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.

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<p>Vertical green is promoted as climate change mitigation and adaptation measure, and it provides green space for the urban population. However, it could be used in urban water management as well if its evapotranspiration, thus its water demand would be predictable.</p><p>For optimal performance, plants need to be provided with water, nutrients, and rooting space. But irregular precipitation, drought periods, and lack of natural water storage necessitate additional irrigation preferably by local water sources (such as rainwater runoff and greywater).</p><p>The amount of water needed for irrigation can be calculated using the Penman-Monteith approach which quantifies evapotranspiration of vegetated horizontal surfaces. For Vertical Green, the Penman-Monteith equation has already been tested. In that way, water demand of VGS can be calculated for hourly time steps based on radiation, wind speed, and vapor pressure deficit expressed by air temperature and relative humidity data.</p><p>The needed meteorological data can be measured on-site or derived, thus adapted – verticalized - from remote climate stations, depending on data availability, and needed accuracy of the results. This study models water demand using (1) on-site measured meteorological data, (2) ‘verticalized’ remote station data, and (3) remote station data. We then compare simulated evapotranspiration with measured lysimetry data for a ground-based Vertical Greenery system of <em>Fallopia baldschuanica</em> monitored in Berlin, Germany.</p><p>This study finds radiation and vapor pressure deficit to have the highest impacts on the variance of the results while wind speed has the lowest impact. In this contribution, we present the developed model, verticalization methods for the input parameters and validate the performance of the model based on measured water demands.</p>

Vertical greening systems (VGS), as proven strategy for adaptation to heat stress, are prohibited for monument-protected buildings and façades. To what extent monument protection effectively limits the implementation of VGS in heat stress affected city centers has not been quantified yet. In this study, the ratio of buildings under monument protection was quantified for Berlin, Germany, using a GIS analysis. It was then compared to a map of potential heat stress. This was done for the entire city and the inner-city area on block-scale.Therefore, maps of the Berlin buildings, monuments and blocks were processed. The ratio of buildings under monument protection was calculated for each block of Berlin. To not deform the outcome, urban green areas were cut out of the maps. Inside the 80 km2 city center of Berlin, which is severely affected by heat stress, the range of monument protection prohibiting VGS in the individual blocks ranges from 0 % to 100 %. However, 25.42 % of the building façades in the city center and 16.20 % for whole of Berlin are protected on average and therefore cannot be greened. Compared to other restricting factors, monument protection does not generally hinder large scale implementation of VGS in Berlin. Nonetheless, 102 potentially heat stress exposed blocks inhabited by 48,122 people cannot be greened due to monument protection. This demonstrates that VGS should be discussed as exception from monument protection that can be justified by a predominant public interest. It also points to the need of minimal invasive and mobile greenery technologies, which would enable heritage-protection conform greening.