**1. Introduction**

Due to rapid urbanization, the urban thermal environment has worsened, and urban heat islands (UHI) have become a common problem in most cities around the world [1,2]. From 1961 to 2000, air temperature has increased 0.16 ◦C per decade in large cities in northern China [3]. Among 419 large cities around the world, the average annual daytime surface urban heat island is 1.500 ± 1.200 ◦C [4]. High temperatures in urban areas not only lead to more energy consumption for cooling [5] but also affect human health [6–8]. High temperatures and heat waves could even increase the mortality rate. In 27 European countries, over 28,000 people die every year due to exposure to extreme heat, which accounts for 0.61% of all deaths in these regions [9]. Therefore, studying how to efficiently mitigate urban thermal issues is essential for adaptive strategy under climate change and rapid urbanization.

In recent decades, various methods including changes to underlying surface materials, optimizing urban planning and designing, and the addition of vegetation have been proposed to mitigate UHI [10–12]. Among them, vegetation is considered one of the most effective mitigating methods [13,14]. Many studies have been conducted on the cooling effects of urban vegetation. Urban parks, urban forests, urban lawns, and green roofs can provide different degrees of cooling [15–18]. Research has shown that just a single tree could save 12–24% of cooling energy for a single-story building [19]. Sixty-three large *Eucalyptus camaldulensis* per hectare could reduce air temperature by 1 ◦C in Mexico City, while 24 large *Liquidambar styraciflua* trees could even reduce the air temperature by 2 ◦C [20]. A 147-hm<sup>2</sup> park in Nagoya was also found to reduce the air temperature by 1.9 ◦C on hot days [21]. These studies showed that vegetation area, vegetation shapes and vegetation compositions could affect the microclimate [22–25], and the cooling effects of vegetation could be attributed to its shading, reflection, and evapotranspiration [26,27]. However, most of these studies focused on the cooling effects under different green space ratios and did not quantitatively estimate their ET rates and energy budget. Therefore, quantitative evaluation of the cooling effects is still a challenge.

Although ET is believed to be the most robust cooling mechanism, as it can consume large amounts of latent heat [12], observing the ET characteristics of urban vegetation is especially difficult [28,29]. As the vegetation is segmented by various artificial underlying surfaces in urban settings, it is difficult to meet the fetch requirements of traditional methods such as the Bowen ratio, eddy covariance and large aperture scintillometers [30]. ET could be estimated on a large scale by satellite remote sensing [31], but its resolution is usually too sparse on the street or neighborhoods scale. Moreover, only one image of an area could be obtained over several days. In contrast, sap flow and lysimeter data can only directly measure individual or small groups of plants [32,33]. Therefore, a fetch-free, high-spatiotemporal-resolution ET estimation method is needed to obtain accurate ET characteristics of urban vegetation.

The three-temperature model was proposed and developed to estimate ET via three temperature data points, net radiation and ground heat flux [34,35]. The surface temperature data could be obtained using thermal infrared images, and the meteorological data are easily available. It has been applied in studies on different scales, including the large catchment scale, the field scale and even on a single plant in growth chamber. It has been validated by the Penman–Monteith method, weighing lysimeter, Bowen ratio, eddy covariance, and water budget methods [36–41]. It has also been used to estimate ET of different vegetation types, such as crops, grass, and shrubs [38,42,43]. In urban area, it was used to estimate a small urban lawn's ET and showed grea<sup>t</sup> consistency with the Bowen ratio method [30]. These results indicate potential applications for the proposed method to estimate ET of different urban vegetation.

Hedges are narrow bands of woody vegetation and associated organisms that separate fields and are generally composed of low dense vegetation including short woody plants, shrubs, and grasses [44,45]. It is a typical vegetation type in urban areas. During the past several years, attention has been paid to urban hedges because of their ecological functions, such as air quality purification, creation of animal habitat, and more [46,47]. However, their ET characteristics and regulation of the urban microclimate are neglected. Therefore, in this study we aim to (1) investigate the ET characteristics of two common urban hedges using the '3T + IR' method in a subtropical megacity, Shenzhen, and then (2) quantify the cooling effects of the urban hedges and quantify the function of ET. This study could provide a new fetch-free and high-spatiotemporal-resolution method for estimating urban ET. It may contribute to understanding the ET of urban hedges, and then the species selection and landscape design in urban planning for urban heat island mitigation.

#### **2. Material and Methods**
