Editorial: Worldwide experience on NbS in the water sector Open Access

The water sector worldwide faces increasing challenges due to climate change, rapid urbanization, and resource depletion. Traditional gray infrastructure solutions are often costly, energy-intensive, and limited in their ability to provide additional benefits besides water management. Nature-based solutions (NbS) offer a pathway to address these challenges holistically by incorporating ecological principles into engineering practices.

This Special Issue of Blue-Green Systems is dedicated to advancing knowledge on NbS for urban and watershed-scale water management by presenting a wide range of studies covering technical, environmental, social, and policy dimensions. The contributions in this issue explore innovative applications of NbS, including solutions for stormwater management, wastewater treatment, non-sewered sanitation, water reuse, industrial water management, and watershed restoration. Beyond addressing technical feasibility, the Special Issue also aims to enhance awareness of the multifunctional benefits of NbS and their role in supporting sustainability in the water sector and contribute to a broader understanding of how NbS can be effectively designed, implemented, and managed across different geographic and economic contexts.

The 12 contributions of this SI provide a wide-angle observation on the application and future needs of Nbs in the water sector. Several papers explore the role of NbS in urban water management and stormwater control, highlighting their capacity to mitigate flood risks and improve infiltration processes. Rossetto et al. (2023) evaluate the long-term performance of a concrete grid paver parking area in Italy, demonstrating that, even after 15 years of operation without maintenance, the system continues to meet infiltration capacity standards. This is largely due to the role of vegetation and soil processes in preventing clogging. Funke & Kleidorfer (2024) provide a model-based analysis of sustainable urban drainage systems (SUDS), assessing their sensitivity to extreme rainfall events and malfunctions. Their findings suggest that while SUDS can effectively manage stormwater under normal conditions, their resilience to extreme events varies depending on design specifications and maintenance practices. A broader perspective on performance evaluation is provided by Roghani et al. (2024), who conduct a comparative analysis of international guidelines for assessing NbS. Their study reveals significant disparities in how different countries evaluate NbS performance, emphasizing the need for harmonized metrics to support decision-making and ensure consistency in implementation. Adding to this, Sippel et al. (2023) present a novel method for assessing urban tree vitality using smartphone-based photography, offering a practical and cost-effective tool for monitoring of NbS. Their findings highlight the importance of accessible tools for long-term ecosystem monitoring in urban environments.

NbS also play a crucial role in wastewater treatment and water reuse, as demonstrated by several studies in this issue. Miyazaki et al. (2023) investigate the impact of aeration strategies on nitrogen removal in hybrid aerated treatment wetlands, finding that intermittent aeration can significantly enhance nitrogen removal efficiency while optimizing energy use. Mendoza et al. (2024) examine the potential of hydroponic systems to treat graywater while simultaneously supporting food production. Their study shows that hydroponic lettuce cultivation using treated graywater is viable, with certain micropollutants being removed efficiently, though further research is needed to assess potential human health risks. For urban runoff, water quality is also an important topic. Beral et al. (2023) investigate the impact of de-icing salt runoff on bioretention systems, demonstrating that while short-term exposure does not significantly affect performance, long-term accumulation could compromise filtration efficiency.

The regional application of NbS in wastewater management is explored by Recalde et al. (2024), who analyze its implementation in the Andean region. They highlight how traditional water management practices align with modern NbS principles and could serve as effective strategies for improving water security in this region.

Beyond water quantity and quality management, this issue also explores the potential of NbS in hydrometeorological risk reduction and climate adaptation. Penny et al. (2024) assess innovation pathways within the EU Horizon 2020 RECONECT project, focusing on the commercialization and upscaling of NbS for flood risk reduction. Gutierrez Caloir et al. (2023) employ machine learning and spatial data techniques to optimize NbS allocation at a large scale, presenting case studies from the Netherlands, Serbia, and Bolivia.

Finally, the governance, policy, and decision-making dimensions of NbS implementation are critically analyzed. The socio-economic dimensions of NbS adoption are examined in Zhang et al. (2023), who assess public willingness to pay for low-impact development in Chinese sponge cities. Their findings highlight the influence of public perception and financial considerations on NbS implementation and provide insights into policy and funding mechanisms. Similarly, Bousquet et al. (2023) explore stakeholder engagement in NbS decision-making, emphasizing the importance of participatory approaches in ensuring the long-term success and acceptance of NbS initiatives.

The research presented in this SI highlights the long-term effectiveness, multifunctionality, and growing recognition of NbS in the water sector. Studies on permeable pavements and treatment wetlands confirm that these solutions can remain functional over decades with minimal maintenance, making them cost-effective and resilient alternatives to conventional infrastructure. However, challenges persist in fully integrating NbS into urban planning, regulatory frameworks, and financial mechanisms. While decision-support tools and valuation methodologies are improving, there is still a need to standardize performance assessment criteria and strengthen economic justifications for large-scale investments.

Stakeholder engagement emerges as a critical factor for successful implementation. Research on China's sponge cities reveals that public acceptance of NbS depends on their perceived effectiveness, particularly in flood mitigation. Similarly, governance studies emphasize that co-design approaches, involving local communities, policymakers, and scientists, can enhance adoption and long-term impact. In developing regions, such as the Andean countries, NbS aligned with traditional water management practices offer significant potential but require better financing mechanisms and technical expertise for wider application.

Looking ahead, scaling up NbS will require clearer policies, robust business models, and greater investment in research focused on resilience to extreme weather events. By addressing these gaps, NbS can transition from pilot projects to mainstream solutions, playing a key role in achieving water security and climate resilience on a global scale.