Nature-based solutions (NBS) can protect, manage and restore natural or modified ecosystems. They are a multidisciplinary, integrated approach to address societal challenges and some natural hazards effectively and adaptively, simultaneously providing human well-being and biodiversity benefits. NBS applications can be easily noticed in circular cities, establishing an urban system that is regenerative and accessible. This paper aims to offer a review on NBS for urban water management from the literature and some relevant projects running within the COST Action ‘Implementing nature-based solutions for creating a resourceful circular city’. The method used in the study is based on a detailed tracking of specific keywords in the literature using Google Scholar, ResearchGate, Academia.edu, ScienceDirect and Scopus. Based on this review, three main applications were identified: (i) flood and drought protection; (ii) the water-food-energy nexus; and (iii) water purification. The paper shows that NBS provide additional benefits, such as improving water quality, increasing biodiversity, obtaining social co-benefits, improving urban microclimate, and the reduction of energy consumption by improving indoor climate. The paper concludes that a systemic change to NBS should be given a higher priority and be preferred over conventional water infrastructure.

Resource depletion, climate change and degradation of ecosystems are challenges faced by cities worldwide and will increase if cities do not adapt. In order to tackle those challenges, it is necessary to transform our cities into sustainable systems using a holistic approach. One element in achieving this transition is the implementation of nature-based solutions (NBS). NBS can provide a range of ecosystem services beneficial for the urban biosphere such as regulation of micro-climates, flood prevention, water treatment, food provision and more. However, most NBS are implemented serving only one single purpose. Adopting the concept of circular economy by combining different types of services and returning resources to the city, would increase the benefits gained for urban areas. The COST Action Circular City aims to establish a network testing the hypothesis that: ‘A circular flow system that implements NBS for managing nutrients and resources within the urban biosphere will lead to a resilient, sustainable and healthy urban environment’. In this paper we introduce the COST Action Circular City by describing its main objectives and aims. The paper also serves as introduction to the review papers of the Action's five Working Groups in this Special Issue.

Cities worldwide are facing several challenges connected to urbanization and climate change. Several cities have identified the implementation of nature-based solutions (NBS) as an option to mitigate several challenges at once. However, can two different aims be reached with NBS in the same location? This question has not yet been addressed. This paper discusses the spatial compatibility of NBS implementation strategies to tackle (1) urban heat island (UHI) effects and (2) water pollution at the same location. The evaluation is based on a spatial analysis of Berlin. We found a positive correlation of high UHI and median high stormwater pollution loads for zinc, total suspended solids, Polycyclic Aromatic Hydrocarbons and Terbutryn. Out of more than 14,000 building/street sections analyzed, 2270 showed spatial matching of high UHI and high stormwater pollution loads. In the majority of building/street sections, stormwater pollution was high for three out of the four parameters. We conclude that the compatibility of NBS implementation for both challenges depends both on the implementation strategies for NBS and on the specific NBS measures. Our spatial analysis can be used for further planning processes for NBS implementation.

In der Bewirtschaftung von Niederschlagswasser setzt sich in den letzten Jahren ein Trend in Richtung dezentraler, flexibler Lösungen und grüner Infrastruktur durch, um besser an künftige Herausforderungen wie den Klimawandel oder die Urbanisierung angepasst zu sein. Bei der Erweiterung oder Neuerschließungen von Siedlungsgebieten wird zunehmend der Erhalt des natürlichen Wasserhaushalts angestrebt. Hier fällt es vergleichsweise leicht, dezentrale Maßnahmen wie Versickerung oder Retention entsprechend dem Stand der Technik und Forschung vorzuschreiben bzw. umzusetzen. Innerhalb bestehender Stadt- und Siedlungsstrukturen ist eine vertiefte Auseinandersetzung mit Bebauung, Flächennutzung und Flächenwidmung erforderlich, um mögliche geeignete Standorte für die Niederschlagswasserbewirtschaftung zu identifizieren. Es wird eine Methode aufgezeigt, um auf Basis bestehender amtlicher Daten räumliche Strukturtypen aus Sicht der Siedlungswasserwirtschaft abzugrenzen. Diese dienen als Basis, um flächenbezogen innerhalb des Bestands Potenziale und Restriktionen für die Niederschlagswasserbewirtschaftung zu identifizieren. Die hier entwickelte Methodik kann einen Beitrag zur fundierten Entscheidungsfindung für die Auswahl und Umsetzung von dezentralen Maßnahmen der Niederschlagswasserbewirtschaftung leisten. The implementation of decentralized flexible measures and green infrastructure for stormwater management is increasing, in order to better adapt to future challenges for example to climate change or urbanization. For settlement expansions and new constructions maintaining the natural water balance is gaining importance for stormwater management. In these areas realizing decentralized state-of-the-art measures is relatively simple and can also be enforced through regulative measures. In existing urban structures, however, a detailed analysis of the built structure, the land use and zoning is necessary in order to identify potential locations for decentralized measures for stormwater management. In this paper a method is presented on how urban structures can be categorized from a stormwater management perspective based on nationwide available official data. The defined urban structures are used to identify potentials and constraints for stormwater management. The developed method can contribute to sound decision making regarding the selection and implementation of decentralized measures for stormwater management.