Bridging CBA and MCA for evaluating green infrastructure: Proposal of a new evaluation model (MAGICA)

Abstract

The evaluation of green roofs and walls has been overlooking their real contribution to the built environment. This limits the application and development of these solutions, that have the potential to improve biodiversity, resilience, and carbon footprint of cities. In addition to the difficulty in measuring some ecosystem services, conventional cost-benefit analyses have proven to be inefficient as they cannot deal with non-monetized benefits. To overcome this gap, this study develops a comprehensive decision-making tool for assessing the value of green roofs and walls. The proposed methodology combines two well-known approaches - cost-benefit analysis (CBA) and multi-criteria analysis (MCA) - giving preference to the economic rationale for project investment optimization. This new model – MAGICA (Modelling the Attractiveness of Green Infrastructure through a Combined Approach) – addresses differently monetary and non-monetary benefits for a more robust and complete consideration of all financial, economic, socio-environmental impacts of green infrastructure as well as the preferences of the investors and users. The methodology covers different scales of implementation, including new and existing built assets, and it can be easily adapted to other nature-based solutions. Despite determining an economic indicator, the model is best suited for making relative comparisons between different solutions and identifying which is most appropriate for a particular project context. A case study is applied to demonstrate the methodology, assessing 16 green roof systems for installation in a university building in Lisbon, Portugal. The analysis is based on the results of an experimental campaign and field surveys of students, professors, and staff.