Sustainable Cities and Society 75 (2021) 103287 Contents lists available at ScienceDirect Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs Assessment of the food-water-energy nexus suitability of rooftops. A methodological remote sensing approach in an urban Mediterranean area Perla Zambrano-Prado 1, 2, *, Joan Muñoz-Liesa 3, Alejandro Josa 3, Joan Rieradevall 1, 4, Ramon Alamús 5, Santiago Gasso-Domingo 6, Xavier Gabarrell 1, 4 1 Sostenipra Research Group (Sostenipra 2017 SGR 1683), Institut de Ciència I Tecnologia Ambientals (ICTA-UAB), Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain 2 Department of Techniques and Construction, Art, Architecture and Design University Center (CUAAD), University of Guadalajara (UdeG), Calzada Independencia Norte No. 5075, Huentitán El Bajo 44250, Guadalajara, Jal, México 3 Department of Civil and Environmental Engineering (DECA), Civil Engineering School (Escola de Camins), Universitat Politècnica de Catalunya - BarcelonaTech (UPC), Jordi Girona 1-3, Building D2, 08034 Barcelona Spain 4 Department of Chemical and Biological Environmental Engineering, Universitat Autònoma de Barcelona (UAB), XRB Campus UAB, 08193 Bellaterra, Barcelona, Spain 5 Àrea d’Observació de la Terra – PCOT, Institut Cartogràfic i Geològic de Catalunya, Parc de Montjuïc, 08038 Barcelona, Spain 6 Department of Projects and Construction Engineering, Universitat Politècnica de Catalunya BarcelonaTech (UPC), Campus Sud, Building H, Diagonal, 647, 08028 Barcelona A R T I C L E I N F O A B S T R A C T Keywords: This work established a framework to identify and analyze the technical feasibility of roofs for integrating urban Rooftop urban agriculture agriculture, rainwater harvesting, and photovoltaic systems using various remote sensing. The framework was Rainwater harvesting applied to a region north of Barcelona. Three levels of solar access requirements for tomatoes, leafy crops, Solar energy strawberries, and microgreens were established. The case study included compact and disperse urban forms, Self-sufficiency residential and nonresidential building uses and various building typologies. It was identified that 8% of the roof Geographic information systems Remote sensing area is feasible for tomato and lettuce production, and production could satisfy the 210% of average intake of tomatoes and the 21% average yearly consumption of lettuce. Rainwater harvesting systems could supply 94.26% of the water requirements for lettuce growing in an open-air system; in contrast, 53% of irrigation could be satisfied for tomato production in rooftop greenhouse systems. The results showed a potential for 80% of roof area to be used for rainwater harvesting systems, representing the average yearly water consumption of 44% of citizens for laundry, showering, toilet flushing, cleaning and irrigation uses. Finally, 50% of the roofs are suitable for photovoltaic panels, representing an average energy consumption of 18% of citizens. 1. Introduction security have become pressing concerns, to supply these essential needs, cities must depend on their hinterlands (McGranahan & Satterthwaite, Cities cover about 1% of the surface area on the planet and house 2003), rendering cities unsustainable. Food, water, and energy demand about 55% of the world’s population (almost 75% in Europe). With are expected to increase by more than 50% in 2050 (International increased urbanization, the proportion of the world’s population living Renewable Energy Agency, 2015). In this context, cities must improve in cities is predicted to rise to 70% globally by 2050, and up to 85% in resource management through sustainable urban planning strategies Europe (European Investment Bank, 2018). Urban areas that are func­ (Bibri & Krogstie, 2017) considering the food-water-energy (FWE) tional as centers of production, consumption, and human settlement nexus. contain a variety of vital driving forces for social, economic, and envi­ ronmental stability and sustainability (Chang et al., 2020). Cities 1.1. The nexus between FWE and land in cities consume about 70% of global resources and emit 70% of all greenhouse gases (European Investment Bank, 2018) which generates environ­ Food production requires energy (and water) in many stages of the mental, social and economic consequences. Food, water, and energy food system. Energy is necessary in irrigation systems, farm machinery, * Corresponding author: E-mail address: perla.zambrano@academicos.udg.mx (P. Zambrano-Prado). https://doi.org/10.1016/j.scs.2021.103287 Received 22 February 2021; Received in revised form 19 August 2021; Accepted 19 August 2021 Available online 21 August 2021 2210-6707/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Table 1 Characterization of rooftops using remote sensing and geographic information systems (GIS) to integrate UA, RWH, or PV. Resource Scale of Data collection and GIS Software Building use Roof data acquired with remote Reference production study case technique used sensors C I R E I Re W A S SR M BF BH F N Building footprint solar map ArcGIS • • • • • • Berger (2013) from LiDAR F Ci Land use map Google Earth ArcMap • • Haberman et al. (2014) F Ci DSM from LiDAR ArcMap • • • • • Saha & Eckelman (2017) F Co DSM from LiDAR; spectral QGIS • • • • • Nadal et al. signatures from LWIR data (2017) W Co Aerial imagery LiDAR data Esri • • • • Grant et al. (2017) W Ci Satellite image DEM from QGIS • • • Lupia et al. (2017) LiDAR W N Building footprint from satellite ArcGIS • • • • • Radzali et al. imagery (2018) E Ci DSM from LiDAR ArcGIS • • • • • • • Bayrakci Boz et al. (2015) E Ci DSM from LiDAR ArcGIS • • • • • Brito et al. (2012) E N Google Earth satellite imagery ENVI EX • • • • • Khan & Arsalan (2016) E Co Building footprint from LiDAR Not • • • • Kodysh et al. specify (2013) E Ci Aerial imagery ArcGIS • • • Wiginton et al. (2010) Resource production: food (F), water (W), and energy (E). Scale of the studies: city (Ci), county (Co), and neighborhood (N). Building use: commercial (C), industrial (I), residential (R), educational (E), institutional (I), recreational (Re), and warehouse (W). Roof data acquired with remote sensors: area (A), slope (S), solar radiation (SR), material (M), building floors (BF), and building height (BH). fertilizer production, greenhouse heating and cooling, packing, trans­ the resilience of and accessibility to food, water, and energy. The Eu­ portation and distribution (FAO, 2015; Midgley et al., 2019). Energy is ropean Investment Bank (2018) emphasizes that, energy systems should required to extract, pump, lift, collect, transport and treat water. The be built around renewable energy sources and local generation (Euro­ agri-food sector consumes 30% of the world’s energy (FAO, 2016). Ac­ pean Investment Bank, 2018). In the case of food systems, urban agri­ cording to projections, by 2050, the demand for energy will double culture (UA) is an approach to increase local food production satisfying globally (International Renewable Energy Agency, 2015). Energy inputs food demand in cities (Hume et al., 2021), and may contribute to food into the food supply chain are likely to increase in the coming decades, insecurity. UA has advantages in different fields, such as social (e.g., leading to increased energy production necessities (FAO, 2016; Inter­ social inclusion), environmental (e.g., transport emissions reductions) national Renewable Energy Agency, 2015). and economic impacts (Lupia et al., 2017; Mok et al., 2014; Parece et al., Nowadays, 24% of total freshwater consumption exceeds the 2016; Thomaier et al., 2015). In addition, rainwater harvesting systems regional capacities (Motoshita et al., 2020), producing consequences (RWHSs) are strategies for managing water resources, providing sus­ such as water resource overexploitation, stress, and scarcity (Campisano tainable water cycles and minimizing water tap demand (Jha et al., & Modica, 2015; Gosling & Arnell, 2016). In addition, climate change 2014; Sojka et al., 2016). Regarding the energy sector, bioenergy, solar has negative repercussions for water cycles (Arnell & Gosling, 2016; photovoltaic (PV) or solar thermal panels can satisfy a relevant part of Gosling & Arnell, 2016). Water is also needed during the production energy demands (Corcelli et al., 2019; Midgley et al., 2019). processes of energy required for water abstraction, distribution, and In urban areas, land is an increasingly scarce resource, and the high treatment, as well as food production and distribution processes. In price and competition for residential buildings, industrial growth or addition, indirect water plays an important role on farm machinery, infrastructure construction represent some of the main threats to the fertilizer production, packing, transportation processes. local production of food, water, and energy (Benis et al., 2018a; Midgley Agriculture consumes 70% of water worldwide (International et al., 2019). In this sense, since rooftops cover half of the impermeable Renewable Energy Agency, 2015). As the largest consumer of freshwater surfaces in cities (Farreny et al., 2011), there are many areas that could resources globally, agricultural practices have substantial impacts on be potential spaces for producing food (e.g., through the installation of water security. Agricultural contamination remains a major source of open-air agriculture or rooftop greenhouses (RTGs)), water collection (e. water pollution. Different agricultural activities, such as fertilization, g., rainwater harvesting systems) and energy (e.g., solar photovoltaic or manure spreading and irrigation, have impacts on surface water and solar thermal panels). groundwater; for example, approximately 70% of the water pollution in China comes from agriculture in the form of runoff from fertilizers, 1.2. Rooftops as productive spaces pesticides and animal waste (International Renewable Energy Agency, 2015). Rooftops have the potential to improve urban metabolism by pro­ Land is also an important resource concerning the FWE nexus and is a ducing resources (for instance, food, water, and energy) (Corcelli et al., finite resource needed for food production and some forms of energy (e. 2019). In the case of promoting local food production, the environ­ g., wind and solar farms) and water (e.g., catchment areas, water supply mental benefits are mainly related to a reduction in transportation re­ infrastructure