Diurnal pattern and driving mechanisms of the thermal effects of an urban pond

•Diurnal thermal behavior of an urban pond was observed in a subtropical city.•Water bodies may induce an adverse warming effect at night.•Heat capacity and evaporation are fundamental drivers for daytime cooling.•Longwave radiation and evaporation are important to nighttime heat release.•Time of day and reference site are critical for thermal assessment of urban water bodies. Water bodies are often considered an effective solution to cool their surroundings in hot summers. However, their potential adverse nocturnal warming effect has not yet been investigated thoroughly or considered in current urban planning strategies. This study evaluates the diurnal thermal effects of an urban pond in Nanjing, China, and explores the underlying mechanisms through energy-balance analysis using the Bowen ratio method. In-situ air temperature/humidity, water temperature profile, and upward and downward short- and long-wave radiation on hot summer days were measured. Results demonstrate that compared with a reference soil-site, the pond exerted a moderate daytime cooling effect (0.6 °C) but a pronounced nocturnal warming effect (1.8 °C). During the day, the energy input Rnet (net radiation flux) comprised 77% QS (heat storage) and 23% QE (latent heat flux), while QH (sensible heat flux) was negligible. At night, a negative QS acted as a heat source from the pond, which was released through QH (5%), QE (51%), and Rnet (44%). Results of this study improve our understanding of the thermal effects of urban water bodies, and can be used to inform urban planners and landscape designers regarding implementation of water bodies in the urban context for creating sustainable cool cities and communities.