Optimization of Tree Locations to Reduce Human Heat Stress in an Urban Park

Trees provide cooling benefits through shading and evapotranspiration; they are regarded as an important measure in heat-resilient urban planning and policies. Knowing where to plant trees for maximum cooling benefits, given practical and resource constraints, remains a challenge in both practice and research. Literature in the field of tree modelling and location optimization is limited, either by the incompleteness in accounting for tree shading, evapotranspiration, and the modifying effect of wind, or by the slow-running speed of the Computational Fluid Dynamics model, making them less applicable in practice. This paper describes a novel method to search for the optimal locations for trees to maximize their cooling benefits in an urban environment. A non-CFD simulation model was applied to assess on-site heat stress under the influences of trees, which was evaluated using field measurements conducted under hot, temperate, and cool weather conditions in an urban park in Hong Kong. It was then linked to a genetic algorithm in search of a near-optimal tree layout. The proposed method was tested in the same park, and it can automatically identify locations to plant new trees to minimize heat stress, subject to practical constraints such as avoiding existing buildings and utilities. It can also identify the optimal layout to rearrange the existing 55 trees, hypothetically, which is expected to cool the park (around 30,000 m2) by up to 0.3 ℃ in the site average UTCI equivalent temperature compared with the worse scenario. Trees can cool the most if they are concentrated on the leeward side of the park relative to the prevailing wind, rather than spread evenly. The new method runs significantly faster than existing approaches, and it can inform research and landscape design practices concerning park cooling as a goal.