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Addressing the Water–Energy–Food Nexus through Enhanced Green Roof Performance

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Urban densification and climate change are creating a multitude of issues for cities around the globe. Contributing factors include increased impervious surfaces that result in poor stormwater management, rising urban temperatures, poor air quality, and a lack of available green space. In the context of volatile weather, there are growing concerns regarding the effects of increased intense rainfalls and how they affect highly populated areas. Green roofs are becoming a stormwater management tool, occupying a growing area of urban roof space in many developed cities. In addition to the water-centric approach to the implementation of green roofs, these systems offer a multitude of benefits across the urban water–energy–food nexus. This paper provides insight to green roof systems available that can be utilized as tools to mitigate the effects of climate change in urbanized areas. A new array of green roof testing modules is presented along with research methods employed to address current issues related to food, energy and water performance optimization. Rainwater runoff after three rain events was observed to be reduced commensurate with the presence of a blue roof retention membrane in the testbed, the growing media depth and type, as well as the productive nature of the plants in the testbed. Preliminary observations indicate that more productive green roof systems may have increasingly positive benefits across the water–energy–food nexus in dense urban areas that are vulnerable to climate disruption.
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Sustainability 2021, 13, 1972. https://doi.org/10.3390/su13041972 www.mdpi.com/journal/sustainability
Article
Addressing the Water–Energy–Food Nexus through Enhanced
Green Roof Performance
Jeremy Wright 1, Jeremy Lytle 2, Devon Santillo 3, Luzalen Marcos 3 and Kristiina Valter Mai 3,*
1 Environmental Applied Science and Management Program, Ryerson University, 350 Victoria Street,
Toronto, ON M5B 2K3, Canada; jeremy.wright@ryerson.ca (J.W.)
2 Building Science Program, Faculty of Engineering and Architectural Science, Ryerson University,
350 Victoria Street, Toronto, ON M5B 2K3, Canada; jeremy.lytle@ryerson.ca (J.L.)
3 Department of Electrical, Computer and Biomedical Engineering, Faculty of Engineering and Architectural
Science, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada;
dsantillo@ryerson.ca (D.S.); lmarcos@ryerson.ca (L.M.); kvmai@ryerson.ca (K.V.M.)
* Correspondence: kvmai@ryerson.ca
Abstract: Urban densification and climate change are creating a multitude of issues for cities around
the globe. Contributing factors include increased impervious surfaces that result in poor stormwater
management, rising urban temperatures, poor air quality, and a lack of available green space. In the
context of volatile weather, there are growing concerns regarding the effects of increased intense
rainfalls and how they affect highly populated areas. Green roofs are becoming a stormwater man-
agement tool, occupying a growing area of urban roof space in many developed cities. In addition
to the water-centric approach to the implementation of green roofs, these systems offer a multitude
of benefits across the urban water–energy–food nexus. This paper provides insight to green roof
systems available that can be utilized as tools to mitigate the effects of climate change in urbanized
areas. A new array of green roof testing modules is presented along with research methods em-
ployed to address current issues related to food, energy and water performance optimization. Rain-
water runoff after three rain events was observed to be reduced commensurate with the presence
of a blue roof retention membrane in the testbed, the growing media depth and type, as well as the
productive nature of the plants in the testbed. Preliminary observations indicate that more produc-
tive green roof systems may have increasingly positive benefits across the water–energy–food nexus
in dense urban areas that are vulnerable to climate disruption.
Keywords: green roofs; stormwater management; urban agriculture; blue roofs; climate change
1. Introduction
Accommodating growing populations within urbanized areas is a difficult task due
to the fact that most cities have already developed the available land. This creates an in-
herent pressure to optimize the amount of livable/usable space within a building, while
still minimizing impact on the surrounding environment. There is increased interest for
new buildings to utilize the technology of green roofs (GR) to provide a tangible economic
and environmental return for both building owners and city communities as a whole.
Although GRs are considered to have multiple benefits, one of the main drivers of the
implementation of green roofs is to combat stormwater.
Green roofs have been used as stormwater management tools in Europe for decades,
with widespread implementation originating in Germany in the late 1960s [1]. A properly
constructed GR system serves two primary functions related to stormwater management;
a GR can reduce the amount of stormwater runoff through its water retention capacity, as
well as delay the peak flow of runoff, alleviating the pressures on stormwater
Citation: Wright, J.; Lytle, J.;
Santillo, D.; Marcos, L.; Mai, K.V.