and ecosystem services) (FAO, 2015; Midgley et al., quirements and the consequent environmental impacts (Cerón-Palma 2019). Agriculture already uses 12% of the world’s land surface for crop et al., 2012; Sanyé-Mengual et al., 2013). The transformation of rooftops production (FAO, 2011). Moreover, the use of some of these areas for the into productive spaces is becoming a common practice in many cities agriculture sector would compromise valuable ecosystem services and worldwide (Gundula Proksch, 2016) (see Appendix 1 for cities biodiversity (FAO, 2011). examples). The production of local resources in cities is a key strategy to ensure Different criteria must be considered to assess the feasibility of 2 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 integrating FWE systems. Climatic conditions are crucial to determining 1.4. Sunlight access for rooftop urban agriculture the potential integration of FWE systems. For food production, sunlight access is the main climate parameter for determining the suitability of Another important aspect refers to sunlight access which has rooftop urban agriculture (RUA), based on the requirements of the received little consideration in the literature, referring to the assessment selected horticultural crop (FAO, 2013). Regarding the integration of of UA on rooftops. This criterion is critical for growing food. Solar ra­ RWHS, it is necessary to consider the rainwater potential, which refers diation is a fundamental energy source for photosynthesis which ulti­ to the entire amount of rainfall in the area under consideration (Worm & mately affects crop yields. However, current methods to identify van Hattum, 2006). Solar energy, as in the case of RUA, requires solar potential roofs for UA areas normally define a target daily light integral radiation on rooftops, and shadows from the built environment play an (DLI) value between 20 – 25 mol/m2/day (originally expressed in MJ/ important role in identifying potential areas. day, Nadal et al., 2017; Sanyé-Mengual, Cerón-Palma, Oliver-Solà, Roof characteristics, such as their geometry, area, slope, solar radi­ Montero, & Rieradevall, 2015). Another study assumed that roof area ation and materials, also need to be considered. In the following section, suitable for PV is also suitable for UA (Berger, 2013). the literature concerning roof characterization using remote sensing DLI target usually satisfies light requirements for single species, (RS) to assess the potential integration of FWE systems is described. usually tomato crops (Benis et al., 2017b; Nadal et al., 2017; Sanyé-Mengual, Cerón-Palma, Oliver-Solà, Montero, & Rieradevall, 1.3. Remote sensing data to assess rooftops for FWE production 2015). Compared to the optimal DLI crop requirements (see Appendix Table A3), this is a high threshold that results in over-energy lighting Remote sensing has been utilized to assess the feasibility of roofs to needs in the majority of crops (Benis et al., 2017b), leading to high implement FWE systems. Roof data acquired using light detection and environmental costs derived from LED systems. Thus, high DLI targets ranging (LiDAR) sensors have been applied to assess the UA, RWHS, or also underestimate the urban potential area that could grow low and PV potential of roofs. LiDAR systems allow us to obtain geometric data medium-DLI crop types. and to compute solar access to roofs. Table 1 illustrates a summary of In this context, the novelties of this approach are the integration of works assessing UA, RWH, and PV potential on roofs using remote additional urban and architectural requirements to existent methods, sensing data. the consideration of FWE systems, the use of two hyperspectral remote Previous works using remote sensing for data acquisition focused on sensors with a high spatial resolution (as a basis for data acquisition), a single vector food, water or energy (Table 1). Toboso-Chavero et al. and the classification of optimal daylight needs for crops in both RTGs (2018) assessed the potential integration of the FWE nexus on residen­ and open-air systems. tial buildings. Architectural and urban requirements were based on the method adapted by Nadal et al. (2017). However, roof data acquisition 1.5. Objectives was conducted by means of architectural layouts, implying difficulties for the application at larger scales, such as counties or cities. The method The main objective of this study is to assess the feasibility of roof proposed by Nadal et al. (2017) uses GIS and RS to obtain and manage areas to integrate food production, rainwater collection, and solar en­ information to identify suitable roof areas on industrial roofs for com­ ergy systems in cities using remote sensing technologies. In this regard, mercial purposes. In this sense, there is a need to assess the viability and the specific objectives are 1) to establish a framework for evaluating the potential roofs for the assessment FWE production from an integrated potential integration and self-sufficiency of FWE systems for commercial framework, based on the acquisition of urban and architectural data and nonprofit purposes; and 2) to apply the procedure in a Mediterra­ using remote sensing technology. Most of the studies considered resi­ nean area. dential, industrial, and commercial buildings to integrate UA, RWHS or PV systems. Especially for RUA integration, industrial followed by 2. Materials and methods commercial buildings have a greater presence in previous studies. To a lesser extent, studies have been carried out for self-consumption pur­ 2.1. Case study poses considering smaller roof areas and educational and housing roofs (Nadal et al., 2018, 2019; Saha & Eckelman, 2017). Non-profit UA The case study is located in a Mediterranean region in the El Vallès spaces can provide educational and self of belonging functions. How­ Occidental, north of the Spanish city of Barcelona. The Vallés Occidental ever, no methodological framework has been found that meets both is composed of 23 municipalities, and some of them are part of the commercial and self-consumption purposes. Roof material criteria is Metropolitan Area of Barcelona. This area was chosen due to its sunny rarely considered to assess the potential of UA, RWHS or PV systems weather predominance, which offers a strong potential to integrate both (Table 1). The knowledge of roof materials is valuable as an indicator of RTGs, open-air rooftop agriculture and solar energy systems. This cli­ the construction systems and their load-bearing capacity to support the matic area can also offer winter crop production without requiring any weight of FWE systems (Nadal et al., 2017; Sanyé-Mengual, Cerón-­ active system to heat greenhouses. There are recent initiatives from the Palma, Oliver-Solà, Montero, & Rieradevall, 2015). Besides, the quality city council to promote productive roofs and institutional and citizen and quantity of rainwater harvested from roofs are significantly affected awareness of UA. Additionally, the case study area includes a pilot RTG by roofing materials (Farreny et al., 2011). However, the process of at the Institute of Environmental Science and Technology (ICTA) and the acquiring rooftop data is complex, laborious, and time-consuming since Catalan Institute of Paleontology (ICP) building, where different types of a detailed database to analyze is needed (Nadal et al., 2017). In addition, crops have been produced since 2015 by the Fertilecity project team data collection through site visits or visual interpretation may limit the (Fertilecity, 2018). Regarding rainwater, several municipalities started scale to be analyzed. Hyperspectral imaging data acquired using Long to approve water-saving regulations (Domènech & Saurí, 2011), Wave Infrared (LWIR) remote sensor has resulted fast, automated and including RWHS, since the amount of precipitation makes the integra­ digital tool for identifying roofs characteristics (Nadal et al., 2017). tion of these systems in this area suitable. However, it has been suggested that the use of two or more sensors operated simultaneously with different spectral ranges, can improve 2.2. Criteria to integrate FWE systems for commercial and nonprofit discrimination of materials in urban contexts (Roberts et al., 2012) see purposes Appendix 2 for more information about this section. The work developed by Nadal et al. (2017) and Toboso-Chavero et al. (2018) was used as a basis for developing the methods of this work and adding other aspects specially climatic, urban, and architectural. 3 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 1. Phases to evaluate the feasibility of roof areas to integrate food production, rainwater collection, and solar energy systems in cities based on Tobo­ so-Chavero et al. (2018) and Nadal et al. (2017). The previous authors considered in their methods the short-term po­ c) Resource consumption. Food, water, and energy consumption are tential implementation of the systems on industrial and residential needed to determine the resource quantity required, these data are rooftops. For this reason, a literature review was performed to detect also used in Phase 3. Both datasets and a literature review are used to and establish the needed criteria to broaden the focus of the framework obtain resource consumption information. for this work Figure 1 summarizes the phases of the method used for this work: phase 1 study area characterization was based on the work Food, water, and energy demand was acquired from regional and developed by Toboso-Chavero et al. (2018) and enhanced with a liter­ local public reports (Ajuntament de Barcelona Medi Ambient i Serveis ature review. Phase 2 urban and architectural requirements, and phase 3 Urbans, 2016; Departament d’Agricultura Ramaderia Pesca i self-sufficiency indicators were based on Nadal et al. (2017) and Alimentació, 2017; Medi Ambient i Serveis Urbans-Ecologia Urbana extended with a literature review and own developed work for deter­ Agència d’Energia de Barcelona, 2019) mine crops day light requirements (see Appendix 3 and 44). Phase 2 Urban and architectural requirements. This phase is divided into two steps and various criteria by each step. For the architectural 3. Development of the Framework step, Figure 2 shows the minimum requirements needed to integrate FWE systems. Figure 2 illustrates the framework developed to identify feasible roof areas to integrate