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Progress in Physical Geography
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The online version of this article can be found at:
DOI: 10.1177/0309133312461032
2013 37: 2 originally published online 6 November 2012Progress in Physical Geography
Andrew M. Coutts, Nigel J. Tapper, Jason Beringer, Margaret Loughnan and Matthias Demuzere
and improve human thermal comfort in the Australian context
Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling
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Article
Watering our cities: The
capacity for Water Sensitive
Urban Design to support urban
cooling and improve human
thermal comfort in the
Australian context
Andrew M. Coutts
Monash University, Australia
Nigel J. Tapper
Monash University, Australia
Jason Beringer
Monash University, Australia
Margaret Loughnan
Monash University, Australia
Matthias Demuzere
Catholic University Leuven, Belgium
Abstract
Urban drainage infrastructure is generally designed to rapidly export stormwater away from the urban
environment to minimize flood risk created by extensive impervious surface cover. This deficit is resolved by
importing high-quality potable water for irrigation. However, cities and towns at times face water restrictions
in response to drought and water scarcity. This can exacerbate heating and drying, and promote the devel-
opment of unfavourable urban climates. The combination of excessive heating driven by urban development,
low water availability and future climate change impacts could compromise human health and amenity for
urban dwellers. This paper draws on existing literature to demonstrate the potential of Water Sensitive
Urban Design (WSUD) to help improve outdoor human thermal comfort in urban areas and support Climate
Sensitive Urban Design (CSUD) objectives within the Australian context. WSUD provides a mechanism for
retaining water in the urban landscape through stormwater harvesting and reuse while also reducing urban
temperatures through enhanced evapotranspiration and surface cooling. Research suggests that WSUD
features are broadly capable of lowering temperatures and improving human thermal comfort, and when
integrated with vegetation (especially trees) have potential to meet CSUD objectives. However, the degree
Corresponding author:
Andrew M. Coutts, Centre for Water Sensitive Cities and School of Geography and Environmental Science, Monash
University, Clayton, Australia.
Email: Andrew.Coutts@monash.edu
Progress in Physical Geography
37(1) 2–28
ªThe Author(s) 2012
Reprints and permission:
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DOI: 10.1177/0309133312461032
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of benefit (the intensity of cooling and improvements to human thermal comfort) depends on a multitude of
factors including local environmental conditions, the design and placement of the systems, and the nature of
the surrounding urban landscape. We suggest that WSUD can provide a source of water across Australian
urban environments for landscape irrigation and soil moisture replenishment to maximize the urban climatic
benefits of existing vegetation and green spaces. WSUD should be implemented strategically into the urban
landscape, targeting areas of high heat exposure, with many distributed WSUD features at regular intervals to
promote infiltration and evapotranspiration, and maintain tree health.
Keywords
Climate Sensitive Urban Design, green infrastructure, human thermal comfort, urban heat island, vegetation,
Water Sensitive Urban Design
I Introduction
Global cities and towns have been facing some
significant environmental challenges over
recent years including extreme weather
events, drought and flooding. Such challenging
circumstances have been evident across
Australian cities and towns. In the week of 26
January 2009 to 1 February 2009, Melbourne
experienced a record heat wave, with maximum
temperatures exceeding 43C over three con-
secutive days from 28 to 30 January, peaking
at 45.1C on 30 January. Health authorities
determined that there were 374 excess deaths
over the week-long period: an increase of 62%
(DHS, 2009) over the number expected. For
heat-related illnesses (heat stroke, heat syncope
and dehydration), there was an 8.4-fold increase
in emergency department presentations, and
during the hottest three-day period emergency
ambulance dispatches increased by 46%(DHS,
2009). Adelaide also experienced a heat wave
around this period, with a mean maximum
temperature of 41C over 26 January to 3 Febru-
ary and a maximum of 45.7C (Mayner et al.,
2010). During this period, emergency depart-
ment presentations increased by approximately
18%(Mayner et al., 2010). The elderly popula-
tion (over 65 years) are particularly vulnerable
to extreme heat (Van Iersel and Bi, 2009) which
is of concern given Australia’s ageing popula-
tion (Australian Government, 2010). Projec-
tions of future climate change from global
warming in Australia suggest that there will be
an increase in the frequency of heat wave
events, but also their intensity, duration and
extent (Alexander and Arblaster, 2009).
In addition to the risk from heat waves, the
presence of the urban heat island (UHI) is
likely to exacerbate the effects of extreme heat.
The replacement of natural, vegetated land-
scapes with impervious infrastructures leads
to excess heat storage which is slowly released
at night. This, along with waste heat from
anthropogenic activities and reduced radiative
cooling in urban canyons, supports warmer
urban temperatures. Research by Morris and
Simmonds (2000) for Melbourne found that
between 1973 and 1991 and when anticyclones
were positioned over the southeast coast of
Australia (which supported optimal UHI
genesis), mean UHI intensities were 3.56C.
Torok et al. (2001) observed a peak UHI inten-
sity of 7.1C between the CBD and the city’s
rural outskirts at 9 pm on the evening of 26
August 1992. Coutts et al. (2010) present an
UHI profile for 1 am on 26 March 2006 show-
ing a peak UHI intensity of around 4C
between the inner city and the rural outskirts.
The UHI keeps minimum temperatures in
developed areas higher than in rural areas, and
can restrict night-time recovery from daily heat
stress. Nicholls et al. (2008) examined heat and
mortality relationships in Melbourne for those
over 65 years from 1979 to 2001. They found
that when daily minimum temperatures
Coutts et al. 3
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exceeded 24C average daily mortality for
those over 65 increased by 19–21%(Nicholls
et al., 2008). A combination of urban develop-
ment and associated increases in UHI intensity,
along with a projected increase in hot nights
(Alexander and Arblaster, 2009) and warming
from climate change, is likely to increase
population exposure to higher temperatures,
and compromise the comfort and well-being
of urban inhabitants (Smith and Levermore,
2008).
Adding vegetation in urban areas has been
shown to reduce urban temperatures and is
regularly cited as a key mechanism for UHI
mitigation (Lynn et al., 2009; Rosenfeld
et al., 1998; Rosenzweig et al., 2009; Zhou and
Shepherd, 2010) as well as climate change
adaptation (Gill et al., 2007). Bowler et al.
(2010) reviewed studies that investigated the
effects of green space on temperature and
found that urban greening in the form of parks
and trees may act to cool the environment. The
review also found that both shading and eva-
porative cooling played a role in lowering
urban temperatures (Bowler et al., 2010). What
receives less attention is the role of water in
influencing urban climates through both irriga-
tion and the support of urban vegetation.
Impervious urban surfaces prevent infiltration,
and runoff is rapidly exported away from urban
environments via the stormwater network. This
produces a deficit of water in urban areas, and
reduces soil moisture levels – a deficit that is
oftenbalancedbyimportedpotablewaterfor
irrigation. Unfortunately, much of Australia
experienced extended dry periods over the last
two decades, particularly in the southern cities
of Perth, Adelaide and Melbourne, which have
placed pressure on city water resources. In a
bid to manage potable water supplies from cen-
tralized sources, State Governments intro-
duced various compulsory and voluntary
strategies to encourage water saving across the
community – including outdoor water restric-
tions. Residents have become highly diligent
in saving water in response to outdoor water
restrictions, but existing urban vegetation has
also struggled with reduced water availability.
Residents have adapted gardening approaches
to cope with less potable water supplies by
planting more drought-tolerant species, and
new developments are more commonly
designed with xeric style gardens. Each of
these factors of drought, water restrictions and
xeric gardens, along with the reduced health of
urban vegetation, may further exacerbate
urban warming and energy demands (Larson
et al., 2009).
An approach that could be implemented to
assist in dealing with these challenges is
improved stormwater management, through
Water Sensitive Urban Design (WSUD).
