Estimation of the transpiration of urban shrubs using the modified three-dimensional three-temperature model and infrared remote sensing

Abstract

Urban shrubs are one of the main components of urban vegetation and play a significant role in mitigating the urban heat island (UHI) through transpiration. As there are no suitable methods to measure the transpiration rate of urban shrubs, many issues related with urban transpiration rate and UHI are still not properly understood. In this study, the two-dimensional (2D) three-temperature (3T) model is extended to three-dimensional (3D) scale and applied to estimate the transpiration rate of typical urban shrubs from May to December in 2018. Three common species of shrubs are observed in this study, namely Lagerstroemia indica cv “Bush”, Podocarpus macrophyllus var. angustifolius, and Ligustrum vicaryi. Estimated transpiration rates are proofed by isotope method. Results show that (1) the modified 3D-3T model is a reliable method for estimating urban shrub transpiration (R2 = 0.61, MAE = 0.10 mm·h−1); (2) urban shrubs have a quite high transpiration rate, ranging from 0.2 to 0.6 mm·h−1 in the daytime; (3) the rate of transpiration varies greatly in different directions of the shrub canopy (top, south, north, east, and west); (4) the 3D-3T model is able to better reflect transpiration characteristics under the heterogeneous condition in urban environments. To the best of our knowledge, the improved 3D-3T model is one of the very few methods that can measuring the transpiration rate of urban shrubs. The results from this study provide meaningful information and a useful tool for measuring urban evapotranspiration and urban planning.