A canopy shading-based approach to heat exposure risk mitigation in small squares

Abstract

The impact of climate change in recent years on public health risks continues to intensify, and rational arrangement of trees can effectively reduce urban heat islands. As a gathering place for residents' outdoor activities, the small squares widespread in a city have a high risk of daytime heat exposure during summer due to their lack of shelter. How to maintain a balance between openness and shading has become a challenge for urban design. The tree canopy plan that forms a continuous shaded area during the hottest hours of a typical summer day is provided based on the principles of planting economy, activity space adequacy, and heat stress relief effectiveness. Taking 5 typical squares in Dalian, China as an example, we calculated the spatio-temporal distribution of the mean radiant temperature (Tmrt) before and after tree canopy planning using the validated SOLWEIG model, and quantified the corresponding changes in risk levels. This study aims to provide climate change adaptation strategies for small squares to deal with health risks through implementing tree canopies. The results showed that the average Tmrt decreased by 3.5 °C -7.7 °C during the hottest period of the day (11:00–15:00, August 14, 2020). The high-risk areas were reduced by 27%–50.4%. The improvement rate of canopy planning is 53%–94% in the high-risk area, but only 36% in the medium-risk area. We concluded that square height-to-width ratios are associated with the degree of heat exposure risk, and squares with high levels of heat exposure risk have greater overall cooling potential after tree canopy planning. Implementing tree canopy planning can create continuous low-risk paths, while keeping the square open and protecting those who need to pass during the hottest hours. Compared with the medium-risk areas planted in squares, the planning benefits of high-risk areas are better.