Quantifying the post-rainfall latent heat flux from urban isolated tree–ground cover combinations using LAD-based radiation transfer and evapotranspiration models

Source abstract

Isolated tree–ground cover represents a typical unit of urban blue–green–grey infrastructure and can improve the pedestrian microclimate through tree shading, transpiration, and ground evaporation, particularly after rainfall. However, the impact of stored rainwater and crown radiation transfer on post-rainfall cooling remains insufficiently explored. Therefore, this study developed a process-based modelling framework combining radiation transfer and evapotranspiration models of an urban isolated tree to quantify latent heat flux ( L E ) for evaluating the post-rainfall cooling associated with evapotranspiration. A 15-year-old Zelkova serrata with an average leaf area density (LAD) of 0.54 m 2 /m 3 was selected as representative tree for field measurements. Crown radiation transfer was analyzed using LAD distribution derived from Light Detection and Ranging (LiDAR). The transpiration model was improved based on measurement results, incorporating a ground evaporation model to estimate L E over three sunny days after rainfall. The results revealed that the target tree reduced solar radiation reaching the ground by 38%. Crown shading decreased the evaporation rate and prolonged the cooling duration. Moreover, assuming all stored rainwater was initially available for evapotranspiration, trees increased cumulative L E over three sunny days after rainfall by 32.2%, 98.4%, 90.0%, and 15.5% when combined with natural soil, impermeable, permeable, and water-retaining pavements, respectively. Accordingly, design recommendations for improving post-rainfall cooling were provided: tree planting in low-permeability areas, integrating trees and permeable pavements in pervious areas, and selecting the combination of trees and water-retaining pavements in high-temperature exposure areas. These nature-based solutions can support urban heat island (UHI) mitigation.