food, water, and energy systems in cities. It is divided a) Urban requirements. Building laws and land uses must be considered into three phases: study area characterization (Phase 1), urban and to ensure that the new infrastructures meet legal requirements. architectural requirements (Phase 2), and self-sufficiency indicators Limitations and regulations regarding urban aesthetics for the (Phase 3). Figure 2 also shows the methods for data acquisition for each implementation of the infrastructure must be reviewed. These data criterion, mainly based on RS. Each of the phases is described below. are obtained using a literature review of urban and architectural Phase 1 Study area characterization. In this phase, the characteristics codes of the city. It is important to know the building use to establish of the study area are obtained. Phase 1 is divided into the following three the purpose of the infrastructure. Two purposes are proposed: com­ steps: mercial and nonprofit (for self-consumption and including educa­ tional and social objectives). Retail and industrial buildings are more a) Urban features. GIS is used to define the boundaries and to conduct a suitable options for commercial purposes, while residential build­ spatial analysis. Data about population density, at the county, city, or ings, schools, and public services buildings are desirable for neighborhood level, depending on the scale of the study area. Urban nonprofit goals. To address legal requirements, Barcelona Metro­ form, building use, and building typology are needed to conduct a politan General Plan regulations and building codes (Metropolitan spatial analysis. Population density was acquired at municipality area of Barcelona, 2018) were consulted. Building uses were iden­ level from a public dataset. Roof area, land use, use and typology of tified from the layers explained previously in section a) of Phase 1: buildings is acquired using GIS. Urban form is acquired using a Study area characterization. literature review, and GIS information. To identify land use and b) Architectural requirements. This step considers four criteria: area, building typologies, two vector layers were used: (1) The Urban slope, solar access, and roof material. The first three (geometric Planning Map of Catalonia (UPMC), downloaded free from the characteristics) were acquired using a LiDAR sensor, and the last one Department of Territory and Sustainability (2019), and (2) roof was acquired using hyperspectral remote sensing. Each of the criteria polygons obtained from a previous work developed by the authors. is described below. Data from these two layers were integrated using the tool join attri­ butes by localization from QGIS software. Roof area. A minimum roof area is needed for food and energy pur­ b) Climatic conditions. Since climatic conditions play an important role poses. For commercial energy infrastructures, a minimum of 100 m2 is regarding the potential of FWE systems, a minimum solar radiation needed, and in the case of food production, a minimum of 500 m2 is of 800 MJ/m2/year is needed for food production (microgreens) in required. With respect to nonprofit goals, energy systems require a open-air systems and a minimum of 300 mm/year of rainfall. To minimum of 24 m2, and food systems require a minimum of 13 m2. obtain this information, datasets from public websites are used. The Roof slope. This criterion is divided into two: if the roof is flat with a rainfall dataset was obtained from a weather station (Rural Cat, maximum slope of 10%, food production systems can be integrated; if 2019) over a period of 21 years (from 1996 to 2017). The average the roof slope is greater than 10%, energy and rainwater harvesting annual solar radiation was obtained from an atlas of solar radiation systems can be implemented. in Spain (Sancho et al., 2012). Solar access. This criterion is divided into four groups, one for energy systems, in which a minimum of 1,900 MJ/m2/year is needed, and three 4 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 2. Workflow proposed to identify feasible roof areas to integrate urban agriculture, rainwater harvesting, and photovoltaic energy systems. The sunlight access criterion of the phase 1 study area characterization refers to the minimum energy radiation for producing microgreens. for food production, according to the crop and food production system. requirements of solar access for food systems are as follows: tomatoes The three levels are high, medium, and low for tomatoes; leafy vegeta­ growing in RTGs at 3,800 MJ/m2/year and those growing in open-air at bles, strawberries, and seedlings; and microgreen crops, respectively. 2,000 MJ/m2/year; leafy vegetables, strawberries, and seedlings culti­ Each level of solar access is divided into two corresponding food pro­ vated using RTGs at 2,800 MJ/m2/year and those using open-air at duction systems: RTGs or open-air. Therefore, the minimum 1,650 MJ/m2/year; and microgreens produced in RTGs at 1,400 MJ/ 5 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 m2/year and those produced in open-air at 800 MJ/m2/year (see Ap­ energy production. pendix 4). In this work, priority was given to roofs with high radiation Phase 3 Self-sufficiency indicators. This phase is divided into two steps: levels for the integration of greenhouses. These infrastructures have the resource production and self-sufficiency potential. Food production advantage of greater climate control in addition to a higher production consists of a variety of systems and methods for growing crops that affect of the crop. When the integration of urban agriculture was possible, growing values and water requirements for irrigation; in the case of solar energy was not considered because current panels diminish light RWH, the quantity of rainwater harvested from roofs is affected by for crops. It was demonstrated that transparent photovoltaic (TPV) roofing materials; and for photovoltaic systems, solar radiation in­ panels are not yet developed for commercialization (Lee et al., 2020). fluences the amount of energy produced. Thus, the first step uses Roof material. The load-bearing capacity to support the weight of the different equations, in which the total roof area that fulfills the criteria system is essential, and a minimum load-bearing capacity of 200 kg/m2 for Phase 2 identified as potential and other parameters are used to is required for crops. A lower capacity can be suitable for energy sys­ obtain the production per year of food, water or energy. In the second tems. The load capacity of the roof does not condition rainwater step, self-sufficiency potential uses results derived from the first step and collection in this sense, and any material has potential for water systems consumption of resource data to obtain how many people can be sup­ with special attention paid to health risks according to roof materials ported by the food, water, or energy supplied. and the future use of water resources. It is necessary to consider the part a) Resource production. Tomatoes and lettuce were considered of the building in which the storage tank will be located. If the tank is suitable crops due to the amount of household consumption data in the placed on the roof, load-bearing must be considered. This framework region of the case study. Tomatoes were chosen because they are the does not consider water location, size, and weight tank. second most consumed vegetable in Barcelona, after potatoes (Depar­ tament d’Agricultura Ramaderia, Pesca i Alimentació, 2017), which • Geometric and solar radiation characterization. LiDAR remote cannot be produced in hydroponic RTG systems. Lettuce represents the sensing technology was used to characterize the geometric properties most consumed leafy vegetable (Departament d’Agricultura Ramaderia, of roofs. Then, solar irradiation on surfaces was computed with Pesca i Alimentació, 2017). Soil-less systems for both open-air and RTG GRASS software, version 7.4. This process considered all of the conditions were considered. Food-crop yield was obtained for RTGs and shadow effects in the surrounding area. The airborne sensor Leica open-air studies from Barcelona. Average yield values are listed in the ALS50-II (LiDAR) was used on a flight conducted in 2013. Based on Appendices Table A1. To calculate the rainwater harvesting potential, high density of points (4 points/m2), a digital surface model (DSM) runoff coefficients of 0.80 (ceramic), 0.90 (metals), and 0.60 (gravel) with a spatial resolution of 0.5 m pixels was obtained. This model were considered (Farreny et al., 2011) according to roof materials in the contained all of the geometric characteristics of the terrain and study case. Multicrystalline silicon (multi-Si) technology was chosen for buildings, such as area and slope. The model also included objects on PV because it is the most common in the market (Paiano, 2015) for rooftops (e.g., air conditioning installations) and objects that can estimating energy production, and a PV panel efficiency (new) of 26% produce shadows over roofs. The point cloud was classified as was considered (Lee et al., 2020). ground, vegetation, buildings, and other objects that were not roof­ To determine the potential production of food, water, and energy, tops. Then, points that were considered noisy, such as powerline the total feasible rooftop area (m2) obtained from the application of the wires, points with low intensities, and points in the air, were framework using remote sensing technology (Phase 2) was used in the excluded (Kodysh et al., 2013; Suomalainen et al., 2017). The following equations: average annual solar irradiation was computed considering climatic conditions with the r.sun library in GRASS software, version 7.4, by • To quantify food production, Equation 1 was used, where FP is the ∑ adding direct and global radiation received at each pixel of the DSM yearly food production (kg), a is the total feasible rooftop area every hour for one day of each week over the whole year. In this (m ), and YV is the average yield value (kg/m2). 