Addressing the Water–Energy–Food
Nexus through Enhanced Green
Roof Performance. Sustainability
2021, 13, 1972. https://doi.org/
10.3390/su13041972
Academic Editor: Elena Cristina
Rada
Received: 7 December 2020
Accepted: 8 February 2021
Published: 11 February 2021
Publisher’s Note: MDPI stays neu-
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tutional affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
tivecommons.org/licenses/by/4.0/).
Sustainability 2021, 13, 1972 2 of 13
infrastructures. These characteristics and effects of GR substrate layers and drainage ma-
terials have been extensively studied [2,3].
A portion of water that is retained in a GR is transferred into the atmosphere through
evapotranspiration (ET) by the vegetation, and by evaporation from the soil surface, thus
reducing the volume of water that flows to a city’s Waste Water Treatment Plants (WWTP)
[4]. Water retention and ET characteristics are affected by the storage capacities of a roof,
the presence and amounts of soil and plants, and by the weather conditions and climate
[5–8]. GRs with deeper soil or growing media, when used for the growth of food crops,
are known as productive GRs. These GRs also have enhanced insulation values, reducing
building energy consumption. Thus, GRs have the potential to address the water–energy–
food (WEF) nexus, in which multiple benefits and sector interactions must be considered
to effectively optimize designs [9].
1.1. Sustainable Development and Green Roofs in Toronto
Toronto has had a large impact on GR implementation in North America. A study at
Ryerson University quantified the potential financial and environmental benefits of green
roofs, which spurred the creation and adoption of a GR bylaw in 2009. This bylaw man-
dates implementation of GRs on buildings exceeding a certain gross floor area (GFA), and
requires a minimum initial retention (abstraction) of rainfall of 5 mm [10]. Since the crea-
tion of this bylaw, multiple North American cities have adopted similar policies that uti-
lize GR’s as a stormwater management tool.
Due to the focus of the GR bylaws around an initial abstraction of rainfall, the major-
ity of projects that are required to install a green roof adopt a bare minimum approach to
these requirements. Synthetic growing media layers composed of woven fabrics, mineral
wool or rock wool [11] instead of soil are often used. These layers focus on meeting the
required water retention capacity while decreasing the added structural requirements for
GRs, ultimately driving the short-term costs down. There is no regulation or validation
that the systems are maintaining the required retention rates over their lifespan. Without
proper assessment and regulation of the long-term stormwater performance of the GR, it
is likely that these systems will not provide sustainable outcomes over the long run.
Considering the finite amount of roof space available within urban areas, GRs should
be designed to maximize their environmental benefits and sustained effectiveness over
their lifespan. Innovation within the GR industry such as the growth of vegetables, in-
creased water retention and support of water harvesting practices should be included in
updated policies.
1.2. Green Roofs and Water
Among the United Nations Sustainable Development Goals (UNSDG), Goal 6 is
Clean Water and Sanitation. Over 40% of the world’s population is affected by water scar-
city, making water management techniques imperative.
GR systems could potentially retain and evaporate an average 60% of annual rainfall [12].
A blue roof void can also create substantial water retention benefit that can be utilized both
under the GR and in hardscaping the area surrounding the vegetation [13].
Multiple studies on the ET of extensive green roofs have been conducted; most ET
studies for GRs utilize lysimeters in isolated GR modules to determine changes in weight.
The presence of small, drought-tolerant plants (such as sedums), has been determined to
cause a maximum daily ET rate of 2.52 mm/day [13]. Green roof systems that utilize pas-
sive irrigation methods synergized with blue roof technology, such as through a wicking
material between the water retention layer and the soil media, have shown increases in
ET rates and overall retention capacities [14]. Increasing the amount of water available to
the plants through capillary action resulted in an ET increase of 2 to 4 mm, leading to
higher plant productivity and a more effective building cooling rate [15]. This increase in
ET also holds potential to reduce the carbon payback period of GR systems, by countering
the embodied energy in the system materials with the reduced energy required for
Sustainability 2021, 13, 1972 3 of 13
building cooling [16]. In the context of ET rates and vegetables, it has also been shown that
an increase in soil depth results in higher ET rates [17]. These interactions between food
and water and energy demonstrate the multi-faceted potential that green roofs hold to
promote sustainability, to reduce the urban heat island helping to make cities livable, and
to mitigate climate change.
