35
Regarding the effects of shading and evapotranspiration on surroundings of trees
(based on measurements taken at 12 m and 5 m from trees), Akbari et al. (1992)
concluded that, for large areas, the cooling capacity depends mainly on evapotranspiration, reaching a distance as far as five times the height of the tree. They also
found that shading contributes up to 95% when directly under the canopy, but its
contribution in terms of reducing the temperature (and consequently the energy
consumption for air conditioning) is around 40% for areas larger than 2 ha.
According to Chang et al. (2007), size contributes to 60% of the cooling capacity,
and indirectly affects the contribution of ETA. Finally, Shashua-Bar and Hoffman
(2000), based on empirical studies, note that in areas smaller than two hectares, the
contribution of shading is around 80% of the total cooling capacity, with the remaining 20% determined by evapotranspiration.
Therefore, in the proposed approach, the overall cooling capacity of UGI is
assessed through a weighted summation of the evapotranspiration and shading
scores, using different weights according to size, followed by a standardization of
the results into a scale between 0 and 100. More specifically, in areas smaller than
two hectares, shading is assigned a weight of 0.8 and evapotranspiration of 0.2,
while in areas larger than two hectares, the weights are of 0.4 and 0.6, for shading
and ETA, respectively. Noteworthy is the case of areas with less than 50% of tree
canopy coverage that may turn to be warm islands instead of cool islands during
some part of the day in very hot summer (Chang et al. 2007). To consider this
remark, the cooling capacity scores calculated for all areas with tree canopy coverage below 50% is marked with a “∗” to highlight that, in some circumstances, they
can also work the other way round.
4.2.3 UGI Typologies and Expected Temperature Change
To define different typologies of UGI, the three components of tree canopy coverage, soil cover, and size, were combined. To this end, tree canopy coverage is classified into five intervals: 0–20%, 21–40%, 41–60%, 61–80% and 81–100%. Soil
cover is classified into sealed (all impervious surfaces), bare soil, heterogeneous
cover (mixed cover of bare-soil and shrubs, typical of vegetable gardens or inner
courts or some vacant lots), grass (fine vegetation), and water, based on the
HERCULES soil-cover taxonomy (Cadenasso et al. 2007). Finally, size was divided
into two classes: below and above two hectares. By combining these classes of the
three components, 50 typologies of UGI are obtained to be further analysed considering the three different climatic regions.
Operationally, to assess the cooling capacity of each UGI typology in each climatic region, data on ET0 and Kc was retrieved from a number of databases, including the CGMS database of the Mars Crop Yield Forecasting System and the FAO
(more details in Zardo et al. 2017). Through a literature review, the cooling capacity
score of each UGI typology was then associated to an expected change in temperature (see Table 4.1). Indeed, the conversion of cooling capacity scores (from 0 to 100)
4.2 Methods to Assess the Cooling Capacity of UGI
Regarding the effects of shading and evapotranspiration on surroundings of trees
(based on measurements taken at 12 m and 5 m from trees), Akbari et al. (1992)
concluded that, for large areas, the cooling capacity depends mainly on evapotranspiration, reaching a distance as far as five times the height of the tree. They also
found that shading contributes up to 95% when directly under the canopy, but its
contribution in terms of reducing the temperature (and consequently the energy
consumption for air conditioning) is around 40% for areas larger than 2 ha.
According to Chang et al. (2007), size contributes to 60% of the cooling capacity,
and indirectly affects the contribution of ETA. Finally, Shashua-Bar and Hoffman
(2000), based on empirical studies, note that in areas smaller than two hectares, the
contribution of shading is around 80% of the total cooling capacity, with the remaining 20% determined by evapotranspiration.
Therefore, in the proposed approach, the overall cooling capacity of UGI is
assessed through a weighted summation of the evapotranspiration and shading
scores, using different weights according to size, followed by a standardization of
the results into a scale between 0 and 100. More specifically, in areas smaller than
two hectares, shading is assigned a weight of 0.8 and evapotranspiration of 0.2,
while in areas larger than two hectares, the weights are of 0.4 and 0.6, for shading
and ETA, respectively. Noteworthy is the case of areas with less than 50% of tree
canopy coverage that may turn to be warm islands instead of cool islands during
some part of the day in very hot summer (Chang et al. 2007). To consider this
remark, the cooling capacity scores calculated for all areas with tree canopy coverage below 50% is marked with a “∗” to highlight that, in some circumstances, they
can also work the other way round.
4.2.3 UGI Typologies and Expected Temperature Change
To define different typologies of UGI, the three components of tree canopy coverage, soil cover, and size, were combined. To this end, tree canopy coverage is classified into five intervals: 0–20%, 21–40%, 41–60%, 61–80% and 81–100%. Soil
cover is classified into sealed (all impervious surfaces), bare soil, heterogeneous
cover (mixed cover of bare-soil and shrubs, typical of vegetable gardens or inner
courts or some vacant lots), grass (fine vegetation), and water, based on the
HERCULES soil-cover taxonomy (Cadenasso et al. 2007). Finally, size was divided
into two classes: below and above two hectares. By combining these classes of the
three components, 50 typologies of UGI are obtained to be further analysed considering the three different climatic regions.
Operationally, to assess the cooling capacity of each UGI typology in each climatic region, data on ET0 and Kc was retrieved from a number of databases, including the CGMS database of the Mars Crop Yield Forecasting System and the FAO
(more details in Zardo et al. 2017). Through a literature review, the cooling capacity
score of each UGI typology was then associated to an expected change in temperature (see Table 4.1). Indeed, the conversion of cooling capacity scores (from 0 to 100)
4.2 Methods to Assess the Cooling Capacity of UGI