WSUD involves technologies and approaches
that aim to retain water in the urban land-
scape through stormwater harvesting and fit-
for-purpose reuse and infiltration into soils
to meet ecological, social and financial
objectives. These objectives include: reduc-
tions in stormwater pollutant loads; mainte-
nance of pre-development hydrology; visual
amenity; water provision for irrigation during
periods of water restrictions; reduced need
for downstream stormwater infrastructure;
and supplementing of centralized water sup-
plies (Hatt et al., 2006). This paper provides
an assessment from existing literature, of the
potential for WSUD to mitigate against the
effects of urban heating and uncomfortable
thermal environments brought about by the
UHI, climate change and more extreme heat
waves/events. The reintegration of storm-
water into the urban landscape through
enhanced infiltration and irrigation using
harvested stormwater has the capacity to
increase soil moisture, increase water avail-
ability for urban vegetation and, along with
green infrastructure, provide a mechanism for
improving urban climates. The set of circum-
stances experienced across Australian urban
environments provides a unique context for
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an assessment of the effectiveness of WSUD,
and demonstrates that urban climate improve-
ment through Climate Sensitive Urban
Design (CSUD) should be added to the list
of objectives for WSUD. Universal guide-
lines are presented on how WSUD might be
most effectively applied to achieve more
thermally comfortable urban environments.
II Water Sensitive Urban Design
and Climate Sensitive Urban
Design
The definition of WSUD varies among practi-
tioners across Australia; however, we use the
definition provided by the National Water
Initiative – ‘the integration of urban planning
with the management, protection and conserva-
tion of the urban water cycle, that ensures urban
water management is sensitive to natural hydro-
logical and ecological processes’ (National
Water Commission, 2004: 30). WSUD is also
commonly known as Low Impact Development
(LID) in the USA or Sustainable Urban
Drainage Systems (SUDS) in the UK (Morison
et al., 2010) and aims to minimize the hydrolo-
gical impacts of urban development (Lloyd
et al., 2002), specifically targeting stormwater.
In a demonstration of the ability of WSUD to
minimize the hydrological impacts of urban
development, Dietz (2007) showed that LID
approaches could retain stormwater on site
(rather than being exported away) and therefore
mimicked pre-development hydrological
function. The definition of WSUD is broaden-
ing towards the consideration of the integrated
management of the urban water cycle with
urban planning and design (Wong, 2006) but
in practice still tends to focus on stormwater
management. WSUD for stormwater manage-
ment involves the collection, treatment and
storage of stormwater through features such as
vegetated bio-retention systems, porous
pavements, wetlands, bio-swales rainwater
tanks and distribution through irrigation
(Fletcher et al., 2008) (Figure 1).
Climate Sensitive Urban Design (CSUD)
(also known as Climate Responsive Design or
Bio-climatic Design) is not as straightforward,
and there is a paucity of definitions available.
Simply interchanging the word ‘water’ for
‘climate’ in the definition of WSUD would
imply that the ‘pre-development’ climate of the
region should be maintained or restored which
is unrealistic and impossible to achieve at the
micro-scale. There are also examples of urban
environments that are more environmentally
acceptable to urban residents than surrounding
rural landscapes (e.g. a well irrigated and vege-
tated city surrounded by desert) and infrastruc-
ture can serve to enhance urban environments
(e.g. building shading). Emmanuel (2005b)
states that the goal of CSUD is captured by the
term ‘bio-climatic design’ and is essentially
about designing for the human being, leading
Climate Sensitive Urban Designers and archi-
tects towards a focus on thermal comfort
(Emmanuel, 2005a). However, CSUD also
requires architecture to be in harmony with
nature (Emmanuel, 2005b) and Oktay (2002)
also suggested designing with a sense of place,
taking into consideration the positive and nega-
tive aspects of a site. Grimmond et al. (2010)
suggest insensitive building developments
require unsustainably large energy resources
to keep inhabitants comfortable. CSUD tends
to have two primary objectives: (1) CSUD in
building design to create more energy-efficient
buildings and comfortable indoor environments
(e.g. Manioglu and Yilmaz, 2008; Okeil, 2010;
Strømann-Andersen and Sattrup, 2011); (2)
CSUD in landscape design to create more
attractive outdoor spaces that target improve-
ments in human thermal comfort (HTC) (e.g.
Erell et al., 2011; Johansson, 2006; Keeble
et al., 1991; Mayer and Ho¨ppe, 1987; Oke,
1988a). This involves designing for tempera-
ture, humidity, wind, and solar access (environ-
mental determinants of HTC; Ho¨ppe, 1999) to
Coutts et al. 5
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maintain thermal equilibrium of the human
body with the environment (Erell et al., 2011).
We suggest that ‘Climate Sensitive Urban
Design creates thermally comfortable, attrac-
tive, and more sustainable urban environments
by enhancing positive natural and man-made
features through architecture, planning and
landscape design’.
Given the Australian context, we propose
that WSUD can provide a mechanism for sup-
porting Climate Sensitive Urban Design by pro-
moting localized cooling at the micro- to local-
scale (Figure 1) through enhanced evapotran-
spiration and surface cooling, and, when inte-
grated across the city, limit UHI intensities
and improve HTC. Figure 1 emphasizes how the
Figure 1. Schematic representation of widespread implementation of stormwater harvesting and Water
Sensitive Urban Design elements at the micro-scale in the restoration of a more natural water balance, along
with increased vegetation cover. This enhances urban evapotranspiration and shading resulting in local-scale
cooling effects that can improve human thermal comfort. (See colour version of this figure online).
6Progress in Physical Geography 37(1)
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implementation of WSUD elements at the
micro-scale restores a more natural water bal-
ance in the urban environment. WSUD
features enable infiltration, thereby supporting
water loss in the outdoor urban environment
through subsurface flows and evapotranspira-
tion, rather than surface runoff (exported storm-
water). We propose that WSUD features
promote micro-scale cooling effects through
evapotranspiration and surface cooling, and
when implemented extensively across the urban
landscape (e.g. Figure 1 – micro-scale), can pro-
mote local-scale cooling (e.g. Figure 1 – local-
scale). This paper reviews previous research to
determine the capacity of WSUD elements to
provide such cooling effects and improve
human thermal comfort, and how to maximize
these cooling effects through urban design.
III Urban climate, vegetation and
water
Urban energy and water balances are con-
nected through evapotranspiration (Grimmond
et al., 1991) and the magnitude of evapotran-
spiration influences the partitioning of convec-
tive energy and hence many of the urban
climate features observed (Grimmond et al.,
2010). Evapotranspiration can directly modify
the urban water balance (Figure 1) and subse-
quently flow through to modifications of the
urban energy balance, which is a fundamental
controller of urban climates. Hence, there is a
potential for modifying evapotranspiration to
control/mitigate urban climate. The urban
energy balance for a surface layer urban vol-
ume is given by:
QþQF¼QHþQEþDQSþDQAðWm2Þ
where Q* is the net all-wave radiation (solar
and terrestrial radiation), Q
F
the anthropogenic
heat flux, Q
H
the sensible heat flux (atmo-
spheric heating), Q
E
the latent heat flux (or
evapotranspiration), DQ
S
the net storage heat
flux, and DQ
A
the net horizontal advective heat
flux (Grimmond et al., 2010). The urban water
balance is given by:
PþI¼EþDþDSðmmh1Þ
where Pis the precipitation, Ithe piped water
supply, Ethe evapotranspiration, Dthe drainage
(comprising stormwater and wastewater) and
DSis the net change in water storage of the
urban system (Mitchell et al., 2008). Hence the
connecting component is evapotranspiration
(Q
E
¼E). Manipulating the water balance
through WSUD to enhance evapotranspiration
(Figure 1) modifies the energy balance, reduc-
ing sensible heat fluxes (or atmospheric heat-
ing). While vegetation cover is considered a
primary control on the urban energy balance,
water availability receives less consideration.
Local-scale observations of the urban surface
energy balance using micrometeorological
techniques (Grimmond et al., 2004; Peters
et al., 2011; Spronken-Smith, 2002) has assisted
in quantifying rates of evapotranspiration.
General patterns have emerged between urban
land use, vegetation cover and evapotranspira-
tion – with comparative studies demonstrating
that, generally, as vegetation cover increases,
evapotranspiration increases (Christen and
Vogt, 2004; Offerle et al., 2006). Increasing
evapotranspiration then limits energy partition-
ing into urban heat storage, as well as sensible
heating of the atmosphere. As such, increasing
vegetation is commonly suggested as a primary
mechanism for UHI mitigation. However, there
are exceptions to this generalization and, as Oke
(1988b) describes, there are spatial and tem-
poral complexities, with a key factor in this
appearing to be water availability. Coutts et al.
