hot humid tropic area. Vegetation has potential to reduce environmental temperature. Its form and configuration influence solar radiation, temperature, air humidity and wind of an urban setting. This study reviews the impact of vegetation in ameliorating the urban air temperature and explores the possibility of vegetation configuration to maximize cooling effect in urban open space.

 

Keywords: vegetation, open space, microclimate, hot humid tropic

 

1.         INTRODUCTION

 

Economic progress in many developing countries results to rapid expansion of cities. Developing of the city usually marked with physical developments including buildings, road and infrastructure systems. The development led to imbalance between land availability and land demand, consequently, results to increase in land value and changes in land use in urban centres.

Environmental benefits and opportunities provide by vegetations as an element of urban open space. Such benefits include amelioration of urban microclimate by shading and wind controlling (Shasua-bar & Hoffman, 2000; Yu & Hien, 2006), reduction of air pollution by absorption the pollutan, and noise reduction.

In hot humid tropic area, high intensity of solar radiation and low wind velocity are the problems experienced by users of open space. Reducing solar radiation and increasing and directing wind are modifications that help to bring  an ideal urban microclimate (Katzschner, 2002). This paper discusses the strategy to reduce solar radiation and directing wind using vegetation in urban area. Specifically, the paper elaborates on configuration of vegetation layout in reducing solar radiation and directing wind. Firstly, it describes climatic termal problems of urban open space in tropical region. Then, it discusses on potentials of vegetation in ameriolating urban micro climate, and, finally, it describes the strategy to configure layout of vegetation to ameriolate the mirco climate.

 

 

2.          CLIMATIC THERMAL PROBLEMS OF URBAN OPEN SPACE IN HOT HUMID TROPIC AREA

 

Climate in hot humid area characterized by high air temperature (27 – 32 OC) caused by high intensity of solar radiation (400 Watt/m2), high relative humidity (75 – 80%) and low wind velocity approximately 1 – 2 m/s (Soegiyanto, 1998; Mansy, 2006). Minimizing direct solar radiation and optimizing wind direction reduce air temperature in urban area.

Direct solar radiation has a closed relationship with ratio of height of surrounding building and width of the open space (H/W ratio) (Scudo, et al., 2004). When H/W ratio is small, it makes the solar radiation intensity on urban open space become high.

Building and road absorb heat from the solar radiation and radiate it to the surrounding, and thus increases ground and surface temperatures. This condition creates uncomfortable setting for people. Re-radiation effect of the incorrect ground cover of urban open space also increases the surrounding temperature (Huang, 2008). Vegetation in open space has low reflectance value that helps to reduce the air temperature in an urban surrounding.

Another climatic thermal problem of urban open space in the hot humid tropic area is how to optimize wind velocity and direction. In theory, increase of wind velocity provides more comfort to pedestrians in urban open space. Air movement incorporates lower air masses. It creates refreshing sensation for pedestrian (Corbella & Magalhaes, 2008). Therefore, wind should be directed to the area needed (Koefoed & Gaardsted, 2004). Abundance of trees in the tropic would allow this effect to take place in towns and cities of tropical countries.

 

3.         THE POTENTIAL OF VEGETATION IN URBAN MICROCLIMATE

 

The urban form, characterized by its built environment, surface roughness, urban geometry, ground surfaces and properties of the surface materials can alter the urban microclimatic profile and influencing the climatic elements (Barbiratto, 2005; Linberg, 2003; Shasua-bar & Hoffman, 2000). Trees could change the H/W ratio that affects urban geometry (Shasua-bar & Hoffman, 2000). Leaves of the tree and ground cover vegetation both have minimum reflectance which reduces the urban albedo.

 

3.1 Vegetation and Solar Radiation

Individual leaves of the vegetation allow some radiation to be transmitted through them (20%), absorb some radiation (55%), and reflect some radiation (25%) (Scudo, 2001). Solar radiation that has been arbsorbed by the leaf then be re-transmitted by evapotranspiration (Hiraoka, 2002). Evapotranspiration increases relative humidity and consequently reduces air temperature.

 

Structure of the crown, dimension, shape and colour of vegetation leaves influence reduction level of solar radiation (Bueno-Bartholomei and Labaki, 2005). With the sample of trees species in Brazil, Bueno-Bartholomei and Labaki found that solar radiation reduction is between 76.3 – 92.8 % in summer. Shahidan, Salleh and Shariff (2006) compared the average heat infiltration under canopy of two Malaysian trees that have difference density value. The result is the average heat infiltration of Messua ferrea is 97 % whilst that of Hura crepitans is 86 %.( Table 1.)