An abundant amount of research encompasses the hydrological performance of ex-
tensive green roof systems [18–21]. However, there is an opportunity to research the effect
that different drainage layers, soil depth, and vegetation types have on ET rates and an
overall stormwater benefit. By establishing an array of blue, green and blue-green roof
testbeds, this study aims to answer some of these questions. This research seeks to con-
tribute knowledge on how enhancing the water balance of a GR can increase the environ-
mental benefit of roofs in general.
1.3. Green Roofs and Energy
The UNSDG Goal 7 is Affordable and Clean Energy. Energy currently contributes
approximately 60% of global greenhouse gas emissions.
The insulative properties and passive cooling capacities of soil-based GRs have an
inherent benefit to reducing energy consumption in buildings [22]. Buildings with GRs
experienced lower indoor temperatures in the summer and higher indoor temperatures
in the winter in comparison to buildings with non-vegetated roofing assemblies [23].
Green roofs reduce energy consumption by altering the energy balance of buildings.
GRs also reduce energy expenditures within WWTPs. Treatment facilities for
wastewater are often considered a major energy consumer within an urban area; 33% of
municipal energy consumption can be attributed to WWTP in the Greater Toronto Area
(GTA) [24,25]. Reducing the amount of stormwater loading to a WWTP will ultimately
result in less energy consumption particularly in cities like Toronto that utilize a combined
sewer system.
Irrigation systems can also affect building energy consumption, as shown through
thermal analysis of GRs [26,27]. As soil depth decreases, energy consumption during win-
ter increases up to 140%. Annually, energy use is affected by soil depth, vegetation height
and leaf area index (LAI). In summer, a decrease in vegetation height and LAI increases
energy use for cooling. For extensive GRs, there is a decrease in cooling energy require-
ment as irrigation flow rate increases.
The aim of this study is to investigate the WEF nexus through observing water bal-
ance variances across different GR types, which can reduce energy consumption while
supporting food production.
1.4. Green Roofs and Food Security
Among the UNSDG, Goal 2 is Zero Hunger. Current estimates show 8.9% of the
world’s population experiencing hunger, with the numbers increasing over the past 5
years around the world. Local food production could contribute to mitigating these dev-
astating conditions.
Despite the economic activity in Toronto, the city also holds refuge to large numbers
of households that experience varying levels of food insecurity [28]. Urban agriculture is
a growing sustainable trend that addresses food insecurity by decreasing the dependency
that urbanized areas have on food systems outside of the city [29]. Green roofs have the
capacity to support farming practices, as exhibited by the Ryerson Urban Farm (RUF),
which generated 9000 lbs of produce over the 2017 growing season [30]. Although a single
rooftop farm does not have the productivity to eradicate food security issues for an entire
city, it does have the capacity to alleviate some of the localized pressures on food demand.
Multiple urban agriculture entrepreneurs and programs are now emerging in Toronto
[31], including the Ryerson Urban Farm Living Lab [32], which will help to address ques-
tions related to farming management, job creation, and maintenance including planting,
irrigation, harvesting, and soil optimization.
Sustainability 2021, 13, 1972 4 of 13
The current Toronto GR bylaw does not promote the inclusion of rooftop farming.
This could be partly due to insufficient local data on the hydrological performance of roof-
top farms. From a practical standpoint, maintenance approaches and structural limita-
tions must be considered and optimized. In connection to the feasibility of GRs to produce
vegetables, Stovin et al. [33] monitored the runoff retention of GR testbeds and showed
that the testbeds with vegetation had better capacity to remove moisture. Deeper, vegeta-
ble-supporting GRs address the WEF nexus through increased water retention, enhanced
insulative properties and urbanized food production.
2. Materials and Methods
The GR systems in this study consisted of an extensive GR (4” growing media), an
extensive GR combined with a blue roof (BR) spacer, and two rooftop farm plots growing
various vegetables. The primary objective of this study is to determine the enhancement
of green roof water retention and evapotranspiration (ET) rates through different subme-
dia drainage systems, and how these systems can address the WEF nexus.
