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. This paper proposes a paradigm shift toward closing urban water cycles through nature-based solutions (NBS), focusing on building greening elements like green roofs (GRs) and vertical greening systems (VGS). Using a life-cycle assessment approach, it evaluates material inputs and ongoing irrigation water demands. Simulations across six diverse cities show that VGS can recycle 44 (Lisbon) to 100% (Berlin, Istanbul) of rainwater runoff when supplemented by greywater during dry periods, while managing 27–53% of building greywater on greened surfaces. The study highlights significant knowledge and policy gaps, particularly the need for comprehensive life cycle assessments quantifying the full „water footprint“ of building greening systems.
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