(2007) found that for three local-scale suburban
tall-tower sites in Melbourne, Australia, evapo-
transpiration rates were relatively low given the
amount of vegetation cover at the sites (35–
47%) and attributed this to the drought and
presence of water-use restrictions. Increases in
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water availability through irrigation have been
found by Oke and McCaughey (1983) to
increase evapotranspiration rates by 40%in a
suburban neighbourhood in Vancouver,
Canada, compared to a nearby rural landscape
during the day. Meanwhile, Grimmond and Oke
(1995) found that variability in evapotranspira-
tion rates across four North American cities was
influenced by vegetation, precipitation and
irrigation and suggested that an inverse relation-
ship between the mean daytime Bowen ratio
(the ratio of sensible heat flux to latent heat flux)
and irrigation may exist. Urban climates are
strongly connected to vegetation cover and
water availability, with vegetation being an
important conduit for water loss to the air (Oke
et al., 1989).
While increasing vegetation cover is com-
monly cited as a key UHI mitigation approach,
an inherent assumption is that vegetation
(including trees and grass) is healthy and water
is available to support transpiration and thriving
canopies to promote shading. In urban areas,
limited water availability due to export of
stormwater, restricted irrigation and drought
can leave vegetation highly stressed. Urban
street trees also face high heat and radiation
loads (Oke et al., 1989), and high vapour
pressure deficits , which can constrain stomatal
conductance (Chen et al., 2011) and can lead to
leaf senescence (Munn-Bosch and Alegre,
2004), restricting transpiration. If tree stress is
very high and if the right meteorological condi-
tions prevail, trees can lose a proportion of their
canopy coverage, reducing leaf area index
(Hsiao, 1973) and transpiration, and become
less efficient at shading urban surfaces
(Shashua-Bar et al., 2010a). Extreme conditions
can lead to embolism and death (Gaspar et al.,
2002). This compromises the ability of vegeta-
tion to act as a cooling mechanism, which is
concerning given that it is commonly during
extreme conditions when the cooling effects of
trees are needed. Ensuring adequate water
availability for tree root systems is critical to
maintaining tree health, especially under addi-
tional pressures from climate change (e.g. heat
and drought) (Allen et al., 2010) and urban
development.
Choosing the most appropriate tree species to
promote cooling while also ensuring tolerance
to current and future climate conditions will
be critical, but there is very little data on tree
physiological controls and responses in Austra-
lian urban environments. Pataki et al. (2011a)
state that tree water is important in influencing
vegetative cooling and has been shown to be
highly species-specific in urban forests. The
choice of tree species and density of tree plant-
ing is important in the context of CSUD and
sustainable water management, but current
understanding of different tree species’ water
use and ‘climate performance’ is limited (Peters
et al., 2010). Thought should also be given to the
resilience of tree species under climate change.
Peters et al. (2010) undertook an assessment of
tree transpiration from dominant tree species in
a suburban neighbourhood of Minneapolis-
Saint Paul, Minnesota, USA, and found that
evaporative responses to climate change in
urban ecosystems were likely to depend in part
on species composition. Pataki et al. (2011b)
warns that large-scale tree planting may place
additional pressure on already constrained
water supply systems due to irrigation require-
ments. Therefore, species selection, along with
fit-for-purpose alternative water sources (e.g.
stormwater) could play a critical role in main-
taining healthy urban vegetation and maximiz-
ing urban cooling.
Stormwater is an abundant source of water in
urban areas and stormwater runoff in Australian
cities can be similar to, or even higher than, the
total potable water consumption (PMSEIC,
2007). In 2010, Melbourne’s annual potable
water consumption sourced from water supply
catchments was 356 GL, while the total avail-
able stormwater runoff was 463 GL (LVMAC,
2011). At present traditional stormwater infra-
structure (i.e. drainage) rapidly exports
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stormwater away from the urban environment
which promotes dry urban landscapes. Separate
stormwater and wastewater removal systems in
Australian cities makes stormwater harvesting a
viable option for providing augmented water
supplies that are fit for the purpose of irrigation
and use in WSUD. The hypothesis is that using
WSUD to reintegrate stormwater back into the
urban landscape helps to restore the water
balance and influence the urban climate by
modifying the urban radiation budget and
surface energy balance. This in turn drives the
environmental parameters that influence human
thermal comfort. While this sounds promising
theoretically, there is limited empirical evi-
dence demonstrating the climatic benefits of
WSUD.
Figure 2 theorizes the key processes involved
in developing urban micro-climates (modified
after Oke, 2009) during warm summertime
conditions between a conventional (water lim-
ited) urban landscape (Figure 2, a and c) and a
water sensitive urban landscape (Figure 2, b and
d), which each exert environmental influences
on HTC (e.g. temperature, humidity, mean
radiant temperature and wind speed). During
the day when aiming to limit heat stress,
promoting shading and limiting atmospheric
heating are important for creating a more com-
fortable thermal environment. The water sensi-
tive scenario (along with healthy vegetation)
serves to increase shading, evapotranspiration
and reduce surface temperatures, thereby reduc-
ing Q
H
and radiative loadings on pedestrians, as
well as supporting an overall reduction in Q
G
(Figure 2b). This is in contrast to a more con-
ventional urban landscape (Figure 2a) where
water is limited and vegetation health is
compromised. Under this arrangement, Q
H
dominates, and intense surface heating and
reduced shading supports higher radiative load-
ing on the human body. This also increases
energy demand for cooling, increasing Q
F
.
At night, promoting long-wave cooling and
ventilation is important for creating a more
comfortable thermal environment, and the
water sensitive scenario (having generally
stored less heat during the day) is less conducive
to supporting urban canopy layer warming
(Figure 2d) than the conventional urban layout
(Figure 2c).
The following synthesis draws on existing
research to demonstrate current knowledge on
climatic benefits of WSUD and support the
processes presented in Figure 2. This review
focuses primarily on the outdoor thermal envi-
ronment, taking into consideration temperature,
humidity, wind and solar access – and the
effects of vegetation. The scope of this synthesis
is primarily limited to empirical observational
studies that attempt to quantify the CSUD
benefits within the urban canopy layer, of
increasing water availability and implementing
WSUD type features throughout the landscape.
The scale of interest focuses primarily at the
micro-scale, as this is the scale of implementa-
tion of WSUD, up to the local-scale where the
effects of landscape irrigation may be seen.
Given that vegetation is commonly integrated
with WSUD, this synthesis also briefly touches
on observational studies that attempt to quantify
the CSUD benefits of vegetation to understand
its role as a component of WSUD (detailed
reviews have focused on vegetation previously;
e.g. Bowler et al., 2010; Chen and Wong, 2009).
Selected modelling studies have also been
included to supplement observational studies,
but only when the study focuses specifically
on WSUD or irrigation. Studies that draw solely
on remote sensing have not been included.
Emmanuel and Johansson (2006) propose that
there is a need for more assessment of climate
sensitive design approaches through both
observational and modelling studies. Given the
growing support for WSUD, it is important to
assess the additional benefits for UHI mitigation
and CSUD from increased water availability,
and to provide insight into how to maximize
these benefits through architecture and land-
scape planning.
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Figure 2. Generalization of key processes in the formation of urban micro-climates during summer for
conventional (water limited) urban landscapes (a and c) and water sensitive urban landscapes (b and d). Day
(a and b) and night (c and d) conditions are presented. Surface radiative and energy balance processes are
presented with arrows denoting direction and relative strength of fluxes. The relative level of human thermal
comfort experienced in each case is also presented by the human and the corresponding human thermal
comfort (HTC) scale. In the conventional urban landscape during the day, a combination of high sensible
heat fluxes and strong radiative heat loading results in a hotter environment for urban dwellers. In contrast,
the water sensitive landscape provides higher water availability to soils and waterways, along with healthy,
full canopied vegetation, compared to conventional water limited, xeric urban landscape. WSUD increases
evapotranspiration and shading, and reduces surface temperatures, limiting radiative and heating loads
resulting in improved human thermal comfort. Reduced heat storage during the day is beneficial at night,
as less energy is available to support ongoing low-level atmospheric warming. Cooler outdoor environ-
ments along with reduced heat transfer into buildings limits the need for indoor air conditioning and asso-
ciated anthropogenic heating. Other factors may also be influential, but are not presented here, such as air
pollution effects on radiation and wind flows. (See colour version of this figure online).
