Urban green space in modern cities displays a cooling effect on hot summer days, based on which its energy saving potential can be estimated. The current work studies the energy saving potential (ESP) of a Chinese traditional garden in Beijing on a hot summer day and provides the corresponding spatiotemporal distribution maps, with a specific focus on the effect of the garden's water body. For this purpose, numerical simulation using ENVI-met model is created and validated by field measurement. The maximum cooling effect in the garden is a change of 1.3 degrees C in air temperature reduction at 14:00. Of this change, over 0.8 degrees C is caused by the water body. The center of the water body has a significant cooling effect due to evaporation, absorption of short wave solar radiation and desirable ventilation conditions. Based on the cooling effect, the spatial distribution and time evolution of heat reduction and ESP for the garden and its water body is obtained. Time evolution of spatial 3D heat reduction and ESP of the entire garden shows that the best performance is achieved at around 14:00 within a vertical height of 0-20.5 m. The corresponding values are 1.0 x 10(8)J and 80.7 kWh. The best performance of the water body occurs at around 14:00-15:00 when the heat reduction and ESP are 5.4 x 10(7)J and 43.6 kWh. Finally, the distribution maps of spatiotemporal heat reduction and ESP caused by the water body and entire garden within a height of 0-20.5 m from 8:00 to 18:00 are constructed. The total heat reduction values caused by the water body and the entire garden were 4.7 x 10(8) and 8.6 x 10(8)J, and the corresponding ESP values were 379.3 and 694.1 kWh. (C) 2019 Elsevier B.V. All rights reserved.