Water in the city is typically exploited in a linear process, in which most of it is polluted, treated, and discharged; during this process, valuable nutrients are lost in the treatment process instead of being cycled back and used in urban agriculture or green space. The purpose of this paper is to advance a new paradigm to close water cycles in cities via the implementation of nature-based solutions units (NBS_u), with a particular focus on building greening elements, such as green roofs (GRs) and vertical greening systems (VGS). The hypothesis is that such “circular systems” can provide substantial ecosystem services and minimize environmental degradation. Our method is twofold: we first examine these systems from a life-cycle point of view, assessing not only the inputs of conventional and alternative materials, but the ongoing input of water that is required for irrigation. Secondly, the evapotranspiration performance of VGS in Copenhagen, Berlin, Lisbon, Rome, Istanbul, and Tel Aviv, cities with different climatic, architectural, and sociocultural contexts have been simulated using a verticalized ET₀ approach, assessing rainwater runoff and greywater as irrigation resources. The water cycling performance of VGS in the mentioned cities would be sufficient at recycling 44% (Lisbon) to 100% (Berlin, Istanbul) of all accruing rainwater roof–runoff, if water shortages in dry months are bridged by greywater. Then, 27–53% of the greywater accruing in a building could be managed on its greened surface. In conclusion, we address the gaps in the current knowledge and policies identified in the different stages of analyses, such as the lack of comprehensive life cycle assessment studies that quantify the complete “water footprint” of building greening systems.

Urban regeneration, which includes renovation of decaying urban structures as well as renovation of dilapidated buildings, is an important economic, aesthetic and cultural factor for sustainable development planning worldwide. The revitalization of urban structures is a complex challenge, influenced by various interdisciplinary aspects, like urban socio-economic development, integrated transport and mobility solutions and others. And it requires involvement of different stakeholders at different levels in identifying and finding solutions. Technological innovations offer a variety of digital tools to address modern urban governance and modelling issues, identifying problem areas with public involvement and participation that allow relevant services to respond quickly, thus improving the urban environment and the quality of life of people. With the help of digital tools, it is also possible to promote the availability of public services, model and forecast environmental risks, as well as perform other functions integral to the organization and management of modern society. However, the wide range of these tools and their mutually asynchronous use often make it difficult to choose the right tool, thus creating the risk of fragmented implementation. The aim of this paper is to propose theoretical approach for modelling of complex interactions affecting urban regeneration as well as summarize and systematize the existing digital information tools which might empower the process of regeneration. The analysis of the use and impact of digital and smart tools allows to identify urban problems, as well as to outline ways to solve them. The results of the research will form the basis for further research stages.