Source: Modified after Oke (2009).
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IV Potential urban climate benefits
of WSUD
Understanding the climate benefits of WSUD is
important, as a number of State and Local Gov-
ernments in Australia are encouraging the
implementation of WSUD through policy and
practical actions to support WSUD objectives.
For example, the Tasmanian State Stormwater
Strategy sets out key principles and standards
for managing stormwater, such as in new urban
developments where ‘All new developments
that create 500 m
2
or more of additional imper-
vious surface, including subdivisions, roads and
other large developments, should incorporate
best practice stormwater management’
(DIPWE, 2010: 2). In the State of Victoria,
Clause 56.07 of the Victorian Planning Provi-
sions requires new residential subdivisions of
two or more lots to incorporate ‘integrated water
management’ principles. This includes the
management objectives for urban runoff of
Clause 56.07-4 which aims ‘To minimize
increases in stormwater run-off and protect the
environmental values and physical characteris-
tics of receiving waters from degradation by
urban run-off’ (VPP, 2006: 2). The following
sections explore empirical evidence for support-
ing the inclusion of CSUD as an objective of
WSUD implementation as highlighted above.
1 Insights from studies on vegetation and
green space
WSUD commonly integrates vegetation into its
design, or provides an alternative water source
(e.g. rainwater tank) for irrigation of vegetation.
There are few studies that consider the climatic
effects of WSUD itself, but insights can be
drawn from studies of vegetation and green
space to infer the likely benefits supporting
CSUD. At the micro-scale, trees have been
shown to be particularly beneficial in lowering
urban temperatures (Tsiros, 2010) and improv-
ing HTC (Georgi and Dimitriou, 2010) due to
both transpiration and shading. Important
determinants of the intensity of cooling from
vegetation include tree location, size and
canopy coverage, planting density and irrigation
management (Pataki et al., 2011a; Shashua-Bar
et al., 2010a) and as much as 80%of the cooling
effect of trees is from shading (Shashua-Bar and
Hoffman, 2000). ‘Grey’ infrastructure (e.g.
buildings and urban infrastructure) is also
important in determining the effectiveness of
vegetation for urban cooling. Trees are particu-
larly effective in east–west oriented street
canyons (Ali-Toudert and Mayer, 2007b) and
in wider, shallower street canyons (Shashua-
Bar et al., 2010a) because they provide more
shading under these arrangements. Water
consumption for irrigation of vegetation is crit-
ical, as water and landscape managers are likely
to seek efficient water use while maximizing
cooling, even when stormwater is used.
Shashua-Bar et al. (2009) compared the water
efficiency of different common urban cooling
approaches, including irrigated grass, shade
trees, and shade mesh in two adjacent court-
yards at the Sede-Boqer campus of Ben-
Gurion University, Israel. They found that a
combination of irrigated grass and shade trees
was the most effective in cooling, while the
vegetation alone achieved the highest cooling
efficiency per amount of water used (Shashua-
Bar et al., 2009). This research demonstrates
how effective vegetation is in urban cooling,
and begins to draw out the most effective
strategies for maximizing cooling effects.
Analysis of the role of green spaces in urban
areas provides further insight into how WSUD
features at a range of scales may act to support
CSUD. Upmanis et al. (1998) have shown that
parks can be several degrees cooler than the
surrounding urban area, in a feature known as
the Park Cool Island (PCI). PCI intensity is
often largest at night (like the UHI) and tends
to increase with park size (Upmanis and Chen,
1999). Parks with extensive tree coverage tend
to be cooler during the afternoon due to shading
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effects, while more open parks with turf are
cooler at night due to greater long-wave radia-
tive cooling (Figure 2) (Chang et al., 2007;
Spronken-Smith and Oke, 1998). Hence, impor-
tant controls on PCI intensity include the park’s
size and characteristics, the nature of the
surrounding landscape, the macro-scale cli-
mate, and irrigation rates. PCIs are often best
developed in drier, hotter climates, because
evapotranspiration is enhanced due to oasis
effects and because of more intense heating of
the urban landscape surrounding the park which
supports greater urban-park temperature con-
trasts. This point emphasizes the importance of
the surrounding urban landscape, as PCI are a
comparative measure, and PCI intensity is
determined not only by the characteristics of the
park, but also by the characteristics of neighbour-
ing urban surfaces. Jauregui (1990) observed
that, after sunrise, Chapultepec Park in Mexico
City was actually warmer than nearby urban
areas because of the high thermal mass of urban
materials which took longer to warm up, though
the night-time PCI was around 2–3C. PCIs can
also exert a downwind cooling effect (Figure 1)
which Spronken-Smith and Oke (1998) suggest
extends to about one park width, but the influ-
ence reduces rapidly with distance away from the
park boundary. The downwind extent is influ-
enced by wind speed and direction, layout and
height of surrounding buildings, and the LAI of
parks (Chen and Wong, 2006). Implementation
of WSUD should be fit-for-place, drawing on
information such as this to strategically design
WSUD into urban landscapes to maximize cool-
ing (Figures 1 and 2).
While many studies tend to focus heavily on
mitigating elevated temperatures, CSUD should
consider all environmental determinants of
HTC. For instance, Clarke and Bach (1971)
compared the environmental parameters of
HTC above grassed and paved surfaces in both
downtown Cincinnati and a suburban area 27
km away. They found that during the day
micro-climates above the grassed surfaces were
consistently more comfortable, and the differ-
ences were larger in the downtown streets
(Clarke and Bach, 1971). While temperatures
under tree canopies may be reduced (especially
during the afternoon), there is evidence that at
times, relative humidity can be elevated, as is
vapour pressure during afternoons and evenings
(Souch and Souch, 1993). This was shown in a
study of a PCI in Mexico City during the rainy
season where again, temperatures were lower
compared to surrounding areas, but humidity
(vapour pressure) can be higher. However, the
net effect on HTC is an overall improvement
because the benefits from reduced temperature
are larger (Barradas, 1991). During the day,
mean radiant temperature is the single most
important meteorological factor influencing the
human energy balance in summertime condi-
tions (Matzarakis et al., 2007) and hence has a
major influence on HTC. Therefore, trees can
provide a substantial improvement to HTC by
blocking solar radiation (Picot, 2004), even if
air temperature reductions are negligible
(Shashua-Bar et al., 2010b). Once again, while
vegetation and green space can have a positive
influence on HTC, it is just one component of
the landscape that influences the human thermal
experience. Street design (including height to
width ratio) and orientation also have an influ-
ence on HTC by influencing shading and wind
flow patterns (Ali-Toudert and Mayer, 2007a;
Johansson, 2006), so Climate and Water Sensi-
tive Urban Design needs to consider both the
natural and built components in designing urban
landscapes. As highlighted earlier, more
research is needed into tree physiological con-
trols and responses in Australian urban environ-
ments, and identifying plant species that can
maximize improvements in HTC by the assess-
ment of plant traits such as LAI, transpiration
rates and resilience to climatic extremes.
Pataki et al. (2011a) suggest that while the
many commonly cited environmental benefits
of urban green space are still poorly supported
by empirical evidence, there is a high potential
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for urban cooling from urban green space. What
is less clear is the influence of reintegrating
water back into the urban landscape and the
capacity for WSUD to support urban cooling
and improving HTC. The provision of water for
outdoor urban environments and irrigation can
be met through potable water supplies, recycled
water and/or harvested stormwater. In the fol-
lowing sections, we focus on the impacts of pro-
viding fit-for-purpose water supply through
stormwater harvesting and WSUD. This
includes the common WSUD approaches of:
stormwater harvesting for supporting irrigation;
treatment wetlands and open water bodies;
bio-retention systems; infiltration systems; and
green roofs and green walls.
