A three-layer evapotranspiration model considering the vertical structure of urban green spaces

Urban green spaces with complex vertical vegetation structures typically have higher capacities for microclimate regulation. The vertical structure comprising "tree + shrub + soil" is a common feature in urban green spaces. The Penman–Monteith and Shuttleworth–Wallace models estimate the evapotranspiration (ET) from trees and soil only, but they do not account for the contribution of shrubs, thereby leading to underestimation of the ecosystem ET. We developed a three-layer model of the vertical structure comprising "tree + shrub + soil" in urban green spaces and its impacts on ecosystem ET. The three-layer model was validated based on data measurements, showing that the root mean square errors of latent heat flux for tree, shrub, and soil were 31.0 W·m^–2, 21.3 W·m^–2, and 19.1 W·m^–2, respectively. The uncertainty of parameters can significantly affect the model’s performance when estimating ET and its components. Sensitivity analysis showed that meteorological parameters, bulk stomatal resistance, and soil surface resistance played important roles in ET estimation using the three-layer model, and these sensitivity coefficients were also influenced by the meteorological conditions and vegetation canopy structure. The three-layer model enhances our understanding of ET in urban green spaces, aiding in their management and planning.