Urban greenery provides several benefits because it dissipates incoming solar radiation on the building structures through its effective shading; it can reduce long-wave radiation exchange between buildings due to the low surface temperatures created by shading of plants and it can reduce the ambient air temperature through evapotranspiration (Wong et al., 2003).
While there is little scientific ambiguity over the temperature reducing attributes of urban greenery, the more practical question often asked by planners, developers and architects is:
“How much greenery is needed to reduce air temperature?”
Herein lies the issue of quantification. In planning and developing an area, quantification is needed as it is closely related to the financial aspect of the development, i.e. investment cost and maintenance cost. All the scientific jibber jabber is pretty pointless if we cannot put a number to it.
To facilitate application, quantification of greenery is necessary. And to do that, a few items have to be clearly defined. They are as follows:
1. Leaf Area Index (LAI)
To simplify, most plants have leaves. To establish how leafy a plant is, we measure the total surface area all the leaves cover over a pre-determined area. Since the result is normalised, we can use it to compare leafiness between different plant species. Figure 1 shows how we can measure the LAI of a plant. In the figure, all leaves within a 0.5 m X 0.5 m area are removed. They are arrayed, scanned and the green pixels are tabulated. The value is divided over 0.25 m2.
Knowing the LAI of the plant means knowing how leafy the plant is. Theoretically, plants with higher LAI values have more leaves and can provide more shade.
2. Green Plot Ratio (GnPR)
Over an area, greenery density is measured as Green Plot Ratio (GnPR). Developed by Ong (2003), it is derived from the average of greenery on a lot, using the leaf area index (LAI), in proportion to the total lot area. It is the sum of the products of the area of each greenery type and its corresponding LAI value, and is divided by the total lot area. Figure 5 demonstrates how GnPR is calculated. GnPR is now has been adopted by Singapore’s National Parks Board (NParks) as a method in quantifying greenery density (Tan and Sia, 2009).
So how do we know urban greenery can reduce air temperature?
Field measurements have been conducted from various locations in Singapore to establish the relationship between air temperature and built environment as a whole (i.e. building, road/pavement and greenery). The complete study can be read here (Jusuf & Wong, 2009).
The study confirms that lower temperatures can be found in areas with denser greenery (higher GnPR). Assuming an ideal urban canyon, the relationship between GnPR and air temperature is illustrated in Figure 3.
Greenery lowers the air temperature through different mechanism in different time of the day. During daytime (shown as maximum temperature – Tmax), shading by its leaves plays an important role in lowering the air temperature. The increase of GnPR affects the openness to the sky. Once the greenery completely covers up the openness to the sky, in this case at GnPR of 5, there is no further reduction in the maximum temperature.
Meanwhile during night-time, greenery provides cooling through evapotranspiration. The higher the GnPR value, the lower minimum temperature (i.e. Tmin) will be. In the graph, it is shown as a straight declining line. On average temperature, greenery provides cooling through both shading and evapotranspiration for GnPR up to 4. Once the density of greenery covers the openness to the sky, the cooling effect of greenery is mainly from the evapotranspiration process.
I have developed a tool to simulate how air temperature can be affect by urban components. These include building footprint, height, tree locations, pavement area, etc. With a site model (Figure 2), we can simulate the air temperature at critical timings (Figure 4).
And so, to answer the question of how much greenery is needed to reduce air temperature, you can find out using a tool I have developed. The Screening Tool for Estate enVironment Evaluation, or STEVE Tool.
– Dr Steve Kardinal Jusuf
Jusuf, S.K. and Wong, N.H. (2009). Development of empirical models for an estate level air temperature prediction in Singapore. In proceedings: Second international conference on countermeasures to urban heat islands, 21 – 23 September 2009, Berkeley, California.
Ong, B.L. (2003). Green plot ratio: an ecological measure for architecture and urban planning. Landscape and Urban Planning, 63, 197-211.
Tan, P.Y. and Sia, A. (2009). Leaf area index of tropical plants: a guidebook on its use in the calculation of green plot ratio. National Parks Board. Singapore.
Wong, N.H., Chen, Y., Chui, L.O. and Sia, A. (2003). Inverstigation of thermal benefits of rooftop garden in the tropical environment. Building and Environment, 38 (2), 261–270.
About the writer:
Steve Kardinal Jusuf has a Ph.D. degree in Building Science from the Department of Building, National University of Singapore. He received the World Future Foundation Ph.D. Prize in Environmental and Sustainability Research for his Ph.D. work on air temperature prediction model within urban climatic mapping method. He developed the Screening Tool for Estate Environmental Evaluation (STEVE), an application that simulates urban air temperature based on the greenery density, building distribution and the amount of hard surfaces.
Visit his website here.