2 Stormwater harvesting for supporting
irrigation
Stormwater harvesting involves the collection
and storage of runoff in urban areas. The
simplest form of stormwater harvesting
involves gutter-pipe systems such as rainwater
tanks, while more complex storage systems may
include constructed wetlands, where urban
runoff can be stored for treatment and drawn
on for irrigation. Stormwater harvesting can
also involve treatment trains, where runoff may
be treated through a bio-filtration system before
storage in a tank, providing higher-quality water
for multiple end uses. Aquifer Storage and
Recovery is another and more advance
approach, where underground aquifers are arti-
ficially recharged by infiltrating stormwater
through permeable media or via direct injection,
and the water is recovered later for indoor and
outdoor use (Khan et al., 2008). These
approaches for stormwater harvesting all
provide a source of water for irrigation.
Large-scale irrigation has the potential to
influence the climate (Oke, 1987) through
modification of the surface energy balance, and
studies have shown that irrigation is an impor-
tant driver of climate variability. Increasing the
amount of irrigated vegetation is likely to
increase evapotranspiration and reduce heat
storage through shading (Figure 2), and hence
support night-time cooling (UHI mitigation)
(Gober et al., 2010). Harvested stormwater can
provide a fit-for-purpose source of water for
landscape irrigation, and when liberally applied
to the landscape can support high evapotran-
spiration rates (Oke and McCaughey, 1983).
Well watered green spaces such as irrigated
parks can support oasis effects, which can occur
whenever there is a cool, moist surface that is
dominated by larger-scale warmer and drier
surroundings. At the smallest scale, this could
be an isolated tree in an urban street (Oke,
1987). This has been observed for both irrigated
parks (where evapotranspiration rates were
three times that of the surrounding residential
neighbourhood) (Spronken-Smith et al., 2000)
and irrigated suburban lawns (Oke, 1979; Suck-
ling, 1980).
In a study of variability of neighbourhood
microclimates during hot, summer conditions
in Boulder, Colorado, Bonan (2000) found that
irrigation mitigated surface heating during the
day, and early afternoon air temperatures in
irrigated areas were cooler than those in non-
irrigated and urban areas. Irrigation also
increases the capacity of the soil to act as a heat
sink and supports lower surface temperatures
through evaporative cooling, in contrast to
impervious surfaces where water cannot infil-
trate. Mueller and Day (2005) showed that sur-
face, soil and canopy air temperatures for a 6
11 m irrigated turf grass (mesiscape) plot were
cooler during the day than concrete, asphalt and
gravel xericscape plots because of evapotran-
spirational cooling, while humidity (vapour
pressure) was also higher above the mesiscape.
This highlights the potential for evaporative
cooling in reducing localized air temperatures,
but may increase humidity. Grossman-Clarke
et al. (2010) investigated the role of irrigation
for cooling during extreme heat events in Phoe-
nix using the Weather Research and Forecasting
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Model (WRF), in conjunction with the Noah
Urban Canopy Model, and found that irrigation
provided cooling of 0.5–1K in maximum
daytime and minimum night-time temperatures
across most of the area, but was more evident in
areas of extensive mesic vegetation (*2K)
than areas of xeric land use (*0.5 K).
While increasing soil moisture may support
surface cooling during the day, higher soil water
contents can actually serve to increase the heat
capacity of soils, meaning they may not cool
as rapidly at night compared to dry soils. In a
comparative study of different parks in Sacra-
mento and Vancouver, Spronken-Smith and
Oke (1998) suggest that irrigation is also an
important control on the intensity of PCIs. In
Sacramento, the state of water availability
(along with shade and surface albedo) was an
important control of park surface temperature
during the day, with the relative coolness of irri-
gated green space favouring PCI development
through evaporative effects. Interestingly, sur-
face temperatures of dry parks during the day
could actually be higher than the surrounding
urban neighbourhoods (Spronken-Smith and
Oke, 1998). In contrast, surface temperatures
of drier parks cooled more rapidly at night due
to lower soil thermal admittance, and appeared
important in supporting slightly larger night-
time PCIs (Spronken-Smith and Oke, 1998).
Meanwhile, Mueller and Day (2005) observed
during summer that canopy air temperatures in
their xeriscape plot during the day were higher
than asphalt and concrete, but were not signifi-
cantly different from the mesiscape plot at
night. So, while irrigation may serve to slightly
slow surface cooling of soils at night, irrigation
is beneficial in reducing surface heating of dry
pervious surfaces during the day (which are
more prominent during periods of drought and
water restrictions) and xeriscapes (rising in
response to low water availability). WSUD
technologies can provide low-energy, decentra-
lized stormwater storage, and fit-for-purpose
water supply for use in landscape irrigation, and
complement centralized supplies to avoid a
trade-off between water consumption and urban
climate degradation. However, the capacity to
meet irrigation needs through stormwater har-
vesting will vary across Australian cities
depending on the regional climate and the city’s
water demand regime. In Brisbane, for instance,
with a subtropical climate, the greatest demand
for irrigation during the year coincides with the
period of highest rainfall. In contrast, Mel-
bourne’s temperate climate leads to a more
seasonal irrigation demand, while rainfall
patterns are more uniform throughout the year,
so storages need to be designed accordingly
(Mitchell et al., 2006).
3 Treatment wetlands and open water
bodies
In order to meet stormwater quality objectives,
such as those prescribed by Victoria’s Clause
56 regulations, a common approach of develo-
pers is to integrate stormwater treatment wet-
lands into residential subdivisions. This also
allows developers to meet public open space
requirements, and wetlands currently tend to
have lower establishment and maintenance
costs than more dispersed source control
methods like vegetated bio-filtration systems.
Similar to the downwind cooling effects of
parks (Upmanis et al., 1998), wetlands are also
likely to provide downwind cooling influences
– a feature known as the Lake Effect (Saaroni
and Ziv, 2003). Similar to irrigated urban parks
(Spronken-Smith et al., 2000), open water
bodies provide a source of moisture to support
oasis effects during the day, especially when the
area is dominated by larger-scale warmer and
drier surroundings (Oke, 1987) of the urban
environment. Even water features (e.g. foun-
tains) have the potential to alleviate high urban
temperatures through enhanced evaporation
(Smith and Levermore, 2008).
A growing number of studies investigate the
effects of open water bodies (whether they are
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treatment wetlands, ponds, rivers or water
features) on the climate of urban areas. Gener-
ally, studies have shown that temperatures
adjacent to and downwind of water bodies are
reduced by around 1–2C compared to sur-
rounding areas nearby, with a maximum tem-
perature reduction seen during the day (Chen
et al., 2009; Nishimura et al., 1998; Saaroni and
Ziv, 2003). Chen et al. (2009) attributed this
temperature reduction to evaporation from the
water body, while Saaroni and Ziv (2003) attri-
bute the downwind cooling influence of the
pond (1.6C at midday) in the warmest part of
the day to reduced sensible heat flux, as the lake
surface was cooler than the surrounding park’s
grass cover and the daytime cooling effects
were evident under both hot-and-dry and sultry
weather conditions. The study by Saaroni and
Ziv (2003) of a 100 m wide pond within an
urban park in Tel Aviv, Israel, also demon-
strated higher humidity and a lower heat stress
during the day downwind of the pond. However,
later in the day when the grass cover cooled to
lower temperatures than the lake surface, eva-
porative cooling was the main driver for
temperature reductions, but this increased
humidity, and therefore increased the heat stress
(Saaroni and Ziv, 2003). These findings suggest
that wetlands implemented for stormwater treat-
ment may also support downwind cooling
effects during the day, but potential cooling
effects are dependent on the condition of adja-
cent micro-climates.
