Are urban water bodies really cooling?

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.

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. This is why urban designers often include them in the design of the urban environment. In hot-weather periods, water bodies are often cooler than the overlying air, especially during the hottest hours of the day (e.g., Broadbent et al., 2017; Gross, 2017). Accordingly, their surface temperatures are often lower than the surface temperatures of surrounding urban structures (e.g., Sun and Chen, 2012; Méndez-Lázaro et al., 2018). 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). However, the latter mechanism also 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). Consequently, water bodies may enhance night-time urban heat islands, in particular in later summer season (Hathway and Sharples, 2012; Steeneveld et al., 2014; Van Hove et al., 2015). So, the question arose if typical urban water bodies would actually have a significant effect on thermal sensation. And in case this effect does not occur, how the environments of urban water bodies could still offer cooling conditions during hot summer days by implementation of shading, evaporation and ventilation interventions and how such interventions can be optimally combined in designs to reduce exposure to urban heat effectively, at least locally. Therefore, the ‘Really cooling water bodies in cities’ (REALCOOL) project explored possible local cooling effects of combinations of shading, evaporation and ventilation interventions for relatively small urban water bodies. Sixteen representative virtual urban water body settings have been investigated and redesigned (combining shading, evaporation and ventilation interventions). These REALCOOL designs considered the water environment as a whole: the waterbody as well as the adjacent spatial features such as vegetation, ground cover, water mist or fountains. For more general information on the REALCOOL project the reader is referred to Cortesão et al., 2017 and http://climatelier.net/projects/research/realcool-really-cooling-water-bodies-in-cities/. Given the comparatively limited evidence on cooling or warming effects by urban water, the first goal of the research presented in this paper was to assess the thermal effects of selected small urban water bodies types on their environment. The outcome of this assessment influenced the generating of new REALCOOL designs to optimize cooling based on shading, ventilation, and evaporation (Cortesão et al., 2019). The second goal was to assess the thermal effects of the resulting REALCOOL designs. The evaluation of thermal effects at larger or smaller scales was beyond the scope of the present study. Thermal effects of the water body environments were computed using the micrometeorological model ENVI-met 4.1.3 (http:// www.envi-met.com/). This ENVI-met version now enables the user to choose the intensity of turbulent mixing of the water. Furthermore, the light absorption characteristics can be specified by the user. Utilizing the new option by adjustment of these properties resulted in more realistic water temperature simulations than in the previous versions. Thermal effects were computed as differences in air temperature and PET, respectively, between the situation with and without water (first goal) and the REALCOOL design (second goal).