Scaled outdoor experiments to assess impacts of tree evapotranspiration and shading on microclimates and energy fluxes in 2D street canyons

Urban trees contribute to summer cooling by offering solar shading and evapotranspiration. But their relative significance is still unclear due to the challenges in directly measuring tree evapotranspiration rate or latent heat flux (Q_E) in actual urban environment. Concurrently, high-quality experimental data are still required for the validation of urban canopy model simulations. For this purpose and as a novelty, this study directly measures the tree evapotranspiration (or Q_E) and shading effects in street canyons (building height/street width, H/W=1 or 2, H=1.2m), using scaled outdoor experiment in a subtropical region, i.e. suburban Guangzhou, China, from August to October 2022. Results show that urban trees (leaf area index: 3.5) can effectively deliver surface temperature reductions beneath the tree canopies up to 5.1℃ as H/W=1 and 8.2℃ as H/W=2, while slightly raise air temperature by less than 2.0℃ above tree canopies. Energy flux comparison indicates tree shading dominates the primary cooling mechanism, with up to 97% of incoming solar radiation (800Wm^-2) intercepted, while tree evapotranspiration plays a secondary role, with a rate of less than 1.6g/min and a latent heat flux below 90Wm^-2. Additionally, trees decrease street air velocity by up to 63.6%, and increase of water vapor pressure by up to 2.77hPa.