Comparison of cooling effect between green space and water body

•Water bodies have stronger cooling effect than tree-based green spaces.•Optimal green and blue space sizes increase with increase of impervious surface.•Economically optimal sizes of green space and water patch are around 0.30 ha.•Simpler and more fragmented green and blue spaces have greater cooling intensity.•Green spaces and water bodies with more complex shapes create larger cooling areas. The cooling effects of green spaces and water bodies are essential in subtropical cities with high temperatures combined with increasing urban heat island effect, while there is insufficient research on the difference between these two categories of cold sources. Therefore, Nanning, China, was selected as the typical case in the subtropical climate zone, and spatial and statistical analysis based on Landsat 8 OLI/TIRS images was conducted to compare the cooling effect and factors of blue and green spaces. Results showed that (1) the area-related green-space and water-body metrics are significantly positively correlated with the cooling effect. An additional 10 % of green space cover leads to a decline in the mean LST of 0.39 °C compared with a decline of 0.42 °C for water cover; (2) green and blue spaces with simple shapes and high fragmentation may have strong cooling effect, whereas green and blue spaces with more complex shapes may cool larger areas, benefiting more residents; (3) the threshold values of efficiency (TVoE) of tree-based green spaces and water patches were both approximately 0.30 ha, increasing with spatial sprawl of impervious surfaces; and (4) water bodies have slightly higher cooling intensity and much larger cooling ranges than tree-based green spaces.