2.1. Overview of Lab Layout
Five testbed modules were constructed on the rooftop of a building within the down-
town core of Toronto, Canada. The property is a three-storey combined office and resi-
dential building with a conventional flat roof approved and built to accommodate the
structural capacities of a GR. The main characteristics of each module are summarized in
Table 1.
Table 1. Test module characteristics.
Extensive (P1) Extensive +
Blue (P2) Aggregate (P3) Productive (P4) Productive +
Blue (P5)
Media Extensive Blend Extensive Blend Dolomite Farm Blend Farm Blend
Drainage Device Drainage Board Blue Roof Drainage Board Drainage Board Blue Roof
Plant Material Sedums Sedums None
Carrot-Beet rota-
tion; clover
cover crop
Carrot-Beet rota-
tion; winter rye
cover crop
Media Depth 100 mm 100 mm 25 + 55 mm infill 175 mm 175 mm
Total System Depth 130 mm 170 mm 90 mm 220 mm 240 mm
Drainage Retention 8 mm 50 mm 18 mm 11 mm 50 mm
Media Retention 35 mm 35 mm 0 mm 79 mm 79 mm
Total Retention 43 mm 85 mm 18 mm 90 mm 129 mm
Saturated Mass 128 kg/m2 175 kg/m2 115 kg/m2 239 kg/m2 278 kg/m2
The test modules were constructed with a footprint of 0.72 m2 (1.2 m × 0.6 m) each
and simulate higher slopes associated with typical flat roofs (4%). The dimensions of the
testbeds were designed to support a drainage reservoir underneath that has the storage
capacity of 50 L; otherwise considered as the equivalent to 67 mm of runoff from rain
events. Each of the testbeds utilize GR-growing media that follow the FLL guidelines, in-
cluding a porosity of 60% and an averaged density of 850 kg/m3 [17]. This was to establish
evapotranspiration and retention rates that can be correlated to multiple GR systems that
are currently being sold in the marketplace.
Sustainability 2021, 13, 1972 5 of 13
Two of the modules are planted with typical GR vegetation composed of drought-
tolerant succulents (sedums) to mimic typical industry practice for an extensive GR. To
determine contrasting water balances provided by vegetable production, two of the mod-
ules are “productive” plots and consist of vegetable plantings. A fifth control module uti-
lizes a 60 mm blue roof spacer with crushed white dolomite to simulate an area of a roof-
top that does not sustain vegetation; under the current green roof construction guidelines
there is a 500 mm wide vegetation-free zone around the perimeter of green roofs [34]. The
layers of the modules are described below in Table 1, with corresponding water retention
specifications.
Test module P1 is considered as the base control group as the industry standard ex-
tensive GR. This system includes water retention in both a base protection layer (5 mm)
and a three-dimensional drainage layer (25 mm). On top of these water retention capaci-
ties, 100 mm of extensive growing media provides additional water retention. Plug
planted sedums, which come in a tray of 72 plants were planted at approximately 25
plants per m2. This system, commonly used amongst various green roof suppliers, meets
green roof construction standards [35].
Test module P2 is similar to P1 with respect to growing media depth and vegetation
but has a contrasting water retention system, as shown in Figure 1. A blue roof (BR) stor-
age void of 65 mm, creates a water retention capacity beneath the growing media, sepa-
rated by a filter sheet to keep the growing media out of the sublayer. Green roofs com-
bined with BR voids are a growing submarket within the GR industry. The benefits of this
type of system are an extensive GR with minimal structural requirements compared to
greater soil depths, but a maximized amount of water retained within the system. Previ-
ous research conducted in the Netherlands concluded that the additional water retention
provided by BR voids contributes to higher ET rates and thus increased mitigation of the
urban heat island [14].
Figure 1. Extensive GR module P2 blue roof sublayer with capillary wicking elements.
The third test module (P3) uses a BR spacer under crushed dolomite, as shown in
Figure 2; this aggregate is typically used in construction materials and is mined across
several locations within Ontario [36].