Urban waterways (e.g. rivers, creeks) are also
open water bodies that could provide a down-
wind cooling effect, and cities and towns have
traditionally been established near rivers
because of the social and economic values they
provide. Rivers are often an iconic feature of
Australian cities, such as the Swan River in
Perth or the Yarra River in Melbourne. How-
ever, due to rapid development and impervious-
ness, smaller waterways have been piped and
buried to facilitate stormwater removal. ‘Day-
lighting’ of urban waterways is considered
WSUD, and research by Kim et al. (2008) com-
pared conditions before and after the restoration
of a 5.8 km stretch of the Cheonggye Stream in
Seoul, Korea. They estimated a 0.4C average
(0.9C maximum) temperature drop during the
day over the stream area itself after the restora-
tion (though synoptic-scale and local-scale
weather conditions during the two periods were
not the same), and observed a downstream cool-
ing effect of up to 1C. In Hiroshima, Japan,
Murakawa et al. (1991) observed downwind
cooling effects from the Ota River of at least a
few hundred metres. Air temperatures near the
270 m wide river were some 3–5C cooler
(between 12 noon and 5 pm) than the surround-
ing area on fine days and the extension of local
cooling from the river was more widespread
when building density was lower and streets
were wider (Murakawa et al., 1991). Daylight-
ing of urban streams and maintaining existing
waterways in new green-field developments has
potential for supporting downwind cooling
effects during the day. However, management
of urban development in the catchment across
Australian cities is also critical to avoid rapid
stormwater removal during rain events, which
leaves stream base flows extremely low during
dry periods. Restoring a more natural water
balance through widespread implementation of
WSUD and flow-regime management through-
out the catchment can support higher base flows
in urban streams (Burns et al., 2012) to support
cooling when needed.
In contrast to urban parks where the PCI is
most pronounced at night, these studies suggest
that lake effects are more pronounced during the
day. This is because water bodies can maintain
warmer temperatures at night due to the high
heat capacity and thermal inertia of water. This
leads to the possibility of treatment wetlands
and open water bodies actually providing a
relative warming effect at night (unless eva-
porative cooling dominates) but, compared to
urban surfaces, they may still provide a cooling
effect. As with the effects of irrigation, water
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bodies may serve to increase humidity down-
wind (Saaroni and Ziv, 2003), but more research
is needed in quantifying the overall effects on
HTC. While Saaroni and Ziv (2003) observed
lake effects even under sultry weather condi-
tions, it is likely that evaporative cooling will
be more pronounced in Australian cities with
warmer and drier climates. While studies sug-
gest that the distance of the downwind cooling
effect of PCIs is similar to the park’s width, less
information is available on the range, intensity
and downwind influence of water bodies. As
with the cooling influence of PCIs, it is also
apparent that the design of the surrounding
urban landscape is important in maximizing
downwind cooling effects.
4 Bio-retention systems
Bio-retention systems have been shown to
reduce peak stormwater runoff flow rates and
volumes, promote infiltration and evapotran-
spiration, and improve water quality (DeBusk
et al., 2011). Bio-retention systems (or bio-
filtration systems, or rain-gardens) do this by
channelling stormwater into vegetated basins
or trenches, with porous filtration media. Water
flows through dense vegetation, and temporar-
ily ponds on the surface before slowly filtering
down through the filter media into underlying
soils, or the treated water is piped to down-
stream waterways or storage areas (FAWB,
2009). Grass swales, and vegetated filter or buf-
fer strips, act in a similar manner, serving to
slow stormwater down and encourage infiltra-
tion – and enhance evapotranspiration which
can modulate the urban microclimate (Mitchell
et al., 2008). Fletcher et al. (2008) observed that
on average 30–35%of inflow into a lined bio-
filtration system built at Monash University in
Melbourne, Australia, was lost via evapotran-
spiration. At the University of Guelph, Ontario,
Canada, Denich and Bradford (2010) con-
structed a 1.89 m
2
bio-retention system with a
lysimeter to measure rates of evapotranspiration,
and showed average daily evapotranspiration
rates 7.7 +0.5 mm day
-1
. Li et al. (2009) exam-
ined six bio-retention systems in Maryland and
North Carolina, USA. For two bio-retention sys-
tems in North Carolina, one system was lined to
prevent ex-filtration, while the other was not
lined – and differences in inflow and outflow
of the system could be attributed to evapotran-
spiration. Results showed that the amount of
water inflow lost to evapotranspiration was
around 19%(Li et al., 2009). This suggests an
important consideration for the design of bio-
retention systems (and all WSUD for that mat-
ter) because if CSUD is an objective of WSUD,
designs should focus on enhancing evapotran-
spiration. A study by Dietz and Clausen
(2005) found that for two rain-gardens in Had-
dam, CT, only 0.4%of the inflow was lost
through evapotranspiration.
While it is clear that bio-retention systems
can reduce stormwater runoff and encourage
both infiltration and evapotranspiration,
research is needed on optimal designs of
bio-retention systems to maximize their cooling
potential. Bio-retention systems need to be resi-
lient to a range of water availabilities seen
across Australian cities, as systems could expe-
rience both extended dry periods and flash
urban flooding. Irrigating bio-retention systems
could help sustain vegetation over drought
periods while ensuring the system provides a
cooling function over an extended period.
Design considerations should be given to vege-
tation types that support CSUD, where bio-
retention systems that support trees can achieve
the benefits of both enhanced evapotranspira-
tion and shading. Bio-retention tree-pits are an
example of this as they promote infiltration of
stormwater into the soil layers surrounding the
root zone of the tree – providing water for trees
to draw on for heat dissipation. Research is also
needed in matching the design of WSUD tech-
nologies with various plant functional types for
Australian urban climates to improve HTC.
Some bio-retention tree-pit designs promote
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surface ponding and saturate the root zone,
which may not be ideal for particular plant func-
tional types.
5 Infiltration systems
Infiltration systems act in a similar manner to
bio-retention systems, but are non-vegetated
technologies. Conventional urban impervious
materials absorb a large amount of solar radia-
tion during the day and, because there is no
evaporative cooling, surface temperatures of the
materials are high, produce a sensible heat
exchange from the surface to the atmosphere,
and support warmer air temperatures (Asaeda
and Ca, 2000). Porous pavements (or permeable
pavements) are a common example of an infil-
tration system and can be very useful where
vegetated systems are impractical: for example,
treating intensely polluted runoff from indus-
trial sites or urban surfaces such as car parks.
Infiltration systems can improve urban micro-
climates by reducing surface temperatures of
porous urban materials (Nakayama and Fujita,
2010) and enhancing evaporation compared to
conventional urban materials, yet provide the
same functionality. In a study of an area of water
permeable pavement at a test site in Coesfeld,
Germany, Starke et al. (2010) found that the
permeable pavement could hold up to 3.8 l/m
2
more water than the equivalent area of
impermeable pavement and evaporation rates
were 16%higher (Starke et al., 2010). Asaeda
and Ca (2000) compared four surface
materials – porous block pavement, dark
non-porous asphalt, natural grass and ceramic
porous pavement – over a period of two days
during hot conditions. They found that surface
temperatures of the ceramic porous pavement
at noon were similar to the lawn. However, the
porous block pavement was actually similar to
the asphalt and about 10C higher. This was
because the porous block made from coarse
grains was unable to retain water for a long time
and so absorbed large amounts of solar radiation
like the asphalt pavement (Asaeda and Ca,
2000). This highlights the need for optimal
design of WSUD systems to maximize their
potential for improving HTC. In an interesting
innovation, He and Hoyano (2010) investigated
the passive cooling effects of a constructed
cooling wall made of highly porous material
with high moisture absorption capacity. Using
capillary action, harvested stormwater that is
supplied to the base of the wall is drawn up wall
to a height of about 1 m. As air passes through
the wall, it provides an evaporative cooing ben-
efit, while also providing shading to reduce
radiative loading on pedestrians (He and
Hoyano, 2010). This passive cooling wall
essentially acts like a tree, drawing moisture
from beneath and acting as a conduit for water
loss to the air (Oke et al., 1989). These studies
demonstrate the capacity for urban materials
and technologies to mimic natural systems in a
way that can support CSUD; however, there is
little observational evidence of their climatic
effects.
