Sponge Watersheds: Nature’s Blueprint for Resilient Cities

What if our cities and landscapes stopped fighting water and instead absorbed, stored, and slowly released rain like a natural sponge? Cecilia Polacow Herzog, Osvaldo Moura Rezende and João Luís Bittencourt Guimarães urge us to embrace water as an ally in building resilient, sustainable, and livable urban spaces.

Sponge watersheds represent an innovative, nature-based approach to managing and adapting to climate change in cities and beyond. By mimicking how nature absorbs and releases water across entire river basin areas, they reduce flood risks in urban, peri-urban and rural areas. At the same time, they offer an effective means to increase biodiversity and ecosystem services beyond flood mitigation, water recharge and others, providing social and economic benefits.

Although large-scale implementation is still new, there is growing evidence that sponge watersheds may be essential in minimising climatic impacts. Since 2014, China has invested estimate of 1.5 trillion US dollars in 657 cities. Now, the country is expanding its “sponge city” approach to watershed-scale planning, prompted by floods caused by upstream stormwater inflow that was not well-managed by less extensive approaches.

While sponge cities are seen as a real response to help cities become more resilient to climate challenges, it is important to recognise that many floods actually begin with storms upstream of cities. For example, in 2024, the southern Brazilian State of Rio Grande do Sul was hit by intense and continuous storms for several days, causing immense material and immaterial losses, including almost 300 deaths. This is only one of the many upstream storms leading to downstream destruction that have happened in recent years around the world.

From Floods to Forests – The Power of Sponge Watersheds

Sponge watersheds work by replicating the natural capacity of landscapes to absorb, retain, detain and gradually release rainwater. Unlike conventional “grey” infrastructure that rapidly channels water away through pipes and drains, the sponge watershed approach uses blue-green infrastructures, and by that fosters infiltration and groundwater recharge, significantly reducing peak flows and mitigating urban flooding. Commonly implemented blue-green infrastructure (BGI) in urban environments includes fluvial parks, natural reserves, bioswales, storage ponds, and rain gardens, among other elements. These components are integrated within a functional system to enhance ecological and hydrological performance. For cities facing the twin challenges of climate change and urban sprawl, the adoption of such resilient, adaptive methods is not just innovative, it is imperative.

A sponge city in action: Nature-based solutions like green roofs, rain gardens, and urban wetlands absorb, store, and slowly release rainwater—making cities more resilient, sustainable, and livable for all. © Herzog

Beyond flood reduction, sponge watersheds also offer advantages in preventing landslides. In regions with steep slopes and eroded soil, the excessive runoff often triggers catastrophic slides. Sponge systems help by holding more water and promoting vegetation growth, which stabilises the soil, thereby protecting vulnerable communities. This technical advantage becomes a lifeline for low-income areas where infrastructure investments are scarce.

Furthermore, sponge watersheds deliver a powerful synergy between climate change mitigation and adaptation. By restoring natural hydrological cycles through natural vegetation recovery, these systems contribute to carbon sequestration, besides the benefits mentioned above. Moreover, by fostering biodiversity and ecological connectivity, they offer long-term resilience against unpredictable climate extremes.

How Nature-Based Solutions Transform Vulnerable Communities

In Brazil’s informal settlements (as favelas), where millions live under precarious conditions, nature-based solutions (NbS) have the potential to transform lives. The Brazilian Ministry of Cities’ “Periferias Vivas” program, for instance, has developed a report to advance policies for NbS in peripheries.[1]

By implementing nature-based techniques— like community rainwater harvesting systems and green corridors, among other locally-adapted NbS typologies—urban planners can achieve many benefits at once. These include reducing flood risks, moderating urban heat islands, fostering social inclusion, and even generating local employment through community-led maintenance programs.

Despite these benefits, the journey toward widespread implementation of sponge watersheds in Brazil is not without challenges. The legal framework remains fragmented, municipal plans often lack explicit guidelines on nature-based solutions, and licensing hurdles and limited fiscal incentives further complicate projects, especially in regions with deep-rooted grey infrastructure practices. To move forward, Brazil not only needs regulatory reforms, but also the development of innovative governance models, with robust, interdisciplinary partnerships between government agencies, private sectors, community organisations and local residents.

Active co-creation is key to enabling the adoption, sustainability, and upscaling of more nature-friendly water management projects. When all stakeholders, including local residents, effectively participate in the decision-making, design, management and monitoring processes, the chances of long-term success increase significantly. It is especially important that women, minorities, and marginalised communities are engaged participants to ensure that solutions are equitable and tailored to diverse needs.

Resilient Cities Begin with Nature-Based Action

The transformative impact of sponge watersheds extends beyond technical efficiency. They represent a shift in mindset—a move away from attempting to control nature to actively collaborating with it.

So, what steps can drive this change? First, urban policymakers must update regulatory frameworks to explicitly integrate nature-based solutions. This involves harmonising federal, state, and municipal guidelines, as well as providing fiscal incentives for green infrastructure projects. Second, community engagement should be at the forefront, with participatory planning processes that empower residents to shape and maintain sponge systems in their neighbourhoods. Finally, sustained investment in research and data collection, through robust performance indicators (KPIs) like infiltration rates, reduced inundated areas, and economic savings from damage prevention, will help fine-tune these interventions over time. Underlying all steps is promoting education and training to enable a paradigm shift towards a cultural connection with nature and natural processes.

In conclusion, sponge watersheds offer a promising, holistic pathway toward creating resilient cities that harmonise human activity with natural processes. They demonstrate that addressing urban water challenges can also drive social inclusion, ecological restoration, sustainable economic development and climate action. By embracing this approach, regional and urban planners, decision-makers, and communities can collaboratively build cities and regions that are not only sustainable and adaptive but also truly liveable for people and all living beings.

[1] Brazil, Ministério das Cidades. SBN nas periferias: avanços na regulamentação de uma nova Política Pública., 2024. Available at: https://www.gov.br/cidades/pt-br/assuntos/publicacoes/arquivos/arquivos/copy_of_SBNnasPeriferias_SNP1.pdf