Test module P4 is considered the baseline productive control group utilizing vegeta-
bles in place of the traditional sedum vegetation used in GRs in Toronto. This assembly
includes 200 mm of growing media, on top of a 40 mm drainage board with an increased
retention capacity; this is the suggested system buildup provided to support vegetable
production on roofs [37]. The system buildup employs a “Wicking Mat” that utilizes ca-
pillary action to increase the distribution of submedia water retention and to increase the
ET rates of the green roof plants [38]. A key effect of the capillary methods of increasing
Sustainability 2021, 13, 1972 6 of 13
moisture in the system is potentially less demand on traditional irrigation systems. Alt-
hough the media blend satisfies FLL requirements, there is an increased percentage of
organic matter in this blend in order to sustain life for the vegetables [39]. Multiple species
of produce will be harvested from this module to simulate crop rotations of the Ryerson
Urban Farm, located a few blocks away. Based on the increased growing media and re-
tention system depth, it is hypothesized that there will be a clear enhancement of the re-
tention capacity of this system. This information is important for future policy decisions
made by cities and could motivate decision makers to incentive rooftop farms rather than
sedum-based GRs.
Figure 2. White dolomite aggregate in module P3.
The final test module (P5), shown with P4 in Figure 3, is also defined as a productive
module that supports vegetable production. The P5 system utilizes a deeper blue roof
retention layer.
Figure 3. Modules P4 and P5 showing beet and carrot plant growth.
Sustainability 2021, 13, 1972 7 of 13
2.2. Description of Lysimeter System
External drainage reservoirs for each of the modules collect the water that flows out
of the system, as shown in Figure 4. To establish the retention capacities of the test mod-
ules and the associated ET rates, a lysimeter was implemented. Digital load cells were
utilized under the four corners of the bed to measure weight to obtain the mass transfer
of water in the modules; there were also 3 loads cells measuring the weight of the drainage
reservoir in order to determine how much rainfall was not retained in the module. Be-
tween these seven sensors the water flow in and out of the modules can be obtained, iso-
lating ET as differences in mass transfer. The data from the load cells were obtained by a
Pi Zero microcomputer, combined with an 8-channel multiplexer to provide communica-
tion for all cells on the same i2c bus [40]. To mitigate the risks of water damage, all elec-
tronics were contained within a PVC conduit assembly. The lysimeter system with the
testbed assembly is shown in Figure 4.
Figure 4. Overflow reservoir and lysimeter system to monitor the hydrological behaviour of the green roof testbed.
2.3. Description of Data Acquisition
All data were transferred via the building’s WiFi to the Google cloud platform, al-
lowing for remote access of information, partly due to the location of the lab as well as the
SARS-CoV-2 pandemic restricting onsite presence. All data were organized into an SQL
database enabling adaptable analysis [40].
3. Results
3.1. Stormwater Retention
Since installation, there has been minimal irrigation provided to the plants in the ex-
tensive modules. This is to build up the species resilience in the microclimate, as well as
to simulate a typical extensive GR implementation which generally does not include irri-
gation.
Figure 5 shows the amount of overflow measured in each of the drainage reservoirs
after three successive rainfall events. The water levels were measured in mm in the trans-
lucent containers, and the reservoirs were emptied after each measurement. As can be
seen, the extensive module P1 and the BR aggregate module P3 show stormwater runoff,
with other modules having no runoff at all after the first two (moderate) rainfall events.
The 7th of August reflects a rare rain event with massive thunderstorms occurring in the
two days prior to the visit to the rooftop. Upon inspection, the drainage reservoir under
the P3 module was filled to the rim, with the lid floating on top—as the container had
been forced open. Ultimately, the only number that can be used to reflect this overflow is
the height of the bin, 152 mm, though it was abundantly clear that much more rainfall
Sustainability 2021, 13, 1972 8 of 13
than this had passed through the non-GR dolomite module P3. After this heavy storm
event, productive module P5 still had zero runoff.
Figure 5. Water levels in the drainage reservoirs reflecting runoff measures over three separate rainfall events in modules
P1–P5.
Lysimeter loading data from September, 2020 are presented in Figure 6 across daily
intervals for the P1 extensive plot; the net mass includes gains from rainfall and losses
from ET. From this daily average dataset it can be observed, for example, that after rainfall
on day 9 ET slowly declined as a function of reduced available moisture content in the
growing media as expected. From these data, the ET rate of the P1 module averaged 2.27
mm/day, with a maximum of 4.42 mm measured on September 15th, as reported previ-
ously by our team [40].
Sustainability 2021, 13, 1972 9 of 13
Figure 6. Extensive grow bed P1 mass changes resulting from rainfall and evapotranspiration.
