The number of extreme heat events has substantially increased across Europe in recent decades, and the length, frequency and intensity of heat waves is projected to increase in the future. They are associated with decreases in general population well-being and increases in mortality and morbidity, especially in vulnerable population groups, unless adaptation measures are taken (EEA, 2016). In addition to health effects, higher temperatures have a very significant impact on energy consumption, comfort and the environmental quality of cities, where impacts from heat waves are worsened due to the urban heat island (UHI) effect. Urban areas are hotter than their surroundings as building materials and pavements absorb and store more heat than vegetated areas.
Actions can be taken to mitigate the UHI effect and adapt to warmer temperatures, thereby creating a more habitable urban environment. Existing UHI mitigation techniques, which have the potential to both enhance thermal comfort and reduce energy demand in cities, recommend the use of cool materials, urban greening, water, and shading as potential solutions to moderate temperatures and increase adaptive capacity. Where to apply and how effective a specific measure or technique is depends on location, density and scale of the urban setting, availability of water and climate zone. One UHI mitigation technique is the use of water to moderate temperatures. Water, in fountains, sprinklers and misting systems, can modify the microclimate and cool outdoor spaces, especially in UHIs. For example, water misters can cool expecting passengers and the air at local bus stops, people can drink from drinking fountains or use the water to cool themselves off, or sit by a fountain in a park. Wetting streets with water also provides a cooling effect. A study in Paris, France, found that street wetting can reduce air temperature by 0.79°C and provide UHI-mitigation up to -0.22°C (Hendel, et al, 2016.)
However, the main constraint for use of these techniques in the Adriatic region is water availability. Quality and quantity of the water needed for each cooling strategy varies, i.e. drinking fountains require that potable water is available on demand, whereas decorative fountains can use circulated water. If not integrated in a broader water management plan, this could lead to an increase in overall water consumption, which could be unsustainable, particularly during the droughts that often accompany heat waves. The trade-off between the implementation of cooling solutions utilizing water to cope with heat waves and other water consumption needs to be carefully considered in situations of actual or potential water scarcity.
The performance of water-based heat mitigation systems is strongly affected by the local climatic conditions; humidity, ambient temperature, wind speed, turbulence, and solar radiation define the capacity to evaporate and the mitigation potential of water-based technologies and techniques (Santamouris, et al, 2016). Evaporative cooling systems and surface water are highly effective in drier climates where surface water can use ambient heat for evaporation, thus lowering the air temperature. Water-based techniques may present a reduced heat mitigation potential when used in humid climates, yet in these cases evaporative misting fans can be effective, where the combination of high pressure and ultra-fine water particles results in a cool and relatively dry feeling (LCL, 2017). A potential downside to non-passive cooling systems is that flowing or sprayed water, in most cases, means higher electricity consumption.
Water, when used alongside other UHI mitigation measures, can yield significant temperature reductions; there is even a considerable increase in the mitigation potential when more technologies are combined (Santamouris, et al, 2016). Complementary strategies include increasing tree and vegetative cover; installing green roofs or high albedo roof surfaces; using high emissivity, high albedo, or high permeability pavement; temporary shading structures or tree canopy. See AO38 Water sensitive urban and building design for water uses such as ponds and pools that are more related to urban landscape design and AO33 Green spaces and corridors in urban areas for information on vegetated areas.
A successful example of the use of water to cope with heat waves in cities alongside other urban cooling strategies comes from Thessaloniki, Greece, where a project to upgrade the bioclimate of the historic centre included the installation of systems promoting water evaporation (water jets, water curtains and sprinklers), together with forced air movement (through use of outdoor fans), installation of new lighting equipment, creation of a pedestrian network, and tree planting (Covenant of Mayors Office, 2017).
In Košice and Trnava, Slovakia, water has also been used to help citizens cope with heat waves. While the city borough of Zapad included the building and restoration of elements of blue infrastructure and water fountains as an action to address the vulnerability of citizens to high temperatures and heat waves in its adaptation plan, in Trnava, a former neglected open space neighbouring a block of flats, kindergarten and elderly care home (thus identified as a location highly vulnerable to heat waves) was transformed through the removal of tarmac, planting of trees, construction of a fountain, and provision of new benches, thereby creating a green space providing relief during heat waves (Climate-ADAPT, 2018).
Costs and benefits
As the use of water to cope with heat waves in cities is typically implemented alongside other urban cooling or adaptation measures, it is difficult to find the stand-alone cost of water measures where they have been used. The extension of water supply services is a low-cost activity. However, in situations of water scarcity the water price will not be the key factor when deciding about competing water uses. There are monitoring and maintenance costs associated with checking water quality, changing filters and keeping fountains and spray nozzles in good condition. There are also electricity costs associated with misters and fountains.
Benefits of water use to cope with heat waves and to lessen the UHI effect include increased outdoor thermal comfort, which leads to reduced energy consumption (e.g. for air conditioning), and in turn less waste heat production. Water uses such as fountains provide additional social benefits of drawing children together to play and people to meet in parks and squares. However, decision on adaptation option for coping with heat in cities should be taken from an integrated prospective. This means that in situations of water scarcity, using water for urban green will contribute more to mitigating climate change than some of the other techniques presented above.
Implementation time and lifetime
The installation of fountains or water misters can be done very quickly, and with regular maintenance, these features can last for some time.
Source for more detailed information
Climate-ADAPT, 2016, Adaptation option: Water uses to cope with heat waves in cities, https://climate-adapt.eea.europa.eu/metadata/adaptation-options/water-uses-to-cope-with-heat-waves-in-cities
Climate-ADAPT, 2018, Case study: Social vulnerability to heatwaves – from assessment to implementation of adaptation measures in Košice and Trnava, Slovakia, https://climate-adapt.eea.europa.eu/metadata/case-studies/social-vulnerability-to-heatwaves-2013-from-assessment-to-implementation-of-adaptation-measures-in-kosice-and-trnava-slovakia
Covenant of Mayors Office, 2017, Bioclimatic upgrade of public spaces Thessaloniki, Greece, https://www.covenantofmayors.eu/index.php?option=com_attachments&task=download&id=362
EEA, 2016, Indicator assessment: Extreme temperatures and health, https://www.eea.europa.eu/data-and-maps/indicators/heat-and-health-2/assessment
Hendel, et al, 2016, Measuring the effects of urban heat island mitigation techniques in the field: Application to the case of pavement-watering in Paris, https://www.sciencedirect.com/science/article/pii/S2212095516300086
Low Carbon Living, 2017, Guide to urban cooling strategies, http://www.lowcarbonlivingcrc.com.au/sites/all/files/publications_file_attachments/rp2024_guide_to_urban_cooling_strategies_2017_web.pdf
Santamouris, et al, 2016, Passive and active cooling for the outdoor built environment – Analysis and assessment of the cooling potential of mitigation technologies using performance data from 220 large scale projects, https://www.sciencedirect.com/science/article/pii/S0038092X16306004
LIFE WATERCOOL – Water efficient systemic concept for the climate change adaptation in urban areas – ongoing project in Spain – In a demonstration area on one avenue, water infrastructure will be used as a vehicle for cost-effective adaptation to heat (by combining cool reservoirs and functional pavements) and to improve efficiency in water use and comfort.