 

Table 1. Attenuation of Solar Radiation of some Tropical Trees

Analyzed Trees Attenuation of Solar Radiation (%) Source
Cingingium jambolana 92,8 1
Senna spectabilis 88,6 1
Caesalpinia peltophoroides 88,5 1
Cassia fistula 87,3 1
Hymenaea courbani 87,2 1
Ficus benjamina 86,3 1
Michelia champacca 82,4 1
Bauhinia forticata 81,7 1
Clitoria fairchildiana-summer 78,6 1
Cedrela fissilis-summer 75,6 1
Tabebuia impegtiginosa 75,6 1
Schinus molle 73,6 1
Clitoria farichildiana-winter 70,2 1
Cedrela fissilis-winter 29,9 1
Hurra crepitans 86 2
Messua ferrea 97 2

Sources : (1) Bueno-Bartholomei and Labaki (2005) ; (2) Shahidan, Salleh and Shariff (2006)

 

 

3.2 Vegetation and Temperature

Cooling effect by vegetation had been observed by researchers such Zahoor (1997), Saxena (2001), Hiraoka (2002), etc. Zahoor (1997) in Pakistan found that vegetation has significant influenced to local temperature and effective in reducing air temperature about 6 –7 °F. While Saxena (2001) by computer simulation also found that temperature on housing neighborhood surrounded by tree has 1.34 deg F lower than without the vegetation.
By computer simulation, Hiraoka (2002) observed the air temperature distribution of around 10 m height single tree (Figure 1). According to the simulation, the maximum range of maximum reduced air temperature is approximately 25 m from the tree. The air temperature distribution still depends on wind velocity and direction.

In their research, Vinet, et al (2001) found that temperature on green area has dropped at 01.00 pm. (Figure 2) Hien, et al (2006) reported that peak temperature difference between densely green area and sparse area in Singapore which has humid tropic climate can be as high as 4 °C at 01.00 pm. Mayer and Matzarakis (2002) confirmed a similar result.

 

 

3.3 Vegetation and Wind

Moreover, vegetation has great potential to control air movement. The effect of vegetation on wind cannot be determined with certainty. We can only make educated guesses, based on theory and observation, to suggest how vegetation can affect the wind (Scudo, 2002).

Vegetations influenced the pattern of air movement through guidance, filtration, obstruction and deflection (Figure 3). Affectivity of air movement controlling depends on vegetation characteristic. Geometry, height, permeability and crown of the vegetation are the structural vegetal characteristic that influenced the controlling air movement. (Scudo, 2002)

Stathopoulos et al. (1994) found that a single row, high density windbreak vegetation reduced air infiltration by about 60% when planted approximately four tree heights away from the building. They also found that the efficiency of windbreaks depends on the direction of dominant winds.

 

 

4.         URBAN MICROCLIMATE CONTROL STRATEGY THROUGH VEGETATION

 

The role of vegetation in controlling urban microclimate can be observed in 3 different levels:building level, street level, and urban and sub-urban level. The effect of vegetation on urban microclimate can be divided in five major effects: shading effect, ground temperature effect, surface temperature effect, short-wave reflection effect  and wind effect (Scudo, 2002) (Table 2).

 

Table 2. Microclimate Control Strategy by Structural Vegetal Characteristics

1 2 3 4 5 6 7 8 9 10 11
STRUCTURAL Geometry Columnar
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Pyramidal
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Horizontal branching
  • ·
  • ·
  • ·
  • ·
Roundhead
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Disposition Continuous
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Irregular
  • ·
  • ·
  • ·
  • ·
Group
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Height High
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Medium
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Low
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Very low
  • ·
  • ·
  • ·
  • ·
Transmission High
  • ·
  • ·
  • ·
Middle
  • ·
  • ·
  • ·
  • ·
Low
  • ·
  • ·
  • ·
  • ·
Permeability High
  • ·
Middle
  • ·
  • ·
  • ·
Low
  • ·
  • ·
  • ·
Crown Deciduous
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
Evergreen
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·
  • ·

(Source: Scudo, 2002)

Note:

1. Shading 7. Reflection control – short wave (decreasing)
2. Surface temperature control (increasing) 8. Wind obstruction
3. Surface temperature control (decreasing) 9. Wind deviation
4. Ground temperature control (increasing) 10. Wind filtration
5. Ground temperature control (decreasing) 11. Wind channeling
6. Reflection control – short wave (increasing)

 

 

5.         DISCUSSION

 

It is rather difficult to select the vegetation characteristic for urban open space in the hot humid tropic city. On one hand, the density have to be more higher to make a greater shadow effect. But in another hand, the higher density value create a wind obstruction effect that should be avoid in hot humid tropic area.

Shading effect of vegetation depends on vegetation characteristic and configuration.Geometry of the canopy, leaf area density and height of the tree are vegetation characteristics that influence the shading effect. Piramidal and roundhead canopy have a greater shaded area than columnar. Vegetations that have more density value, have a greater heat infiltration value (Shahidan, Salleh and Shariff, 2006).

Vegetation configuration also play an important role in defining shaded area on urban open space. Distance between the tree should be kept in shadow range of the tree. Thus, it will make a contiuous shading area on the urban open space. Orientation of the row of vegetation should be concerned with the sun position (Shasua-Bar & Hoffman, 2000).

In order to optimize the wind potential of hot humid tropic climate, vegetation chanells and deviate wind. Density and configuration of the tree can drive the wind direction rather than geometry of the canopy. However, high density of tree canopy may block the wind direction rather than directing it.

 

 

6.         CONCLUSION

 

Main climatic thermal problems in urban open space in hot humid tropic area are how to minimize high solar radiation and to optimize the wind. Vegetations have potentials in ameliorating urban open space microclimate. Vegetations configuration is one of determining factors to get benefit of vegetation potential in hot humid urban open space.

Further researchs are needed to find out the effect of vegetation configuration to microclimate condition and the ideal configuration of vegetation for hot humid tropic urban open space.

 

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