Cooling wisdom of 'water towns': How can urban river and lake networks shape city climate?

'Water town', a city design idea featuring buildings sitting along rivers/lakes and their associated riparian vegetation, serves as an effective nature-based solution for citizens to escape the dual challenges of heatwaves and urban heat islands. We selected two Chinese megacities, i.e., Shanghai, a city encompassing rich river networks and Wuhan, a city of a hundred lakes as study areas and investigated the cooling potential of rivers and lakes via field monitoring, remote sensing and numerical modelling. It was found that the width of rivers, the confluence of river tributaries, coverage ratio, density, and morphology of river networks are key factors affecting the cooling potential of rivers. In contrast to rivers, lakes usually possess a relatively large surface area and can influence local and regional climate to a greater extent. Particularly, the temperature difference between built-up areas and lakes can induce wind circulation, i.e., the lake breeze circulation. Rivers were found to be all-day cool spots compared to adjacent city streets with greater daytime cooling intensity than at night, while lakes may induce daytime cooling and nighttime warming due to large thermal inertial. In addition, the combination of blue-green spaces and building forms in a neighborhood should be optimized to create a perfect 'water town' with good ventilation conditions, abundant tree shading effects, and effective evaporative cooling effects, so that the synergistic effect of urban heat and moisture islands can be mitigated. Our study can offer significant reference for the planning and design of water sensitive cities.