2 calculation, all the shadows generated by buildings, vegetation, to­ ∑ pographies, etc., were considered. FP = a × YV (1) • Roof material characterization. Previous work developed by the authors on rooftop classification was used, the work included hyperspectral data acquired during a day flight in 2018 with two • To determine the water harvesting potential, Equation 2 was used, imager spectrometers: an Airborne Imaging System for Different where WHP is the yearly amount of rainwater harvested from roof­ ∑ Applications (AISA) Eagle II and a Thermal Airborne Spectrographic tops (L), a is the total feasible rooftop area (m2), Ptot is the total Imager 600 (TASI-600). Data obtained by remote sensors were pro­ annual precipitation (mm), and RC is the runoff coefficient (har­ cessed using a cluster algorithm (K-means) to classify roof material vesting efficiency of the system, depending on roof material; see data. In the present work, two classes of roofs were totally excluded above). ∑ to integrate FWE systems. Fiber cement roofs were excluded because WHP = a × Ptot × RC (2) of their low load-bearing capacity to support RTGs, open-air UA, and PV panels, as well as health risks for collecting rainwater; addition­ ally, green roofs were excluded considering that these roofs already • To determine solar energy production, Equation 3 was used, where have a use that contributes to more sustainable cities. It should be PVEP is the yearly amount of photovoltaic energy production (kWh/ noted that the location and weight of the water tank for RWHS were m2/year), ∑ a is the total feasible rooftop area (m2), and SR is the not considered in this work. global solar radiation potential (kWh/year). ∑ Data layers with geometric characteristics (area and slope) and PVEP = a × SR (3) nongeometric characteristics (solar radiation, roof materials, use and typology of buildings) were integrated into a single GIS layer with QGIS software using the joint attributes by location tool. Once all of the data were integrated into a single layer, potential roofs were identified using b) Self-sufficiency. Fresh tomato and lettuce consumption of 14.9 select by expression. With this tool, roof requirements for each system and 4.4 kg/per capita/year, respectively, was considered (Departament (food, water, and energy) were filtered and then extracted into a new d’Agricultura Ramaderia Pesca i Alimentació, 2017). Water consump­ layer of roof polygons that fully fit the established criteria. In this work, tion of tomatoes and lettuce production in Barcelona in open-air and potential areas for food production were prioritized with respect to RTG systems and the soil-less method were considered for calculations 6 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 3. Location and study area characterization. Building typologies of housing in the study area included low rise buildings in nuclei and center populations (R1), compact and open building blocks (R2), blocks with interior patios (R3), isolated blocks or towers (R4), detached houses (R5), terraced houses (R6), and building blocks for public equipment housing (SD). of water self-sufficiency for irrigation crops (see Appendices Table A2). • To quantify potential solar energy self-sufficiency, Equation (6) was In addition, the water demand for domestic use was acquired from the El used, where PESS is the self-sufficiency of total PV energy produc­ consum d’aigua a Barcelona L’aprofitament i els usos dels recursos hídrics tion, PVEP is the total photovoltaic energy production (obtained (water consumption in Barcelona The use of water resources) report, from Equation 3), and ED is the average energy demand (MWh/per considering an average of 105 L/household/day, including laundry, capita/year). showering, toilet flushing, cleaning and irrigation (Ajuntament de Bar­ PESS = PVEP/ED (6) celona Medi Ambient i Serveis Urbans, 2016). The average yearly energy consumption over a period of 19 years was estimated from data obtained from the Balanç d’energia amb efecte d’hivernacle i emissions de gasos de Barcelona 2017 (Energy Balance and Greenhouse Gas Emissions of 4. Results Barcelona 2017) report (Medi Ambient i Serveis Urbans-Ecologia Urbana Agència d’Energia de Barcelona, 2019). 4.1. Study area characterization To determine the potential of self-sufficiency (# persons), the following equations were used. Figure 3 illustrates the study area location in the Valles Occidental region north of Barcelona, Spain, and its main characteristics. The study • To quantify potential food self-sufficiency, Equation (4) was used, area has approximately 67,000 inhabitants and compromises 15 km2 where PFSS is the self-sufficiency of total food production, FP is the with compact and diffuse urban forms and a total of 3 km2 of roofs food production (obtained from Equation 1), and FD is the average ranging from 0.25 m2 to 13,480.5 m2. Roofs were in different land uses: food demand (kg/per capita/year). housing (75.0%), industrial (16.6%), services (5.9%), mix (0.6%), and PFSS = FP/FD (4) non-urban (0.5%). Some roofs were not identified with respect to the land use location (1.5%). Different building typologies of housing were found: 4.3% corresponded to low rise buildings in the founding nuclei • To quantify potential water harvesting self-sufficiency, Equation (5) and centers populations (R1); 25.1% to compact and open building was used, where PWSS is the self-sufficiency of the total water blocks, corresponding to historical growth of a structure before 1950 collection of roofs, WHP is the water harvesting potential (obtained (R2); 0.4% to multifamily blocks with interior patios, corresponding to from Equation 2), and WD is the average water demand (L/per modern extension (R3); 12.9% to blocks or towers configured from an capita/year). isolated multifamily building (R4); 38.4% to detached houses for single families (R5); 18.6% to terraced houses for single families (R6); and PWSS = WHP/WD (5) 0.1% to building blocks for public equipment housing (SD). The average global radiation was 4.6 kWh/m2/day, and the average annual rainfall over the study area was 592 mm. 7 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 4. (a) Potential roofs over the study area. (b) Details of identified potential roofs on industrial and services buildings. (c) Proportion of potential roofs for PV and RWHS, UA & RWHS and RWHS. Food production is allowed for self-consumption. Land use types do indicated in section 3 Development of the Framework, rooftops classi­ not include agriculture use. For this reason, UA for commercial purposes fied as green roofs were discarded from the analysis of potential, pro­ is not permitted. On some rooftops, RTGs were not permitted due to the duction and self-sufficiency. maximum height and volume restrictions. In this sense, a case-by-case A total of 823 roofs and 0.2 km2 were identified for integrating UA review by local technicians is required. To integrate RWHS and PV and RWHS (Figure 5a). The results of potential areas were identified by systems on rooftops, no restrictions were found. According to the different colors according to the growing system and crop type, as shown Spanish Technical Building Code, it is mandatory to include minimums in detail in Figure 5b. The distribution of potential roofs for UA was as for electricity and sanitary hot water self-sufficiency in new buildings follows: RTGs for tomato production 100,610.5 m2 (49.0%); RTGs for and developed building extensions (except for residential buildings) lettuce crops 7,410.2 m2 (3.6%); open-air for growing tomatoes when the constructed area exceeds or is increased by more than 3,000 97,043.25 m2 (47.3%); and open-air for lettuce production 89 m2 m2. (0.04%) (see Figure 5c). Figure 6a shows potential roofs for UA and their location distribution by land use. It can be observed that most rooftops were located in service 4.2. Analysis and quantification of potential roofs for integrating FWE land use areas. Regarding each growing system distribution of feasible systems rooftops according to land use, they were as follows: RTGs for harvesting tomatoes: 56.6% services, 41.5% industrial, 0.7% housing, and 1.2% After applying the proposed framework, potential roofs were ob­ mix; RTGs for growing lettuce: services 91.1% and 8.9% housing; open- tained. In this section, the results are presented in layers according to the air system for tomatoes: 59.4% housing, 23.4% services, 14.3% indus­ systems studied. A total of 38,575 roofs (2.6 km2) representing 87% of trial, 0.4% mix, and 2.5% without data; and finally, open-air lettuce: the total roof area were identified for integrating UA, RWHS, and PV services 68.9%, and 31.1% without data. Figure 6b also shows the po­ (Figure 4a). These results were identified by different colors according tential roof distribution by building typology of housing use (60%). Most to the systems, as shown in detail in Figure 4b. Result showed that 8% roofs belonged to typology R4 with 61.1%, followed by R6 with 30.1%, (0.2 km2) of total roofs identified as potential could integrate both UA R2 with 6.1%, R5 with 2%, and R3 with the fewest potential roofs with systems (commercial and self-consume) and RWHS. The 50% (1.3 km2) 0.5%; no roofs were identified as typology R1. of potential areas could integrate PV and RWHS systems. The 42% (1.1 Figure 7 shows the results of tomato and lettuce production and self- km2) of feasible roofs were potential for integrating RWHS (Figure 4c). It sufficiency. The total roof area for tomato crops could produce 2,097 can be observed that 78% of potential rooftops were residential housing, tons per year, 73.4% by RTGs and 26.6% by open-air systems. Produc­ 13% industrial, 6% services buildings, mix (0.5%) and nonurban (0.3%) tion could satisfy the average intake for 140,620 persons, representing were recognized in similar proportion, and 1.4% of roofs did not have 210% of the population over the study area (Figure 7a). Regarding let­ data about building use (Figure 4d). A total of 92 green roofs were tuce crops, the results showed that a total of 62.3 tons per year could be detected, of which 7 roofs were identified as potential for tomatoes crop produced, 99.5% by integrating RTGs and 0.5% by open-air systems. using open-air system and 9 for solar energy. These potential areas This production could satisfy the consumption of 14,000 inhabitants, represent 0.013% for RUA and 0.016% for PV of the total roof area. As 8 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 5. (a) Potential roofs to integrate rooftop agriculture and RWHS over the total study area. (b) Details of identified potential roofs on educational buildings from the Autonomous University of Barcelona. (c) Distribution