Sustainability assessment of urban rooftop farming using an interdisciplinary approach

Urban agriculture (UA) is blooming around cities of the developed world as a response to the increasing urban population, the growing environmental awareness of the industrial food system and the need of addressing social gaps. These new local food systems aims to develop sustainable pathways that re-establish the relations between producers and consumers while boosting local economies and minimising food-miles. Furthermore, the recent financial crisis and the spread of vacant lands have revitalised UA projects, not only at the self-managed level (i.e., community, private) but also at the commercial one. In particular, UA practitioners and farmers have found in the roofs of the city a vacant space for placing food production leading to the development of urban rooftop farming (URF). Consequently, rooftop farms, rooftop greenhouses and rooftop gardens have colonized buildings. Nevertheless, specific assessment of the potential implementation and the sustainability performance of different URF forms, cultivation techniques and crops, are necessary. To address these gaps, this dissertation seeks to answer two main research questions ? What is the potential of urban rooftop farming in qualitative and quantitative terms? and ? What are the environmental impacts and economic costs of urban rooftop farming systems? With this goal, a methodological framework is proposed and three case studies are analysed, which are pilot experiences of different forms of urban rooftop farming. Food production in cities is a complex system that involves several stakeholders, has multiple scales and affects the three dimensions of sustainability (environment, economy, society). Thus, a comprehensive assessment might combine different disciplines to approach such topic. This dissertation follows an interdisciplinary framework that includes (a) qualitative research, to deepen in the perceptions of the different stakeholders related to UA and URF; (b) geographic information systems (GIS), to identify and quantify the available and feasible roofs for implementing rooftop farming; (c) life cycle assessment (LCA), to quantify the environmental burdens of rooftop farming systems; and (d) life cycle costing (LCC), to quantify the economic costs of URF. This framework enables to approach URF from the city scale (e.g., planning perspective) to the system scale (e.g., food products). A stakeholders? analysis through qualitative interviews unravelled that the development of rooftop farming in Barcelona is currently facing some limitations mainly due to a constrained support from some stakeholders. The main barriers to supporting urban rooftop farming are the lack of a common definition of urban agriculture, the specific origin of UA in Barcelona and its urban morphology and the limited social acceptance of some food production techniques. However, stakeholders valued the sustainability benefits (i.e., environmental, economic and social) linked to urban rooftop farming, particularly in the context of the development of a local green economy. In quantitative terms, urban rooftop farming shows a great potential for increasing the current local production and reducing the environmental burdens of the city? s ? foodprint? A multicriteria set is needed to identify the technically and economically feasible roofs for the implementation of commercial rooftop greenhouses (RTGs) (i.e., availability of space, sunlight, resistance and slope, and legal and planning requirements). Industrial parks and retail parks are here analysed and compared. Retail parks show a greater short-term potential (53-98%) than industrial parks (8%) due to a more resistant architecture, although industrial parks are of great interest for large-scale URF implementation plans due their extensive area. The potential implementation of integrated rooftop greenhouses (i-RTGs) which take advantage from the residual flows from the building (i.e., residual heat and CO2, rainwater) is an innovative way of rooftop farming. Benefits of i-RTGs vary in warm regions (e.g., Mediterranean), where unheated production can be performed, and cold regions (e.g., The Netherlands), where greenhouses requires heating. The preference between regions for implementing i-RTGs is based, thus, on whether the goal is increasing food production (i.e., higher crop yields in warm areas) or reducing environmental burdens (i.e., substitution of energy consumption for heating in cold areas). From a life cycle perspective, the rooftop greenhouse lab (RTG-Lab) (Bellaterra, Spain), the community rooftop garden in Via Gandusio (Bologna, Italy) and a private rooftop garden in the city centre of Barcelona (Spain) are analysed. URF can become an environmentally-friendly option for further develop urban agriculture and local food systems in cities. However, results depend on the type of rooftop farming, the crop and the growing system. The pilot projects assessed in this dissertation unravelled some trends and drawn some recommendations for the development of rooftop farming. Regarding food production in rooftop greenhouses, the greenhouse structure plays a major role in the environmental impacts and the economic costs (41.0-79.5%), as in conventional agriculture. Although the greenhouse structure of RTGs have greater environmental impacts than multi-tunnel greenhouses (between 17 and 75 %), tomatoes from an RTG in Barcelona are more environmentally-friendly not only at the production point (between 9 and 26% lower) but also at the consumer (between 33 and 42 % lower). Although tomato production results in 21% higher cost than conventional tomatoes, the consideration of the entire supply-chain highlights the competitiveness of RTGs as local food systems. Regarding rooftop gardens, crop inputs are the most contributing elements. The community garden employed re-used elements in their design (e.g., pallets) and irrigation was the most contributing stage (60-75%). In the private garden, fertirrigation (between 33 and 46%) and the structure of the garden (between 28 and 35%) (i.e., made of raw wood) were the main contributors. Rainwater harvesting for supplying the water demand of the crops and the integration of re-used elements in the cultivation structures might enhance the sustainability of gardens by decreasing the resources consumption of the system. The comparison of different growing techniques in the community garden highlighted the higher eco-efficiency of soil production, when compared to hydroponic techniques (i.e., nutrient film technique, floating system). The assessment of different crops showed the same pattern in the community and private rooftop gardens. Fruit vegetables have lower environmental burdens than leafy vegetables since they yield better. However, these rooftop farming forms perform polyculture, the design of which is commonly oriented to fruit vegetables, resulting in a low plant density for leafy vegetables. An improved design, which divides the garden, could then improve and balance these divergences among crop types. This dissertation contributes to the comprehension of the development process of competitive and sustainable urban agriculture and urban rooftop farming in cities of developed countries by developing methodological aspects and generating new data on the topic. The methods and results advance in the knowledge and understanding of rooftop farming, urban agriculture and local food in order to support decision-making processes in the design and development of future rooftop farming projects. Future research and strategies might focus on assessing the perceptions of stakeholders in other case studies, while focusing on specific aspects such as social acceptance; quantifying the potential of rooftop farming in other urban areas and cities; and assess more case studies and URF forms from a sustainability perspective, paying particular attention to the integration of the social aspects.