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RESEARCH PRODUCT

Are urban water bodies really cooling?

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Abstract Small urban water bodies, like ponds or canals, are often assumed to cool their surroundings during hot periods, when water bodies remain cooler than air during daytime. However, during the night they may be warmer. Sufficient fetch is required for thermal effects to reach a height of 1–2 m, relevant for humans. In the ‘Really cooling water bodies in cities’ (REALCOOL) project thermal effects of typical Dutch urban water bodies were explored, using ENVI-met 4.1.3. This model version enables users to specify intensity of turbulent mixing and light absorption of the water, offering improved water temperature simulations. Local thermal effects near individual water bodies were assessed as differences in air temperature and Physiological Equivalent Temperature (PET). The simulations suggest that local thermal effects of small water bodies can be considered negligible in design practice. Afternoon air temperatures in surrounding spaces were reduced by typically 0.2 °C and the maximum cooling effect was 0.6 °C. Typical PET reduction was 0.6 °C, with a maximum of 1.9 °C. Night-time warming effects are even smaller. However, the immediate surroundings of small water bodies can become cooler by means of shading from trees, fountains or water mists, and natural ventilation. Such interventions induce favorable changes in daytime PET.