of potential roofs according to growing system and crop type. Figure 6. (a) Potential roofs identified to integrate rooftop agriculture and their distribution by land use and (b) by building typology of housing. representing 21% of citizens in the study area (Figure 7b). population in the study area (Figure 8d). The results showed that a total Self-sufficiency analysis for crop irrigation showed that the integra­ of 1,124,000 m3 of rainwater per year could be collected on roofs tion of RWHS could supply 94.3% of the total water requirements for (considering potential roofs from figures 8a and 9a) representing the lettuce crops in open-air systems and 82.1% for the same crop in RTGs, average consumption of 44% of the population in the study area for as well as 63.2% for tomatoes in open-air and 53.0% for the same crop in laundry, showering, toilet flushing, cleaning and irrigation uses RTGs (Figure 7c). (Figure 8d). A total of 8,088 roofs (1.3 km2) were identified to integrate PV panels A total of 1.1 km2 of roofs were identified as feasible for integrating and RWHS (Figure 8a). Potential roofs were assigned the yellow color. RWHS (Figure 9a). These results did not consider potential roofs for Details of the identified roofs of housing and service buildings are shown agricultural irrigation (shown in Figure 5a) and neither did feasible in Figure 8b. Feasible roofs were located in different land uses, as shown roofs for RWHS and PV (shown in Figure 8a). The results of potential in Figure 8c. Most of the rooftops belonged to industrial (65.1%); roofs for integrating RWHS are shown in blue. Details of the identified housing roofs represented 20.1%; service rooftops represented 12.6%; roofs of housing and service buildings are shown in Figure 9b. It was polygons without land use information represented 1.3%; and few roofs identified that most rooftops belonged to industrial (43.1%) and housing were mixed and nonurban at 0.6% and 0.4%, respectively. (40.2%) uses; service rooftops represent 14.1%; some polygons (1.6%) The results showed that a total of 140,300 MWh/m2/year could be did not show information related to land use; and FWE roofs were mixed produced on roofs, representing the average consumption of 18% of the and nonurban at 0.7% and 0.4%, respectively (Figure 9c). 9 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 7. Rooftop urban agriculture production and water self-sufficiency by crop. (a) Tomato production in open-air and RTG systems and self-sufficiency. (b) Lettuce production in open-air and RTG systems and self-sufficiency. (c) Water self-sufficiency for irrigation regarding the type of crop and system. Potential areas for integrating UA were considered for rainwater collection for crop irrigation. 5. Discussion agriculture. This result is similar to those obtained by Sanyé-Mengual, Cerón-Palma, Oliver-Solà, Montero, & Rieradevall, (2015) in a logistic 5.1. Rooftop potential to integrate FWE systems and building use and industrial park area located in Barcelona where a 8% of roofs were found feasible for RTGs representing 13 h. Compared to Sanyé-Mengual, In relation to feasible roofs for UA and food production, this work Cerón-Palma, Oliver-Solà, Montero, & Rieradevall, (2015) work the found that 8% of roofs in the study area could integrate urban feasible roof area in the present study was 7 ha greater, even so, tomato 10 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Figure 8. (a) Potential rooftops for solar energy production by PV panels and RWHS over the study area. (b) Details of identified potential roofs in housing and service buildings. (c) Potential roofs identified to integrate PV panels and RWHS and their distribution by land use. (d) Energy and water self-sufficiency. Figure 9. (a) Potential rooftops for integrating RWHS over the study area. (b) Details of identified potential roofs in housing and services buildings. (c) Potential roofs identified to integrate RWHS and their distribution by land use. 11 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 production was not significantly higher in our results, this difference considered). could have occurred because 26.6% of roofs were feasible for open-air With respect to the RWHS for crop production, the results showed systems, and the yield production was less than that with RTG sys­ that it could be possible to supply at least 63.2% of the water re­ tems. Concerning other works, quantitative differences were found. In quirements for lettuce crops using both RTGs and open-air and tomatoes an industrial area of Barcelona, was found that 3% of roofs were feasible growing in open-air systems. The results of this work are better, espe­ while another study carried out in the same city in retail parks showed cially for lettuce growing in open-air systems, which found 94.3% water great potential 53% (Nadal et al., 2017; Sanyé-Mengual et al., 2016). self-sufficiency compared with that found for a case study in Rome for The difference about potential roofs could be related to the rooftop horticulture gardens, where 57% of water could be supplied by rain­ building materials, roof systems from retail parks tend to be more water collection on rooftops (Lupia et al., 2017) (see Appendix 6 for resistant than in industrial parks (Sanyé-Mengual, Cerón-Palma, details). The water supply by rainwater for growing tomatoes using RTG Oliver-Solà, Montero, & Rieradevall, 2015). systems was 27% lower than that in previous experimental works per­ In this study, services and housing rooftops were the most feasible for formed in Barcelona, the water efficiency for crops can be improved integrating RUA, which can be attributed to the structural requirements with the use of leached recirculation, including an extension of the crop (for more details on this part of the discussion see the Appendix 5). production season to obtain 80% of the water requirements for tomato It was noted that the R4 building typology showed greater potential crops (Sanjuan-Delmás et al., 2018) which were no considered in this than R5, R2, and R6, which had higher representation over the total work. With respect to domestic water self-sufficiency, in this study, it study area. These typologies showed a large number of shadows and was found that the RWHS could meet up to 44% of household demands. pronounced slopes, especially the R6 housing typology. Blocks or towers This result is lower than those reported in previous works. According to configured from an isolated multifamily building were found to be the (Domènech & Saurí, 2011) 60% of the water demand for irrigation and most promising typology for RUA purposes. The work developed by laundry can be meet with the implementation of RWHS in detached and Toboso-Chavero et al. (2018) already concluded that block-isolated multi-family housing buildings (see Appendix 7 for more details). multifamily buildings have the potential to integrate open-air and Annual irrigation water requirements and urban-scale water collec­ RTG systems. These results show that building morphology and urban tion were calculated in the model. However, a downscaled analysis in configuration could play an important role in rooftop UA potential. It temporal terms affects the overall self-sufficiency results (analysis re­ was found low percentage (1.5%) of roofs without data information sults and their discussion can be found in Appendix 8). In future about land use, which could be due to the use of airborne datasets research, the model can be improved, including analysis at smaller provided by different institutions and acquiring data on different dates temporal and spatial scales and relate to the size and location of the which could generate some outdated data. water tank, which play important roles regarding the feasibility of In the present work, were detected a low proportion of green roofs. RWHS and environmental impacts (Angrill et al., 2012, 2017; Petit-Boix As mentioned in Development of the Framework section, green roofs et al., 2018). were discarded from the analysis of potential roofs. However, discarding Results showed an energy-sufficiency of 18% which is lower than these roofs implies a decrease in the potential of roofs for the integration reported in previous studies (ranging from 30% to 46%) from roofs in of food, water, and energy systems, especially, in the case of pioneers different districts of Barcelona (Riyahi Alam et al., 2008; Toboso-Cha­ cities that support the integration of green roofs and those where green vero et al., 2018). However, differences were found in the data taken as a roofs are mandatory by law such as London or Toronto (City of Toronto, reference for energy consumption, while this work considered con­ 2009; Grant & Gedge, 2019). While green roofs provide environmental sumption of 11.42 MWh/inhabitant/year (Medi Ambient i Serveis benefits, roofs with food, water, and/or energy systems provide several Urbans-Ecologia Urbana Agència d’Energia de Barcelona, 2019), a functions that may be a valuable support for developing resilient cities. previous study considered a lower consumption, 2.96 MWh/inhabi­ Productive roofs can contribute to several goals of the 2030 Agenda for tant/year (Riyahi Alam et al., 2008). Sustainable Development, emphasizing the important role they may play in urban sustainability (Cristiano et al., 2021). Therefore, for future 5.3. Solar irradiation and daylight requirements works, it is important to consider the possibility of larger areas of green roofs with the potential for food and solar energy systems integration. The use of LiDAR technology made it possible to identify the solar radiation needed for solar energy and food production at a building scale 5.2. Food, water, and energy self-sufficiency in an urban scale extension. The method used included the meteoro­ logical characteristics of the site. Previous works has concluded the need This work demonstrated food self-sufficiency of 210% (140,620 in­ to consider climatic data and field measurements (Kodysh et al., 2013; habitants). These results are comparable to those reported in previous Suomalainen et al., 2017). However, this work has not made a com­ work by Sanyé-Mengual, Cerón-Palma, Oliver-Solà, Montero, & Rier­ parison of the results with other methods used to compute roof radia­ adevall, (2015) in Zona Franca Park (Barcelona) where the total feasible tion. This could be future work. Suomalainen