3.2. Agricultural Productivity
Each productive module (P4 and P5) was planted with alternating rows of carrots
and beets in two identical crop rotations. Growth was charted by measuring the height of
several plants on each measurement day and taking the average. The growth curves of
the two harvest cycles over a period of 2.5 months can be seen in Figure 7.
Figure 7. Height of vegetable plant in productive modules P4 and P5 over two harvest cycles.
Sustainability 2021, 13, 1972 10 of 13
4. Discussion
The modules presented in this work provide the ability to test a wide range of GR
systems for optimization of WEF-related factors. They can provide quantifiable patterns
and levels of response in key variables (e.g., stormwater storage/retention/runoff, ET, and
growth/productivity).
The urban agriculture productive test modules P4 and P5 demonstrated zero and
almost zero overflow during three consecutive rain events. This demonstrates the poten-
tial for green roof urban agriculture as a tool for stormwater management as well as food
production and energy use reduction (through the reduced load on municipal WWTPs
and through reduced building cooling requirements in summer resulting from increased
ET). In addition, the urban agriculture modules produced two harvests of hyper-local
food contributing to reduced food transportation. These two productive test modules to-
taled only approximately 1.5 m2 of growing area, and need to be scaled up to achieve a
meaningful contribution to the population food requirements.
The preliminary results of the ET rates of the P1 extensive green roof module ob-
tained from the lysimeter load cell data are similar to other ET research on sedum-based
roofs found during the literature review [41]. It is anticipated that the productive modules
will yield similar results to the study performed by Whittinghill et al. while having a
higher ET rate compared to the sedum roofs [42]. The potential for productive GRsto ex-
hibit higher retention and ET rates was observed in productive modules P4 and P5, which
had almost zero overflow, and demonstrated high-level stormwater management. How-
ever, additional studies are required to validate stormwater management and ET on larger
scales.
Using the blue roof test modules to determine the overall water balance of systems
that harvest water (P2 and P3) could also hold the opportunity to influence future green
roof policies in cities such as Toronto. The research to date describes the benefit of utilizing
ponding elements on drains underneath green roofs in synergy with blue roof voids. The
results can be seen directly in the way that module P2 had minimal runoff when compared
to P1. This demonstrates the benefit of adding retention sublayers under conventional
extensive green roofs for stormwater management. Even with its comparably high vol-
umes of runoff, the aggregate blue roof system holds value to existing rooftops that may
not have the capacity to support a vegetated assembly but can still mitigate the effects of
stormwater runoff. The preliminary results of the manually measured runoff volumes in
the water reservoirs do support the hypothesis that roofs with deeper retention systems
and deeper growing media will result in decreased runoff. The addition of porous grow-
ing media and living plants to urban surfaces allows rainwater to be absorbed and evap-
orated locally, limiting the load on city infrastructure.
Enhanced ET could be provided by the vegetables compared to the sedum vegeta-
tion. Additional load cell data will be needed to compare the profile of P2 to the produc-
tive modules P4 and P5, and to evaluate trade-offs with structural, maintenance and cost
factors. There is a strong correlation between our demand for food, energy and water and
overall demand is increasing. The relationship of how the domains of water, energy and
food interact is often described as a nexus [43]. As the demand for both food and energy
trends upwards, so does the requirement for water within these systems [44].
Urban agriculture also reduces the carbon footprint of food supply chains by decreas-
ing the distance traveled for produce [45]. From the economic aspect, there is a positive
Net Present Value (NPV) when there is a short food supply chain on GR production [46].
Despite a limited growing season, there is additional savings in energy through the insu-
lation value produced by plant ET. Thus, productive GRs will help reduce energy con-
sumption and increase food security.
GRs are effective tools to manage stormwater [47] and this research provides insight
on how these tools can be enhanced to address stormwater better while decreasing energy
consumption and producing food. Though an extensive GR can also support vegetable
Sustainability 2021, 13, 1972 11 of 13
growth [42], a green roof with deeper growing media will be more effective in addressing
the WEF.
With rooftop real estate a finite resource in urban areas, environmental considera-
tions warrant consideration of its use for renewable energy systems. This appears to com-
pete with GRs, however, synergistic effects have also been shown. In a study of agrivolta-
ics, the co-location of solar photovoltaics and agriculture demonstrated that soil areas
shaded by PV panels retained 15% excess moisture content, which supported enhanced
fruit production in tomato and chiltepin peppers compared to the unshaded control [48].
