Closing water cycles in the built environment through nature-based solutions: The contribution of vertical greening systems and green roofs

Water in the city is typically exploited in a linear process, by which most of it becomes polluted, 68 treated and discharged – in the process losing valuable nutrients rather than cycling them back for 69 urban agriculture or green space. The purpose of this paper is to advance a new paradigm for closing 70 water cycles in cities through the implementation of nature-based solutions (NBS), with a particular 71 focus on building greening elements such as green roofs (GR) and vertical greening systems (VGS). 72 The hypothesis is that such "circular systems" can provide substantial ecosystem services and 73 minimize environmental degradation. Our method is twofold: we first examine these systems from 74 a life-cycle point of view, assessing not only the inputs of conventional and alternative materials but 75 the ongoing input of water that is required for their irrigation. Secondly, the evapotranspiration 76 performance of VGS in Copenhagen, Berlin, Lisbon, Rome, Istanbul and Tel Aviv, cities with 77 different climatic, architectural and sociocultural contexts, have been simulated using a verticalized 78 ET0 approach and assessing rainwater runoff and greywater as irrigation resources. The water 79 cycling performance of VGS in the mentioned cities would be sufficient to recycle 44% (Lisbon) to 80 100% (Berlin, Istanbul) of all accruing rainwater roof-runoff, if water shortages in dry months are 81 bridged by greywater. Then, 27 – 53 % of the greywater accruing in a building could be managed 82 on its greened surface. In conclusion, we address gaps in current knowledge and policy that were 83 identified in the different stages of analysis, such as the lack of comprehensive life cycle assessment 84 studies which quantify the complete "water footprint" of these building greening systems.