Using water to cope with heat waves in cities

Temperatures across Europe are rising and heatwaves caused more than 85% of fatalities in the period 1981-2020. In addition, in dense urban areas, having sealed surfaces with high heat retention, the urban heat island (UHI) effect can lead to higher average temperatures. Surface temperatures can be 10-15°C warmer in local hotspots within cities than their surrounding areas (EEA, 2023). Investments in water supply services and infrastructures can help cities to become more resilient to the negative effects of global warming and of heat waves. Besides nature-based solutions in urban areas, different packages of grey interventions may be considered, such as:

• Building, repairing and maintaining fountains for drinking and cooling;
• Cooling by water spray (fountains and recreational water features, such as splash pads and spray parks).

Repairing historic drinking fountains and installing new ones creates more opportunities for people experiencing the negative effects of heat in the city. People can use water for drinking when feeling thirsty or to cool down. The use of water for cooling effects decreases the air temperature by evaporation, absorption of heat and transport of heat. The cooling effect of flowing water is greater than that of water that is standing still, due to the process of mixing flowing water with air and transport of heat. A water spray from a fountain has an even greater cooling effect because of the large contact surface of the water and air, which stimulates evaporation. Cooling effect by evaporation also occurs when water spray is in contact with the skin, decreasing body temperature. Fountains and recreational water features can also have positive social effects as they enhance the attractiveness of public spaces. For example, the city government of Budapest has revived its “Cooling programme”, setting up cooling islands and drinking fountains to protect residents and guests from prolonged exposure to high temperatures. All drinking fountains in Budapest can be easily found through an online map available at the municipal website. Fountains also serve as meeting places.  Similarly, fountains that combine water distribution for drinking and misting have been installed in Paris, to help citizens to cope with heat stress.

Improving the wellbeing of people by reducing thermal discomfort is also a solution to preserve tourism economy during unfavourable climate conditions in the hottest months. Most likely, excessive heat would keep tourists away from some cities, especially in Southern Europe. Fountains and recreational water features may enhance the attractiveness of public spaces and decrease heat stress for both residents and tourists. 

As extreme heat events become more common across Europe, technical solutions like heat warning apps become more widespread. They provide information on heat related risks and on actions that can be undertaken by people. For example they provide instructions on how to get to the closest cooler place with drinking water within a city. These apps can use real-time satellite data, along with other models and city-specific data to estimate the temperature, humidity, and discomfort index (e.g. EXTREMA project in Athens).

There is an evident trade-off between the implementation of above-mentioned cooling solutions and water consumption. Therefore, the options mentioned above cannot be implemented in situations of water scarcity. Indeed, there is a huge need for cities to adopt sustainable cooling strategies that do not cause additional pressure on valuable water resource. 

For this reason, it is highly recommended that cities, using water features to manage heatwaves, adopt all precautions to minimise water consumption. This can be achieved by:

• integrating water use with green infrastructure options that provide cooling through the shading effect and microclimate offered by vegetation, tree canopy cover and urban water bodies in densely built urban areas with high proportion of paved soil. Climate-resilient tree species are especially suitable to this (see also the options Urban green and blue infrastructure and Water sensitive urban design (WSUD);
• making use of innovative design for the construction of drinking water fountains, minimizing water consumption (systems to avoid permanent water flowing);
• maximising the adoption of closed water cycle for no-drinking purposes. Rainwater or domestic and municipal wastewater may be re-used for cooling (e.g. wetting of streets), after proper treatment (see Water re-use option).

