Optimizing an efficient urban tree strategy to improve microclimate conditions while considering water scarcity: a case study of Cairo

Abstract

This study aims to develop an efficient urban tree strategy (UTS) to enhance the microclimate conditions of cities that suffer from heat stress and strong solar radiation, such as the metropolitan area of Greater Cairo. Cairo recently lost its limited greenery to enhance traffic. The proposed UTS aims to achieve a balance between enhancing microclimate conditions and considering the city’s water scarcity. It seeks to consider all strategic factors suitable for local conditions, including the selection of tree species (Step 1), the utilization of new technologies for irrigation (Step 2), and the optimization of the usage of an efficient number of trees (Step 3). When applying the strategy’s recommendations to a study area within Cairo’s downtown center and when testing different tree coverage percentages within urban canyons of various aspect ratios and orientations using ENVI-met, the microclimate conditions are significantly enhanced in certain streets during summertime compared to wintertime. Applying the UTS not only enhances thermal comfort but also helps to create a better comfort zone during certain hours. In one street, for example, there are average physiological equivalent temperature (PET) reductions of − 5.18° and − 6.36° at 16:00 and 17:00, respectively, which also changes the thermal comfort zone from extreme heat stress to very heat stress. The results show a strong positive correlation between thermal comfort enhancement and a reduction in the total mean radiant temperature (TMRT), verifying that shading plays a primary role in enhancing the microclimate conditions of urban canyons. Applying the UTS to the study area significantly enhances the microclimate conditions. Furthermore, through the implementation of irrigation technologies that are part of the UTS, water demand is reduced to only 15% when trees with larger canopies are used. Additionally, when the tree coverage percentage reaches 35 to 50% in some streets, it results in a significant enhancement in the PET.