et al. (2017) found an area for RTGs can satisfy the average tomato demand of 130,000 in­ underestimation of the annual solar radiation of approximately 5% on habitants. Benis et al. (2017b) determined using a simulation that the the sunniest spots on the roof compared to solar radiation based on integration of RTGs in buildings in Lisbon can have an efficiency of a measured. The model of this work considered the shading of vegetation, factor of four in the case of tomato production. However, differences however, in the case of deciduous vegetation, in winter solar radiation were also found concerning other studies developed in industrial and could increase in these areas. From the data obtained with LiDAR and residential areas of Barcelona, in which self-sufficiency was reported the computation of irradiation for solar energy, a calculator which in­ from 50% to 69% (Nadal et al., 2017; Toboso-Chavero et al., 2018). cludes economic indicators could be generated for decision making by These differences can be partially attributed to a smaller feasible pro­ stakeholders use. duction area (67% less). Most of the rooftops identified as potential for urban agriculture can The low feasibility of roofs to integrate lettuce crops in both RTGS integrate tomato crops (RTGs and open-air) which require high solar (3.6%) and open-air (0.04%) systems is related to the priority given to access. A small portion was feasible for lettuce crops, categorized with a roofs with high light levels and consequently the consideration of to­ medium-light requirement. No potential areas were detected for crops mato crops, due their consumption in the region is higher than lettuce. such as microgreens (low light requirement). However, in other loca­ However, the roofs that were found to be feasible for tomatoes are also tions where solar radiation is low, crops with lower light requirements potential for growing lettuce, either year-round or in combined crops could be integrated. According to the calculations made in this work, a with tomatoes (in the months when tomato production is not minimum of 3,800 MJ/m2/year is necessary for tomato crops in RTGs, 12 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 which differs from the light requirements (1,900 MJ/m2/year mini­ remote sensing technology is the basis of the defined framework. In this mum) used in previous studies (Nadal et al., 2017; (Sanyé-Mengual, sense, the availability of data is important. The identification of roof Cerón-Palma, Oliver-Solà, Montero, & Rieradevall, 2015; Toboso-Cha­ materials can be estimated through the proposed method and the use of vero et al., 2018). In future research, the analysis and consideration of remote sensors in the present study. The lack of this information could intercropping and economic issues are recommended. In addition, to be a limitation for determining the potential adequate load capacity for develop new tools using BIM design for modeling and integrate FWE the installation of greenhouses and could constitute an information gap infrastructures on buildings (Benis et al., 2017b; Khan et al., 2018). in evaluating the potential integration and self-sufficiency of FWE sys­ tems in different cities. A lack of information concerning the consump­ 5.4. Population density building typologies and self-sufficiency tion of products and production at the local or regional level could restrict or lead to less accurate production and self-sufficiency analyses. This work considered the characteristics of each building. However, In relation to this limitation, experimental research on rooftop agricul­ the potential roofs and self-sufficiency at urban scale were analyzed. The ture for both RTGs and open-air systems in different regions and population density of each municipality can affect the self-sufficiency considering a diversity of crops and according to their consumption potential (for more details of the analysis see Appendix 9. It is impor­ products is needed. tant to note that not all municipalities have the same typologies and uses In Mediterranean regions, where high radiation potential should be of buildings. This related to population density has implications at the taken as advantage for crops with high lighting requirements without municipal and building scale in the potential and self-sufficiency. For use of artificial light support. example, service and residential buildings showed higher potential for Urban morphology and its characteristics influence the feasibility of urban agriculture compared to industrial buildings. In the municipality potential production, especially for food and energy. Thus, the rela­ with the lowest population density was found the largest amount of tionship between urban morphology and building typologies regarding service building area. In this municipality, is located the Autonomous FWE systems must be performed more deeply. This issue could be University of Barcelona representing the largest area of educational addressed by characterizing urban structure types using remote sensing services. Contrary, most of the building housing with potential are in the technology due to the potential for efficient derivation of mapping urban county with the highest population density. land at the city scale. Concerning the integration of two systems: photovoltaic and rain­ The results of this work indicate that housing and services buildings water harvesting, industrial roofs were found more feasible. Seventy-six could be a better location for RUA than industrial buildings. In addition, percent of the potential industrial area was identified in the municipality RUA could represent social cohesion and educational values more of Barbera del Valles, while in the most densely populated municipality, directly. However, in other cities building typologies and uses with the there was no industrial buildings. Therefore, in future research, it is greatest potential may be different. recommended to include analyses on a smaller scale, considering the Regarding water self-sufficiency for crop production, the case study variables mentioned in the previous paragraph. demonstrated good performance for most of the systems studied. How­ ever, this performance is variable according to the rainwater amounts of 5.5. Other considerations for future works the case study location. Some implications can be improved for future works, for example, considering the recirculation of leachate and the The conflict of prioritizing a roof use solar energy generation vs food factor of occupied area by crops in the case of open-air systems, special production could be solved with the emergence of new technologies and temporal scales as well as with respect to water tanks, size calcu­ such as TPV integrated into RTGs. These new types of panels will allow lations, and feasibility locations, which could be restrictions for imple­ the coexistence of both energy generation and food production. How­ menting RWHS regarding the tank weight and associated environmental ever, although TPV has been extensively researched as a renewable impacts. energy source for urban areas, there are still several challenges that do It is critical to include social, environmental, and economic in­ not seem to be fully covered yet. For commercialization of the TPV some dicators to carry out a complete sustainability assessment and to guar­ main perspectives should be considered high-power conversion effi­ antee economic sustainability of the infrastructures. ciency at the same average visible transmittance, aesthetic factors, and It is important that information about potential roofs be accessible feasibility for real-world applications, including modularization and and easy to identify. In this regard, an interactive map with the location stability evaluation. In this regard, it has been suggested that a high- and information of these areas has already been implemented in some performance TPV cannot be realized commercially yet (Lee et al., cities, for example, the rooftop project maps of Melbourne city (City of 2020). However, in future research, it is recommended to integrate them Melbourne, 2020). Developing this type of interactive map could in methos for feasibility and self-sufficiency analysis considering both represent a valuable contribution to decision making for planning. food and solar energy systems in the same infrastructure. This work explored potential rooftops already built, but the inte­ According to Toboso-Chavero et al. (2018) the combinations with gration of FWE systems into new buildings is also important. The larger CO2 eq savings showed higher self- sufficiency in electricity and expansion of cities continues, and the nexus of FWE systems should be hot water, whereas the combinations with lower environmental impacts considered a part of new buildings. In this sense, the integration of water displayed higher self-sufficiency in food systems. Benis et al. (2018b) and energy flows as well as UA in the phase of project design is an found that food production is more beneficial than energy production important consideration and easier to integrate if the project is for both financial return and local job creation. These important envi­ conceived from the beginning with the integration of these technologies ronmental and economic impacts must be taken into account in future in mind. For these reasons, it is crucial that FWE studies also focus on the studies. development of new projects and designs that will strongly depend on the geographic location. 