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

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 R_net (net radiation flux) comprised 77% Q_S (heat storage) and 23% Q_E (latent heat flux), while Q_H (sensible heat flux) was negligible. At night, a negative Q_S acted as a heat source from the pond, which was released through Q_H (5%), Q_E (51%), and R_net (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.