34
meters is here overlooked, as they do not provide shade that is useful for human
beneficiaries. Nevertheless, such vegetation has a significant contribution in terms
of evapotranspiration.
Tree canopy coverage, soil cover and tree species are the three main components
that jointly affect evapotranspiration (Taha et al. 1991; Akbari et al. 1992; Souch
and Souch 1993; Allen et al. 1998; Bowler et al. 2010b; Schwarz et al. 2011;
Larondelle and Haase 2013). In this context, the focus is however on the first two
components mainly because information of tree species is hardly available at city
scale, whereas, at this scale, differences in evapotranspiration across different combinations of species can be considered negligible (Souch and Souch 1993). On the
other hand, the climatic region is included as a crucial factor to consider given that
it greatly affects the evapotranspiration: in warm and dry areas, evapotranspiration
is more effective than in humid or cool climates (Taha et al. 1991; Akbari et al.
1992; McPherson et al. 1997; Bowler et al. 2010b).
Following the approach by Allen et al. (1998), the evapotranspiration is calculated as:
ET K ET
c
c
=
• 0
where ET c is the tree or soil cover evapotranspiration (ETA) under conditions of
unlimited presence of water in the ground (irrigated), K c is the tree or soil cover
coefficient, and ET 0 is the reference evapotranspiration, which takes into account
the climatic region of the study area.
Operationally, to estimate the evapotranspiration potential of an UGI, its soil
cover and tree canopy coverage are analysed separately to determine the related Kc
coefficient. Hence, the evapotranspiration is estimated by multiplying the Kc coefficient by the climate-specific value ET 0 , again analysing separately the contribution
of trees and of the soil cover (e.g., Kremer et al. 2013; Larondelle and Haase 2013;
Schwarz et al. 2011). In the proposed approach, the overall evapotranspiration value
of the UGI, obtained by adding the different contribution that are expressed in mm
d
−1
, is then standardized into an evapotranspiration score in the 0–100 range.
4.2.2 Cooling Capacity Assessment
The extent to which shading and evapotranspiration contribute to the overall cooling
capacity of the UGI is determined by the size of the UGI itself (Chang et al. 2007;
Cao et al. 2010; Bowler et al. 2010b). In fact, shading and evapotranspiration jointly
reduce the air temperature, but the impact of evapotranspiration becomes predominant as the area gets larger (Akbari et al. 1992). Specifically, green areas larger than
2–3 ha are cooler than their surroundings, whereas green areas smaller than 2 ha
have a limited effect (Chang et al. 2007). Thus, several studies identify the threshold
between small parks and large parks around a value of 2 ha (e.g., Bowler et al.
2010b; Cao et al. 2010; Chang et al. 2007; Shashua-Bar and Hoffman 2000).
4 Developing Ecosystem Service Models for Urban Planning: A Focus…
meters is here overlooked, as they do not provide shade that is useful for human
beneficiaries. Nevertheless, such vegetation has a significant contribution in terms
of evapotranspiration.
Tree canopy coverage, soil cover and tree species are the three main components
that jointly affect evapotranspiration (Taha et al. 1991; Akbari et al. 1992; Souch
and Souch 1993; Allen et al. 1998; Bowler et al. 2010b; Schwarz et al. 2011;
Larondelle and Haase 2013). In this context, the focus is however on the first two
components mainly because information of tree species is hardly available at city
scale, whereas, at this scale, differences in evapotranspiration across different combinations of species can be considered negligible (Souch and Souch 1993). On the
other hand, the climatic region is included as a crucial factor to consider given that
it greatly affects the evapotranspiration: in warm and dry areas, evapotranspiration
is more effective than in humid or cool climates (Taha et al. 1991; Akbari et al.
1992; McPherson et al. 1997; Bowler et al. 2010b).
Following the approach by Allen et al. (1998), the evapotranspiration is calculated as:
ET K ET
c
c
=
• 0
where ET c is the tree or soil cover evapotranspiration (ETA) under conditions of
unlimited presence of water in the ground (irrigated), K c is the tree or soil cover
coefficient, and ET 0 is the reference evapotranspiration, which takes into account
the climatic region of the study area.
Operationally, to estimate the evapotranspiration potential of an UGI, its soil
cover and tree canopy coverage are analysed separately to determine the related Kc
coefficient. Hence, the evapotranspiration is estimated by multiplying the Kc coefficient by the climate-specific value ET 0 , again analysing separately the contribution
of trees and of the soil cover (e.g., Kremer et al. 2013; Larondelle and Haase 2013;
Schwarz et al. 2011). In the proposed approach, the overall evapotranspiration value
of the UGI, obtained by adding the different contribution that are expressed in mm
d
−1
, is then standardized into an evapotranspiration score in the 0–100 range.
4.2.2 Cooling Capacity Assessment
The extent to which shading and evapotranspiration contribute to the overall cooling
capacity of the UGI is determined by the size of the UGI itself (Chang et al. 2007;
Cao et al. 2010; Bowler et al. 2010b). In fact, shading and evapotranspiration jointly
reduce the air temperature, but the impact of evapotranspiration becomes predominant as the area gets larger (Akbari et al. 1992). Specifically, green areas larger than
2–3 ha are cooler than their surroundings, whereas green areas smaller than 2 ha
have a limited effect (Chang et al. 2007). Thus, several studies identify the threshold
between small parks and large parks around a value of 2 ha (e.g., Bowler et al.
2010b; Cao et al. 2010; Chang et al. 2007; Shashua-Bar and Hoffman 2000).
4 Developing Ecosystem Service Models for Urban Planning: A Focus…