6. Conclusion Declaration of Competing Interest This study contributes to defining criteria and procedures for assessing the feasibility of rooftops for integrating urban agriculture, No potential conflicts of interest were reported by the authors. rainwater harvesting systems, and PV panels. Data acquisition from 13 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Acknowledgements Appendix 2. Remote sensing data and criteria to assess rooftops for FWE production This work is part of the Fertilecity II project supported by the Spanish Ministry of Economy and Competitiveness (CTM2016-75772-C3-3-R Area, slope, and solar radiation have been the principal criteria for and CTM2016-75772-C3-1-R, AEI/FEDER, UE); from the Spanish Min­ assessing UA, RWHS or PV systems (e.g., Berger, 2013; Haberman et al., istry of Science, Innovation and Universities. This research has received 2014; Kodysh, 2013; Lupia et al., 2017). In addition to roof geometry, financial support from the European Union’s Horizon 2020 research and some works have integrated other criteria, such as maximum building innovation program under grant agreement No 862663. The publication floors (Berger, 2013) and building height (Saha & Eckelman, 2017), to reflects the author’s views. The Research Executive Agency (REA) is not identify hospitable heights for plants and logistical safety concerns with liable for any use that may be made of the information contained rooftop access for people for UA purposes; roof materials (Nadal et al., therein. This work was supported by the “María de Maeztu” program for 2017) and construction year (Berger, 2013) have also been identified. Units of Excellence in R&D (CEX2019-000940-M). Large surfaces (minimum 465 and 500 m2) have been considered for Authors want to thank to Assumpció Termes and Fernando Pérez- commercial (Berger, 2013; Haberman et al., 2014; Nadal et al., 2017; Aragüés from the Institut Cartogràfic i Geològic de Catalunya for their Salvador et al., 2019; Sanyé-Mengual et al., 2015, 2016). valuable support, to the Greenhouses to reduce CO2 on Roofs (GROOF) UA in schools, for example, provides environmental education and project, to the reviewers and editor for their observations, also to the social cohesion, and according to the main goal of the Global Education Secretaria d’Universitats i Recerca del Departament d’Economia i Con­ 2030 (Agenda UNESCO), it is necessary to acquire sustainability com­ eixement de la Generalitat de Catalunya for the award of a research petencies as a core of Education for Sustainable Development (Leicht scholarship (FI-DGR 2018) to Joan Muñoz-Liesa and to the University of et al., 2018). Guadalajara (Mexico) for awarding a research scholarship to Perla Zambrano-Prado. Appendix 3. Criteria to integrate FWE systems for commercial and nonprofit purpose Appendix 1. Examples of cities that transformed rooftops into productive spaces Phase 1: Study area characterization The characterization of the study area is based on the following New York, Paris, Vancouver, Toronto, Barcelona and Melbourne are criteria: some of the cities that have started to convert roofs into potential pro­ ductive spaces, integrating food production and solar photovoltaic and a) Urban features. It is important to know the urban context, such as solar hot water systems (City of Toronto, 2009; The New York City urban form, population density, use and typology of building. Council, 2010; Ville de Paris, 2019). b) Climatic conditions. Resource potential is crucial to determine the Tables A1 and A2. feasibility for integrating food, water, energy (FWE) systems. Sun­ light amount is necessary for growing food and determine suitable crops, directly affecting crop yields. Solar radiation reached by plants directly impacts the photosynthetically active radiation (PAR), which describes the generally accepted light wavelengths Table A1 between 400 and 700 nm useful for photosynthesis. The relationship Average yield values and water recquirmets for selected crops between solar radiation (normally measured in global horizontal Growing Growing Crop Crop growing Reference radiation, in W/m2h) and PAR (measured with the Photosynthetic method System type yield season Photon Flux Density, in μmol/m2s) cannot be determined accurately (kg/m2) unless values for each individual wavelength are known (Langhans & RTGs soil-less Tomatoe 15.3 Spring- Sanjuan- Tibbitts, 1997). Here, an approximation conversion value is calcu­ summer Delmás et al. lated using global broad bandwidth and PAR measurements taken (2018) Open-air soil-less Tomatoe 5.8 Spring- Boneta et al. during the 2019-2021 period in the Barcelona area in order to give a summer (2019) more precise on-field data. The resulting average conversion factor RTGs soil-less Lettuce 8.36 Spring and Rufí-Salís from recorded daily solar radiation to PAR values used in this study autum et al. (2020) was 2.21 ± 0.01 (CI 95%) to integrate average annual atmosphere Open-air soil-less Lettuce 3.85* Anually Boneta et al. conditions that influence this factor. This factor is in line with the (2019) 2.29 and 2.12 theoretically reached in daylight and blue-sky condi­ * Lettuce crop yield value in open air considers a policultive of vegetables tions respectively (Thimijan & Heins, 1983), which is normally production. Production in RTGs considers passive heating. referred in literature. This allow to calculate the cumulative mea­ surement of total daily photons reached by plants in MJ/m2d (known as day light integral, DLI, normally expressed in μmol/m2s) in order Table A2 to explain the ideal light requirements that saturate the leaf net Water requirement for tomatoes and lettuce irrigation according to growing photosynthetic rate to maximize plant growth (Kozai et al., 2015). systems. Growing Growing Crop growing Water Reference Rainfall over the year plays a key role in determining whether RWHS method System type season use (L/ can compete with other water supply systems, as a general rule, rainfall kg) should be over 50 mm/month for at least half a year or 300 mm/year RTGs soil-less Tomatoe Spring- 66 Rufí-Salís (FAO, 2014; Worm & van Hattum, 2006). summer et al. (2020) Phase 2: Urban and architectural requirements Open-air soil-less Tomatoe Spring- 130.5 Boneta et al. To identify suitable roofs, this phase considers criteria at urban and summer (2019) RTGs soil-less Lettuce Summer 77.6 Rufí-Salís architectural scales. and et al. (2020) autumn a) Urban. Planning and building laws and codes must be considered to Open-air soil-less Lettuce Spring- 130.5 Boneta et al. ensure that the integration of FWE systems meet legal requirements. summer (2019) Cities are composed of diverse building types and a variety of uses, 14 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 such as educational, housing, health, public services, commercial groups of crops are classified according to their optimal daylight and industrial. Building use is important criterion that can define the needs (i.e., maximum yields), even lower light levels will linearly purpose of the FWE systems. For example, educational buildings can decrease yields (Kozai et al., 2015) within acceptable crop yields and integrate these systems for educational and self-sufficient purposes environmental performance (Rufí-Salís et al., 2020) Grouped DLI (Nadal et al., 2019), commercial and industrial buildings for com­ requirements are expressed in mol/m2 and in MJ/m2 daily and mercial purposes (Nadal et al., 2017; Sanyé-Mengual et al., 2015, annually in order to facilitate comprehension and applicability of 2016). this workflow (see light conversion factor section). Hence, the Building uses are usually defined in urban planning and local approach here is to adapt the crop species to the available urban ordinances. solar radiation, since adapting the light requirements with additional b) Architectural. The characterization of buildings roofs (area, slope, LED lightning would lead into an excess of energy needs with high material, radiation and sunlight access) is needed to identify feasi­ environmental costs (Benis et al., 2017). bility areas. • Roof material. Roof materials are essential data to integrate these • Roof area. A minimum roof area of 500 m2 is needed for integrating systems, if a roof does not have the necessary load-bearing capacity UA for commercial purposes (Nadal et al., 2017). A minimum roof to support the weight of the system, it will not be possible to install a area of 13 m2 can satisfy the vegetal demand for one person using soil system on it. A minimum load-bearing capacity 200 kg/m2 is less agriculture systems (Boneta et al., 2019) this was considered for required (Nadal et al., 2017). In the case of RWH, the quality and nonprofit purpose. quantity of rainwater harvested from roofs are significantly affected Taking into account the energy consumption and the yearly by roofing materials (Farreny et al. 2011). operation and maintenance cost, photovoltaics (PV) suitability re­ quires at least a minimum roof area of 100 m2 for commercial use Phase 3: Self-sufficiency indicators and 24 m2 for residential use (Spertino et al., 2013). The established Production of FWE systems must be calculated to obtain the pro­ areas can be geographically sensitive due to major consumption duction and the self-sufficiency potential. To do this, the following three typical at latitudes with less sunlight hours. steps are required: a) Resource production. Potential production is used • Roof slope. For integrate UA systems, slope of roofs must be flat ≤ as indicator to assess the FWE systems integration and to estimate the 10% (Nadal et al., 2017). For rainwater harvesting systems (RWHSs) degree of self-sufficiency. Suitable crops must be identified based on and PV systems, roof slope was not considering, assuming that PV household typical food diet, for this reason, suitable crops can be panels are adjustable. geographically sensitive. Irrigation water requirements of crops data are needed. Water demand, depend on the type of crops and growing Solar radiation. Energy systems require solar radiation higher than methods an systems, if this information is not available for the selected 13 to 14 MJ/m2 per day (Nadal et al., 2017). The amount of radiation is crop in and the geographic region of the study area, data of evapo­ essential for growing food, in this work was established three levels of transpiration (ETo, mm) is required to calculate (Khangaonkar & day light requirements for crops. Daily light requirements (measured Mehaute, 1991). The following considerations should be taken to with DLI) vary depending on the crop and its photoperiod, which range quantify resource production: between 6 to 35 mol/m2d. Current methodologies to identify potential urban agriculture areas normally define a target DLI value between 20 – • Food. To quantify crop yield values, growing method (RTGs or open- 25 mol/m2d, originally expressed in MJ/day (Benis et al., 2017; Nadal air) and growing system (soil less or soil-based) must be considered. et al., 2017; Sanyé-Mengual et al., 2015). • Water. Average rainfall data and runoff coefficient (RC) is required to estimate the potential rainwater harvesting on roofs. RC varies • Other literature refers to the minimum average solar radiation of 8.5 