Cooling wisdom of ‘water towns’: How urban river networks can shape city climate?

Abstract

‘Water town’, a city design idea featuring buildings sitting along rivers and their associated riparian vegetation, serves as natural refuges for citizens to escape the dual challenges of heatwaves and urban heat islands. We investigated the multi-scale cooling effect of river networks in Shanghai, a typical modern city widely known as an assemblage of water towns. At the neighborhood scale, we conducted a series of field experiments along various rivers with different characteristics during the 2022 heatwave periods in central Shanghai. At the city scale, we first quantified the cooling impact of urban river networks by integrating remotely sensed data and fine-resolution urban river network maps. We found that the width of rivers, coverage ratio, density, and morphology of river networks are the key factors affecting the cooling potential. The confluence or proximity of river tributaries can also bring an enhanced cooling effect than standalone ones. Rivers were found to be all-day cool spots compared to adjacent city streets, with greater daytime cooling intensity (1–3 °C) than at night (0–0.6 °C). Compared to vegetation which has more spatial coverage within the city, rivers are constrained by a linear shape along certain paths but possess the ability to flow, which is beneficial for cooling by carrying cool water from upstream parks and offering wind corridors. The atmospheric cooling capability of 20% of river coverage is equivalent to 30% of tree canopy coverage in a neighborhood. The city-scale cooling effect yields a 36.9% cooling area with a maximum cooling extent of 5.5 °C. The disparities in cooling effect between river networks and standalone rivers primarily manifest in confluence areas and the reinforced cooling within dense river networks. Each additional confluence in local river networks leads to a mean increase of 3% in cooling area and 0.01 °C cooling intensity. Our study implies that urban watercourses cannot be neglected in urban climatic studies and should be incorporated into a new conceptualization of water-included urban local climate zone classifications in the world urban database.