15th Water Research Horizon Conference

Challenges Towards Sustainable Urban Water Cycles

Urban water systems face increasing pressure from climate change, pollution, and the ongoing transformation of cities. These challenges demand solutions that transcend disciplinary boundaries, integrating hydrology, ecology, engineering, public health, socio-economics, and community engagement. The 15th Water Research Horizon Conference (WRHC15), hosted by the Water Science Alliance (WSA) at Ruhr-Universität Bochum (RUB) on 29-30 September 2025, embraced this complexity by gathering researchers, practitioners, policymakers, and civil-society actors for a strongly inter- and transdisciplinary exchange on current state of research and the future of sustainable urban water cycles.

Framed by the context of the Ruhr metropolitan region, the conference offered a programme developed collaboratively between the WSA chairs, the WSA Office, and local RUB scientists. Thematically structured into three interlinked sessions on 1) urban water-body targets, 2) water-energy coupling, and 3) sponge-city concepts - alongside a dedicated science-communication track, WRHC15 highlighted how achieving resilient and equitable urban water systems requires knowledge integration across diverse fields and active dialogue between science and practice. An introductory keynote on nature-based solutions (NBS) and sponge-city experiences in China illustrated the global dimension of these debates and sparked intensive discussion among participants: How do approaches chosen in Germany and China compare - what can we learn from each other to improve the sustainability of the urban water cycle?

The first session “Targets for urban water bodies” underscored how urban water bodies represent uniquely heterogeneous and complex blue-green systems, shaped by highly dynamic hydrology, changing water quality, and an unparalleled range of societal demands. Against this backdrop, the speakers explored what realistic developmental targets for urban aquatic environments can look like. Presentations from Dresden, Berlin, and the Ruhr area demonstrated the need for cross-disciplinary approaches to define realistic developmental targets that reconcile the respective ecological, climatic, recreational, and economic goals. Case studies such as Lake Phoenix in Dortmund showcased how integrated visions can drive multifunctional urban water transformations. The speakers emphasised that the rapid, event-driven changes that are characteristic of urban waters require continuous, high-resolution monitoring. This should involve a combination of hydrological sensors, low-cost IoT networks, environmental DNA, and functional biological assays. And the speakers from various projects demonstrated impressively, how these sensor networks can be developed beyond prototypic show cases. Citizen-science initiatives further illustrated the value of transdisciplinary cooperation, as local knowledge and public participation enrich scientific understanding. Together, these tools enable a system-wide assessment of how blue-green infrastructure supports biodiversity, climate resilience, and societal well-being.

The second session “Coupling water and energy for a circular economy” examined how innovative connections between water and energy systems can support regional decarbonisation, particularly in post-industrial urban landscapes. Examples from the Ruhr and Ore Mountains demonstrated how flooded mine workings - traditionally regarded as environmental liabilities - can become low-carbon thermal resources. Advances in hydraulic engineering, materials science, and subsurface microbiology enable safe extraction of mine water heat, while mine-thermal energy storage linked to microbial processes shows promise for reducing contaminant risks. At a broader systems scale, contributions on wastewater-based electrochemical CO₂ conversion and river heat pumps along the Rhine highlighted how circular-economy principles can be embedded into water infrastructure. The session speakers made clear that realising these potential benefits is a challenge that goes beyond research alone. While the interdisciplinary research exchanges across hydrology, chemistry, microbiology, and energy engineering are key, success is only possible through transdisciplinary co-design with municipalities, industry and regulators. The case studies clearly showed that urban water systems can become key drivers of climate-neutral and resource-efficient energy transitions when incorporated into a well-planned system.

The final thematic session “Climate-resilient, sustainable, and health-promoting urban transformation – sponge city aspects” focused on the systemic implementation of water-centred NBS for climate-resilient, sustainable, and health-promoting cities. Presentations illustrated how the sponge-city principle - restoring natural water cycles through decentralised infiltration, retention, and evapotranspiration - can mitigate floods, buffer droughts, and provide cooling during heatwaves. A broad portfolio of blue-green elements, from wetlands to green roofs and tree swales, demonstrated the versatility of NBS across different scales and land-use types. A central theme was the need for transdisciplinary collaboration: effective sponge-city implementation requires not only hydrological and ecological expertise, but also urban planning, governance innovation, social sciences, and active engagement with private landowners. The “Land4Climate” initiative, for example, showed how voluntary cooperation and co-benefit sharing can activate privately owned land for public climate adaptation goals. A hydro-meteorological study from Kilimanjaro further highlighted how local knowledge, participatory monitoring, and understanding of land–soil–water interactions strengthen water management in both urban and rural settings. Collectively, the session underscored that successful climate-resilient transformation depends on integrated planning that spans sectors, scales, and societal actors.

In addition to the thematic sessions and interactions, the WRHC15 also presented this year’s awardee of the Kurt Eberhard Bode Foundation: Maria Magdalena Warter from IGB Berlin was selected from a research panel to become the next Bode Junior Research Group leader and receiving a price of 510,000 €. In his laudatio, Tim Bode, a member of the curatorium of the Kurt Eberhard Bode foundation, emphasized the innovative nature of Dr. Warter’s research on climate resilient cities. 

It was possible for the participants of the WRHC15 to engage with each other and on the subject during the long poster session. Here also simple demonstration of flooding models as well as environmental DNA monitoring from water took place to highlight novel approaches to assessing impacts on extreme events on the urban landscape and demonstrate mitigation options through NBS. Two final highlights concluded the WRHC15. First, participants from academia, agency, PR department, funders and science communicators engaged in a “fish bowl” discussion to explore the intricate field of communicating scientific insights to diverse audiences, emphasising the need for new formats, participatory approaches, and co-production of knowledge. It became evident that the credibility of scientists speaking about their work is much greater than through indirect media, yet the correct literacy is essential on both research side (policy literacy) and the policy side (science literacy) to allow for smooth dialogue. Last, the participants engaged with two workshops. The first used exactly what the fish bowl discussion concluded and trained the participants on how to get started with science communication using social media. Through specific examples and simple advice by an expert trainer the participants familiarized themselves on how to get started with their water-related communication options. The second workshop specifically compared approaches used in China and Germany for dealing with extreme events and the mitigation of these in urban environments through sponge city concepts. After 1.5 full days of presentations and discussion it became obvious that a) urban water bodies are characterised by their complexity, necessitating the definition of specific targets and the consideration of context-specific needs. Research in this field is essential for the development of effective monitoring methodologies to e.g. improve process understanding, and in turn to inform the evaluation of the success of implemented measures.  The conference b) showed that a lot of potential awaits to be lifted to couple urban water and energy nexus, many research questions need to be tackled, yet the potential is high. Finally, sponge city concepts exist and are being implemented, yet increasing their effectiveness requires a targeted approach. Furthermore, the involvement of legal and societal entities is often necessary to maximise their practical application. It is evident that inter- and transdisciplinary approaches are imperative in addressing the grand challenges to improve the urban water cycle.