according to roof material from 0.9 to 0.6 (Farreny et al., 2013). MJ/m2d defined for Mediterranean crops as reported by Nisen et al. • Energy (PV). The technology for PV for example the most commonly (1998). These DLI targets usually satisfy light requirements for in the market. multiple selected crops or single species like tomato plants, which is a reference crop for agriculture studies. However, these are high c) Self-sufficiency. For this phase, two criteria are necessary: potential compared to the optimal DLI requirements for other valuable crops production and household consumption. The potential food for self- as microgreens (see Table A3), underestimating the urban potential sufficiency is calculated by dividing the potential production (food, area that could grow low and medium-DLI crop types. Here, three water, energy) by the average consumption of the food Table A3 Crop classification according to DLI requirements. Day light requirements (DLI MJ/m2•year OA / Crop specie Optimal DLI (mol/ Reference target) RTG m2•day) Low > 6 mol/m2 > 2.7 MJ/ > 800* > 1400* Microgreens 6—12 Kozai et al. (2015); Verlinden m2 (2020) Small vegetative crops, green < 12 Kozai et al. (2015) shoots Lettuce 12—17 Albright et al. (2000) Medium > 12 mol/m2 > 1650* Leafy crops 12—17 Albright et al. (2000) > 5.4 MJ/m2 > 2800* Vegetable seedlings 13 Fan et al. (2013) Strawberries 13 Kozai et al. (2015) Young / low-wire tomato (e.g. 13—17.3 Ingestad et al. (1994) cherry) High > 22 mol/m2 > 2000** > High-wire tomato Avg 22—26 Up to Spaargaren (2001) 3800*** 30—35 >10.0 MJ/m2 High-wire tomato Avg 22—29 Up to Schwarz et al. (2014) 30—40 * Considering 10 and 12 months of crop season for OA and RTG farming, respectively. ** Considering 6 months of crop season equivalent to one short tomato crop. *** Considering 8 months of crop season equivalent to one extended tomato crop. 15 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 (kg•person•year), water (lts•person•year) and energy (kWh•person•­ Grouped DLI requirements are expressed in mol/m2 and in MJ/m2 year) in the study area, resulting in the total number of people whose daily and annually in order to facilitate comprehension and applicability demand is satisfied. of this workflow to other solar measurement sites. It is important to note that vegetative crops do not require significant lower DLI during the Appendix 4. Light conversion factor and sunlight access for early stage of the plants development, while fruiting crops like tomatoes rooftop urban agriculture require significant lower levels (from 2 to 13-20 mol/m2d) during the vegetative growth stage compared to the fruiting phase up to 40 mol/ The cumulative measurement of total daily photons reached by m2d (Schwarz et al., 2014). Therefore, when calculating the annual light plants is known as daily light integral (DLI) expressed in mol/m2/day, needs for tomato plants in the high DLI group, an average value between and explains the linear relationship between light and plant growth rate 20 and 25 mol/m2d is assumed (i.e., considering that the vegetative needed to saturate the leaf net photosynthetic rate (Kozai et al., 2015). phase lasts the same period as the fruiting phase (Philips Lighting Hor­ DLI requirements vary depending on the crop and its photoperiod, ticulture, unpublished work). which range between 6 to 35 mol/m2/day The growing period has been considered according to the average Light conversion factor has been studied with on-field measurements months in the Mediterranean area, excluding the coldest months in the Urban Agriculture facilities in the Institute of Science and Tech­ (December and January) (FAO, 2013). nology (ICTA-UAB) in the compounds of Universitat Autònoma de Similarly, one short tomato crop lasting 6 months has been also Barcelona in the city of Cerdanyola del Vallès, in the Barcelona region. considered according to common practices in the Mediterranean climate The weather station in this facility is equipped with a global solar ra­ while an extended tomato crop of 8 months has been considered for diation (Hukseflux LP02, second class pyranometer) and a PAR Quan­ rooftop greenhouse farming. Finally, an average solar transmissivity tum sensor (SKP215 with a ±5% accuracy, typically <±3%) which radiation of 70% has been considered, even this could vary according to measures incident quanta between 400 and 700 nm. A CR3000 data­ roof slope and orientation (Castilla, 2005). logger from Campbell Scientific measures data every 5sec, recording the averages at hourly intervals during the 2019 to 2021 period. Appendix 5. Roof materials and structural discussion In order to study the Day Light Integral of crops, the sum of all incident radiation of both sensors has been summed per day and then Industrial roofs were present in a larger proportion of the total study divided in order to calculate the conversion factor from global radiation area than service roofs. However, a smaller proportion of potential in­ (W/m2) to PAR (PPFD, in μmol/m2s). The obtained values in a box plot dustrial roofs was observed to integrate UA. Generally, this type of roof are presented in detail in Figure A1. Note that significant differences is characterized by large polygons and a high amount of solar radiation, exist between year quarters, being the second and third quarter greater which makes the integration of RTGs feasible. Even so, most of them than the first and fourth. However, as the assessment here is annual have low load-bearing capacity, such as metal decks and light or metal based, a unique average value of 2.21 ± 0.01 (CI 95%) for all year has coverings and fiber cement sheets. Low-resistance materials are a barrier been used. to integrating UA systems on roofs in the immediate term, and an adaptation concerning structural reinforcement of the roof is needed for it to be a candidate for UA integration. Similar results were found by Nadal et al. (2017) in an industrial zone of the Metropolitan Area of Barcelona, where roofs met the minimum area and solar radiation but not the load-bearing capacity of the roof material. Service and housing buildings are usually composed of higher load-bearing capacity struc­ tures, with non-transitable roof systems composed of concrete and gravel surface finishing or other concrete systems finished with asphalt sheets. These materials meet structural resistance requirements but require a roof finishing adaptation to allow the installation and opera­ tion for growing and harvesting food. Regarding housing buildings, roofs were characterized by a variety of the previous roof systems described, in addition to ceramic roofs, which can fulfill the resistance criterion because concrete systems in general are presented in ceramic roof finishing covers. However, some roofs with too much slope could have metal structures that might not fulfill the resistance requirement Appendix 6. RWHS for crop production These differences can be related to the following reasons: (1) catchment area, this work considered all roof areas as catchment sur­ faces; (2) RC value, this work considered a variety of RCs (0.9,0.8, and 0.6) according to roof materials in the area, while a conservative value of 0.6 was considered for all potential areas by Lupia et al. (2017); and (3) water requirements, establishment of water consumption for irri­ gation plays an important role, and soil-less systems, such as perlite, require less water than soil based systems Figure A1. Annual and quarter conversion factor values. 16 P. Zambrano-Prado et al. Sustainable Cities and Society 75 (2021) 103287 Appendix 7. Domestic water self-sufficiency 201 to 1000 m2 for toilet flushing and laundry (Petit-Boix et al., 2018); in Barcelona city a 7 m3 cistern size could supply water demands These differences could be due to the RC values used; previous work (Toboso-Chavero et al., 2018). It is necessary to evaluate water tank size considered 0.85 for all roofs (Domènech & Saurí, 2011). The present at the building level on a case-by-case basis or as a whole for those with study incorporates the identification of roofing materials which allowed the same typologies. distinguishing the spatial distribution of them and therefore, consid­ ering a variety of RCs from 0.6 to 0.9. Farreny et al. (2013) reported Appendix 9. Population density of each municipality and the water self-sufficiency ranging from 80% to 90% from water collected self-sufficiency potential from rooftops, in addition to paved covers at ground surface (roads, car park, and paved pedestrian areas) which represented 67% of the Table A4 shows a comparison between the highest and lowest den­ catchment area in retail parks from Barcelona. Therefore, considering sity municipalities in the study area and the differences in self- water harvesting in ground-level areas could increase the potential for sufficiency by system. The greatest difference is in the food (total to­ water self-sufficiency, however various implications of the system would mato production) and water, exceeded by up to 65 and 13 times, have to be studied. respectively, by the municipality with the lowest density. In addition to population density, other factors can affect the potential, the urban Appendix 8. Downscaled analysis water self-sufficiency irrigation Table A4 The Figures A2 and A3 show the water self-sufficiency per system Population density and self-sufficiency by system and crop per month. For tomato cultivation in greenhouses, self- Self-Sufficiency (number of times) sufficiency ranges from 28% to 48%; tomato in an outdoor system Municipality Population Food Food Water Solar from 34% to 75%; lettuce in greenhouses from 44% to 100% and lettuce density Tomato Lettuce Energy in the open air from 51% to 100%. (inhabitants/km2) A smaller-scale analysis contributes to a better understanding of Badia del 14,426.88 9.7 1.0 2.0 0.9 water tank sizing and its relation to weight (aspects not included in this Vallès work). Other studies carried out at a smaller scale (building and Cerdanyola 1,889.39 74 7.4 15.5 6.5 del Vallès neighborhood) found for the city of Callafel (Catalonia, Spain) tank sizes of 4 m3 for roof area less than 200 m2 could only meet 20% and 57 m3 For self-sufficiency analysis the potential area of the total study area is water tank could supply from 76 % to 99% according to roof area from considered. Figure A2. 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