6 Green roofs and green walls
Green roofs are probably the best studied
WSUD technology in terms of their contribu-
tion to CSUD. Research on green roofs has
demonstrated their ability to substantially
reduce heat flow through roofs (Liu and Minor,
2005; Niachou et al., 2001), reduce building
energy requirements (Getter and Rowe, 2006;
Solecki et al., 2005), aid in cooling roof surface
layers and internal spaces on warm days (Sim-
mons et al., 2008) and generally improve HTC
at both street level and roof level (Alexandri and
Jones, 2008). Chen and Wong (2009) have
extensively reviewed climatic effects of green
roofs and green walls. In Australia, very few
green roofs have been implemented partly due
to lack a lack of scientific data on their local
applicability (Williams et al., 2010) and
because of the harsh Australian climate. They
are primarily designed to meet stormwater
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quality objectives or amenity, rather than
CSUD. Sedum species and other low-growing
succulent vegetation types are commonly cho-
sen (MacIvor et al., 2011) because of their abil-
ity to survive in harsh, water stressed
environments. This is convenient during times
of water restrictions, and this plant selection
improves the chances of the roof remaining
‘green’. However, more research is needed on
plant types that can best survive Australian con-
ditions, as some Sedum species struggle under
extended periods of hot weather (Williams
et al., 2010), and a balance needs to be met
between plant survival and other green roof ben-
efits, such as transpirational cooling (Wolf and
Lundholm, 2008).
Studieshaveshownthatgreenroofscan
reduce air temperatures above the roof by sev-
eral degrees (Wong et al., 2003). However, if a
green roof is designed to maximize potential
for CSUD, an alternative vegetation selection
might be considered, with species that actively
transpire during the day to sustain evaporative
cooling. Transpiration aids in cooling the roof
surface while removing water from the grow-
ing medium (Lundholm et al., 2010). MacIvor
et al. (2011) investigated the performance of
dryland and wetland species in modules form-
ing an extensive green roof and found that the
dryland species generally out-performed the
wetland species in terms of plant cover, surface
temperature reductions, and higher albedo.
However, planting wetland species, particu-
larly K. polifolia, showed higher water losses
(an indirect measure of evapotranspiration),
but also lower water capture. Modules contain-
inggrowthmediumonlywerealsoinvesti-
gated and showed some of the best results for
water capture and loss (MacIvor et al., 2011).
High water use vegetation, and even Sedum
under Australian conditions, may require a
supporting irrigation regime (using harvested
stormwater) to achieve suitable plant coverage.
This may also be beneficial for building energy
consumption as wet soils can provide
additional insulating effects (Wong et al.,
2003) because of evaporation of water. Sfakia-
naki et al. (2009) found an average 1.4Csur-
face temperature reduction when a green roof
soil was watered compared to dry soil on a
sunny day in Athens, Greece. Lazzarin et al.
(2005) investigated the evaporative cooling
effects of a green roof in Vicenza, Italy, and
found that, for a dry soil in summer, heat trans-
fer into a building could be reduced by 60%
compared to traditional roofing, but evapotran-
spiration was very limited. Once the soil was
wet, heat transfer into the building was
removed, and even reversed, and soil surface
temperatures were reduced from as much as
55C when dry to below 40Cwhenwet(Laz-
zarin et al., 2005). Any green roof irrigation
regime would need to be carefully designed
to ensure that there is sufficient capacity in the
growth medium for water retention from rain-
fall events to meet stormwater runoff objec-
tives. If the bulk of the insulating benefit of
an extensive green roof comes from the soil
layer, more research is needed to determine the
additional cooling benefits of transpiring vege-
tation or regular watering to warrant additional
costs.
Green walls may also be considered to be
WSUD, but only when their design supports
stormwater objectives of attenuated flows and
pollutant removal. This can be achieved if green
walls sprout from bio-retention systems, or rain-
water is diverted through a vertical green wall
system. Monitoring of green walls or facades
have shown that they provide shade to minimize
heat transfer through walls, reduce wall tem-
peratures and act as a wind barrier (Pe´rez
et al., 2011; Perini et al., 2011; Wong et al.,
2009). Wong et al. (2010) compared eight dif-
ferent vertical greenery systems in the tropical
environment of Singapore and found maximum
temperature reductions of 11.6C compared to
the control wall under clear sky conditions.
High foliage density and healthy growth were
observed to be important in reducing wall
18 Progress in Physical Geography 37(1)
at Katholieke Univ Leuven on March 17, 2014ppg.sagepub.comDownloaded from
surface temperatures, but other factors such as
substrate type and substrate moisture condition
all influenced the thermal performance of verti-
cal greenery systems (Wong et al., 2010). Alex-
andri and Jones (2008) used a simple 2D model
to assess the thermal effects of green roofs and
walls across nine cities in different climates.
They conclude that, for all nine cities, green
walls were more effective in cooling tempera-
tures within the urban canyon than green roofs.
Green roofs and walls were most effective when
introduced into hotter and drier climates, and in
narrower streets (Alexandri and Jones, 2008).
More research in the Australian context is
needed to determine the optimal design of green
roofs and walls for micro-climate benefits under
Australian climatic conditions. Simmons et al.
(2008) suggest that green roofs need to be
designed according to specific performance
goals (such as CSUD) and Schroll et al. (2011)
argues that regionally relevant designs and plant
selections should match specific environmental
and management constraints. Also, the effective-
ness of green roofs and walls for urban cooling
across a range of urban densities has not been
clearly identified in terms of the benefits they
provide for canopy layer UHI mitigation and the
HTC benefits for pedestrians at street level.
Installing green roofs on skyscrapers, for
instance, is not likely to provide thermal benefits
at street level. The direct spatial cooling influ-
ence of a green roof is also difficult to assess.
7 Comparison of WSUD approaches
In probably the most targeted study of the
effects of WSUD on urban climates, Mitchell
et al. (2008) investigated the effects of urban
design and the impacts that a range of WSUD
strategies might have on urban air temperatures.
By linking urban water balance and urban
energy balance models, Mitchell et al. (2008)
undertook an analysis of six different WSUD
scenarios for the typical suburban area of Maw-
son, in Canberra, Australia, in comparison with
a conventional urban layout. These included the
addition of a 1.45 ha wetland, 2 ha of lined grass
swales, 100%conversion of roofs to unirrigated
green roofs, a full WSUD treatment train (roof,
swales and a wetland used in series), a full
WSUD treatment train along with a 50%reduc-
tion in garden watering, and a full WSUD
treatment train along with a complete cessation
in garden watering. Scenario predictions
showed that the full vegetated treatment
train resulted in the highest annual rates of eva-
potranspiration, increased by 55 mm yr
-1
above
that of the conventional urban layout (Mitchell
et al., 2008). Interestingly, complete cessation
of garden watering could be almost offset by the
introduction of a full WSUD treatment train as
annual evapotranspiration only decreased by 9
mm yr
-1
. Mitchell et al. (2008) then used a sim-
ple boundary layer model to infer impacts on air
temperature from changes in the convective
heat fluxes – compared to a control desert land-
scape (with no vegetation and no evapotran-
spiration). In comparison to a desert landscape
used as a baseline, the conventional urban lay-
out reduced air temperatures at 3 pm by 4.6C,
due to garden watering, and the scenario of a
50%reduction in garden watering along with
the full WSUD treatment train could achieve the
same amount of cooling (Mitchell et al., 2008).
The scenarios that included the large areal
extent of the implementation of vegetated roofs
were most effective at cooling, though Mitchell
et al. (2008) acknowledged the ambitious nature
of this scenario. Nevertheless, this study
demonstrates how the widespread implementa-
tion of different arrangements of WSUD at the
micro-scale can influence urban climates at the
local-scale (Figure 1) in an Australian city. If
such modelled temperature reductions are rep-
resentative of cooling potential of WSUD, then
this could have large implications for HTC and
heat-related illness and mortality for urban
populations across Australia. Evidently, more
research is needed in this field, drawing on
observational, modelling and remote sensing
Coutts et al. 19
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techniques to help quantify the benefits of
WSUD on HTC and design and test scenarios
of WSUD implementation to maximize cooling
in a practical and cost-effective manner.