Strongly committed city leadership and supporting local Mayors, interested in the adaptation agenda, can be key drivers for sustainable solutions. Holistic urban planning, mapping of social vulnerability and placing new systems based on holistic analyses enhance the social and environmental sustainability of cooling systems while also supporting their uptake. Therefore, the measures should be included in the city planning instruments and in particular in its Adaptation Plans and Heat Health Action Plans. The success significantly depends on the level of integration with other measures, e.g. replacing clean water consumption by waste-water reuse for watering gardens. . Its success also depends on dissemination initiatives to make fountains and other cool places known and accessible by residents and city visitors. The use of mobile apps that provide early warnings on heatwaves and information about how to react are therefore fundamental. If not integrated in a broader water management plan, this measure could lead to an increase in water consumption, which would be unsustainable during droughts and heat waves. Another problem is that sustainable cooling is a cross-sectoral effort that requires collaboration between sectors and disciplines. Very often there are no clear “owners” of the measure, or no one takes responsibility for it. One important limiting factor is the lack of information on sustainable urban cooling practices, including heat-resilient urban design and the lack of available tools to support the implementation of this option. The role of education, training and sharing experiences is crucial, especially for policymakers and professionals, to ensure that norms, practices and standards, designs and plans are aligned with the technical possibilities and appropriate measures for sustainable urban cooling. Inter-departmental collaboration is needed for successful planning and implementation. 

Strong commitment from the city leadership and support from the local Mayor interested in the adaptation agenda can be a key driver for sustainable solutions. Holistic planning as a part of other urban planning, mapping of social vulnerability and placing new systems based on holistic analyses will enhance the social and environmental sustainability of cooling systems. The measures should be included in the city planning instruments and in particular in its Adaptation Plan. The success significantly depends on the level of integration with other measures, e.g. waste-water reuse for watering gardens, to save clean water to be used for this measure. If not integrated in a broader water management plan, this measure could lead to an increase in water consumption, which would be unsustainable during droughts and heat waves. Another problem is that sustainable cooling is a cross-sectoral effort that requires collaboration between sectors and disciplines. Very often there are no clear “owners” of the measure, or no one takes responsibility for it. One important limiting factor is the lack of information on sustainable urban cooling practices, including heat-resilient urban design and the lack of available tools to support the implementation of this option. The role of education, training and sharing experiences is crucial, especially for policymakers and professionals, to ensure that norms, practices and standards, designs and plans are aligned with the technical possibilities and appropriate measures for sustainable urban cooling. Inter-departmental collaboration is needed for successful planning and implementation. 

Direct costs can vary substantially depending on the solution. For example, the cost of fountains and spray systems is low compared to complex solutions. The latter combine nature-based solutions with engineering-driven techniques (e.g. different combinations of shading, evaporation and ventilation around water bodies and mist nozzles to increase cooling effect). Fountains require regular monitoring and maintenance for the water quality, filters and spray nozzles. Costs can increase in situations of water scarcity and conflicting demands for water resources, with other uses and users. 

The New European Bauhaus Facility is a new funding tool. It aims to revitalise neighbourhoods by promoting solutions that “are not only sustainable, but also inclusive and beautiful”. Funding opportunities to re-think and re-design urban spaces, also including water elements for drinking and cooling uses, can be accessed through the dedicated funding webpage.

Cooling cities by using water can provide many benefits, most importantly for improved wellbeing and health, especially for the elderly. Installing new drinking fountains can also have an environmental benefit, since citizens and city guests are encouraged to refill reusable bottles with tap water, instead of buying new plastic water bottles. Positive impacts on tourism economy can also be achieved in hottest months, preventing people to choose different destinations less prone to heatwaves.

There is no strong regulation for sustainable urban cooling by using water in the EU or EU member states. Local governments (municipalities) are responsible for city water and climate adaptation planning, including measures extending water supply services like (drinking) fountains. 

Implementation time is dependent on solutions and can vary from a few months up to a few years depending on the complexity and size of the system constructed. Local cooling systems based on man-made systems such as fountains or water showers do not take so long to complete once the decision to construct them has been made. Implementation time can be longer (several years) if these solutions are integrated in a plan that also includes more complex blue-green infrastructure such as restoration of urban brooks or developing networks of cooling places at the city scale. 

As they are part of the built environment, established cooling systems such as fountains are usually long lasting, over 10 years. On the contrary, other cooling systems such as wetting streets or spraying public open spaces have short term effects, and require repeated implementation, when needed.