Dupraz et al. showed a 35–73% increase in overall land productivity when strategically
combining photovoltaics and agriculture [49]. These results are consistent with those of
Dinesh and Pearce, who calculated over 30% in additional economic value to farm opera-
tors [50]. Green roofs provide the foundation for agrivoltaic systems to proliferate in the
urban context as a solution to the WEF nexus.
This study presents customizable testbeds for quantitative evaluation of WEF nexus
variables. Understanding the enhanced water retention capacities of submedia drainage
layers will motivate more GR projects to incorporate systems that support the production
of vegetables, decrease the amount of energy consumed in WWTPs, and decrease the
amount of water needed to sustain GR vegetation. This research also supports multiple
aspects of the United Nations Sustainable Development Goals, including Goal 2: Zero
Hunger, Goal 6: Clean Water, and Goal 7: Affordable and Clean Energy [51].
4.1. Future Work
Further research activities for the rooftop lab are planned. These include rainwater
harvesting and overflow water re-use for irrigation purposes. Soil and overflow water
quality testing will be performed. Importantly, more data will be collected to improve the
reliability of the findings. Replicated modules, soil moisture and vapour pressure meas-
urement, and lysimeters for all modules will be added. A new, more accurate weather and
camera station for real-time rainfall and local temperature data is also planned.
Further research on the associated effects of submedia drainage layers should include
the effect of ET rates on plant productivity, nutrient runoffs, delayed peak flows, etc. Our
study suggests an associated stormwater benefit from green roofs with larger storage res-
ervoirs or more productive plants, versus traditional setups such as module P1 or P3. A
second full growing season will provide more insight into the long-term water balance
metrics of the various subdrainage assemblies, which will be beneficial to city policy mak-
ers and persons within the local GR industry.
This research suggests that the ET and retention of green roof systems is affected by
the interval and magnitude of rainfalls, which in turn promotes the use of systems that
have larger submedia drainage reservoirs. The expectation is that using a system with a
larger submedia drainage layer will enhance the performance and resilience of green
roofs. Focusing on increasing the resilience and environmental benefit of green roof sys-
tems is increasingly important given climate change impacts. This is why harnessing
knowledge on how to go above and beyond the current practices will motivate necessary
innovation within the green roof industry.
Author Contributions: All authors (J.W., J.L., D.S., L.M., and K.V.M.) contributed equally to the
conceptualization and methodology. J.W., with inputs from the other authors, prepared the origi-
nal draft writing, and all authors contributed to editing and review of the writing. All authors
contributed to development, data curation, validation and resources. K.V.M. supervised and ad-
ministered the work. All authors have approved the published version of this manuscript. All
authors have read and agreed to the published version of the manuscript.
Funding: J.W.: J.L, D.S., L.M., and K.V.M. acknowledge the support of the Ryerson Urban Farm
Living Lab through the Ryerson Urban Farm operations and the financial support of Andrew and
Valerie Pringle, and the support from the Environmental Applied Science and Management Pro-
gram at Ryerson University
Sustainability 2021, 13, 1972 12 of 13
Acknowledgments: J.W.: J.L, D.S., L.M., and K.V.M. acknowledge the support of Adam Johnstone
in facilitating the research, Laura Minkowski for her inputs and advice, Arlene Throness, Jayne
Miles and Sharene Shafie from the Ryerson Urban Farm for their collaboration and advice, Fiona
Yeudall at the Centre for Studies in Food Security at Ryerson University and Andrew Laursen at
the Environmental Applied Science and Management Program at Ryerson University for their
support, and Tom Hinckley for his recommendations on this manuscript.
Conflicts of Interest: Jeremy Wright is employed in the Green Roof sector by ZinCo Canada. The
funders had no role in the design of the study, in the collection, analysis, or interpretation of data,
in the writing of this manuscript, or in the decision to publish the results.
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... In general, the green process is presented in such a way that all concerns are linked and that all parts of every decision must be addressed, and thus the idea of reviewing the principles independently is in opposition with it. Green building design is one of the solutions in the field of sustainable architecture and green architecture to harmonize with the climate and minimize excessive use of reserves and natural resources [43,44]. ...
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