V Conclusions – WSUD: a key
mechanism for enhancing human
thermal comfort
WSUD is a novel approach for helping restore
natural water balance regimes that are able to
support healthy urban vegetation, and purpose-
fully modify the urban energy balance to
support CSUD through enhanced evapotran-
spiration. While there is a strong theoretical
basis for why WSUD should help meet CSUD
objectives, and a small body of research devel-
oping that tends to support this argument, more
research is needed to quantify the intensity of
cooling and improvements to HTC from WSUD
features. This synthesis has outlined that WSUD
can support CSUD through three key mechan-
isms: (1) the creation of oasis effects through
enhanced evapotranspiration; (2) providing
water for healthy vegetation to support cooling
through shade and transpiration; and (3) sup-
porting a reduction in surface radiative tempera-
tures. However, WSUD is just one component
in the realization of CSUD, because the degree
of cooling or improvements to HTC will vary
not only on the design of WSUD features, but
also on the nature of the surrounding landscape
(such as urban geometry and morphology), city
form and climatic zone. Figure 3 sets the context
for the contribution of WSUD in improving
human thermal comfort, and identifies the
Figure 3. Conceptual diagram demonstrating the connection between Water Sensitive Urban Design and
the environmental parameters influencing human thermal comfort – through Climate Sensitive Urban Design
and urban land surface-atmosphere interactions.
20 Progress in Physical Geography 37(1)
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connections between the built and natural envi-
ronment in the urban landscape. Through
CSUD, and intentionally modifying urban land
surface-atmosphere interactions, urban planners
and architects can create thermally comfortable,
attractive and more sustainable urban environ-
ments. WSUD must be integrated with, and
delivered through, urban planning and design,
together with urban forestry, while considering
local and regional climates (Figure 3). Figure
3 also highlights the inherent complexity of
urban landscapes and necessary considerations
in creating thermally comfortable urban
environments.
The evidence suggests that WSUD provides
a mechanism to help address some of the long-
term challenges facing Australian urban envir-
onments, including extreme heat events and
drought. Implementing a mix of WSUD
approaches can help restore a more natural
water balance (Burns et al., 2012) and support
evapotranspiration (e.g. Mitchell et al., 2008)
across Australian cities. In addition, irrigating
green infrastructure and WSUD features can
further drive evaporative cooling and support
healthy vegetation. Limiting the occurrence
of extreme heat events and mitigating the UHI
will reduce the exposure of urban populations.
This is critical given anticipated urban growth,
future climate change and an ageing population
in Australia cities. As this review has high-
lighted, limiting exposure is as much about the
urban landscape itself as it is about WSUD
(Figure 3), so a mix of mitigation approaches
will be needed, including WSUD, to provide
a significant benefit to HTC and subsequently
human health. WSUD presents a particularly
attractive option due to the multiple benefits
it provides (e.g. augmenting water supplies,
improved stormwater quality runoff, and
amenity). However, inter- and intra-city urban
climates vary significantly across Australian
urban environments, as cities like Melbourne
and Sydney experience a more temperate cli-
mate while cities like Brisbane and Darwin
experience subtropical and tropical climates.
While evaporative cooling may not be particu-
larly strong under humid conditions, providing
water for vegetation to promote shading and
reduce surface radiative temperatures will still
be beneficial. Given the example of threshold
temperatures by Nicholls et al. (2008), even
small temperature reductions of 1–2Ccould
make a difference in mortality rates. Research
that helps quantify climatic improvements
fromWSUDcanhelppolicydevelopment
extend beyond urban runoff quality improve-
ments alone.
Maximizing the benefits of WSUD to help
meet CSUD objectives truly comes down to
smart architectural and landscape design as
highlighted in Figure 3. Specific guidance is
needed in urban planning and design on imple-
mentation of WSUD in the context of improving
HTC. Integrating WSUD into urban landscapes
relies on an understanding of use of space. Cool-
ing should be optimized based on when a space
is used, such as providing shading in public
spaces where daytime events are held. Using
WSUD to help achieve CSUD capitalizes not
only on the concept of fit-for-purpose storm-
water, but also on fit-for-place design solutions,
which consider local opportunities and con-
straints. Bowler et al. (2010) found in their
review of green space literature that the reported
research did not provide information on exactly
how vegetation should be best incorporated into
the urban landscape in terms of abundance, type
and distribution. This is currently also true for
WSUD, although snapshots of guidance are
provided. For example, Saaroni and Ziv
(2003) point out that water bodies, even of the
scale of 100 m, can reduce heat stress during the
day, and suggest that for maximum benefit they
should be located at the edge of a park or in the
middle of the city to provide a microclimatic
benefit to nearby built-up areas. To implement
fit-for-place design solutions, it is important
that the local context is well understood, as each
city is likely to have its own set of unique
Coutts et al. 21
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circumstances, as demonstrated by the Austra-
lian illustration here. Careful design of WSUD
is needed, along with recognized benchmarks
against which improvements are judged.
While this assessment of the capacity for
WSUD to meet CSUD objectives was placed
firmly within the Australian context, some
universal guidelines on the implementation of
WSUD for support of CSUD can be drawn from
this review. Considering the objectives of
CSUD of human thermal comfort, attractive-
ness and building energy efficiency, we suggest
that the implementation of WSUD should:
Aim to maximize the cooling potential of
existing green infrastructure first. WSUD
and stormwater harvesting can support fit-
for-purpose water for widespread landscape
irrigation to maintain healthy vegetation and
accentuate urban cooling and HTC benefits.
Irrigation is also likely to be particularly
effective due to the ability to apply water
across a large proportion of the landscape,
as well as controlling the times of irrigation.
Target dense urban environments with little
or no vegetation. WSUD is most effective
under warm and dry conditions. This is
convenient because these are commonly
areas of highest heat exposure that can place
vulnerable populations at risk. Areas of poor
energy efficiency should also be targeted to
improve indoor HTC and minimize energy
consumption.
Harness the cooling and HTC benefits of
trees. WSUD should be combined with
increased tree cover to maximize cooling
via both evapotranspiration and shading.
Trees are also an efficient means of water
use to provide cooling and HTC benefits.
Increased tree cover should also target areas
of high solar exposure.
Aim for many, smaller, distributed technol-
ogies and features at regular intervals
throughout the urban environment to retain
stormwater in the urban landscape and
promote widespread infiltration into soils.
Atmospheric cooling and HTC effects of
WSUD are likely to be highly localized, so
while a treatment wetland can meet storm-
water quality objectives for a residential
subdivision, it may only provide a HTC ben-
efit for those residents in the immediate
vicinity. Distributing WSUD throughout the
landscape should provide a larger areal
extent of cooling than large concentrated
green areas (Chen et al., 2011; Clarke and
Bach, 1971; Honjo and Takakura, 1990).
This approach is also aligned with stream
ecology benefits, where distributed infiltra-
tion of stormwater is facilitated to support
near-natural frequency of runoff to mini-
mize urban catchment effects on stream
health (Walsh et al., 2005).
Work with the built environment to accent-
uate cooling influences. WSUD should be
strategically designed into the urban land-
scape, with features implemented upwind
of target areas and where urban canopy layer
cooling is maximized (within urban canyons
where people reside). Urban spaces should
be sensitive to local and regional climatic
influences (such as sea breezes) and main-
tain natural cooling mechanisms such as
ventilation and trees.
More information on the climatic effects of
WSUD and especially on the extent of localized
cooling influences is needed. Information is also
needed on optimal design approaches to maxi-
mize evapotranspiration from individual WSUD
features and targeted irrigation regimes, while
meeting other WSUD objectives such as supple-
menting centralized water supplies. The impor-
tance of species selection has also been raised,
highlighting the synergies between water avail-
ability and climate exposure on tree physiology
and capacity for meeting CSUD objectives.
Seasonal effects of WSUD on urban climate also
need further consideration, whereas the focus
here has been on warm, summertime conditions.
22 Progress in Physical Geography 37(1)
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Finally, more research is needed on the positive
feedbacks to population health from WSUD
interventions and research tosupport cost/benefit
analysis of different WSUD implementation
approaches, particularly against other available
approaches for mitigating excess urban heating
such as increasing surface albedo. Nevertheless,
based on the evidence presented here, we suggest
that CSUD should be added to the list of objec-
tives for WSUD.
Funding
This research was funded by a large number of
government and industry partners as part of the
collaborative Cities as Water Supply Catchments
research program. A list of funding partners can be
found at http://www.waterforliveability.org.au/
?page_id¼89
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