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.
Highlights
Small urban water bodies, like ponds or canals, are often assumed to provide effective cooling during hot periods and to improve thermal sensation in the neighboring spaces and over the water
Local thermal effects near individual water bodies were assessed as differences in air temperature and Physiological Equivalent Temperature (PET)
The present research confirms that water in small urban water bodies, like the ones considered in the present study, has a small thermal effect on its surroundings
Summary
Like ponds or canals, are often assumed to provide effective cooling during hot periods and to improve thermal sensation in the neighboring spaces and over the water. This is why urban designers often include them in the design of the urban environment. For water bodies with a depth of at least half a meter, these temperature differences are expected because of the large heat capacity of water in combination with the ability of water to transport heat away from its surface by turbulent mixing (Oke, 1987) The latter mechanism implies that water takes longer to cool down, which may result in water bodies being warmer than the air during the night (Steeneveld et al, 2014). On an annual basis, evaporation may be considered an important energy exchange mechanism for larger water bodies (Gunawardena et al, 2017), and large fractions of water environments may help to reduce the urban heat island intensity by its impact on the urban energy budget (Oke, 1987)
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