Abstract
Urban evapotranspiration (ET) is an effective way to mitigate ecological challenges resulting from rapid urbanization. However, the characteristics of urban vegetation ET, especially how they respond to meteorological factors and soil water, remains unclear, which is crucial for urban ET regulation. Therefore, this study measured the actual ET rate of an urban lawn (ETlawn) using the Bowen ratio system and an urban tree (Ttree) by a sap flow system in the hot summer of a subtropical megacity, Shenzhen. The results showed that the daily ETlawn was more restricted by energy (Rs) and diffusion conditions (vapor pressure deficit, VPD), while the daily Ttree was more restricted by VPD and relative extractable water (REW) in the urban area. The daily Ttree decreased when the REW was lower than 0.18, while the daily ETlawn started to decrease when it was lower than 0.14. When REW was lower than 0.11, the Ttree stayed at a relatively low level. The impacts of VPD was more evident on the diurnal Ttree than on the diurnal ETlawn. Wind speed had a scarce impact as it was relatively low in urban areas. This study clarifies the different responses of ETlawn and Ttree to meteorological factors and soil water based on actual ET. The results are of great significance for the knowledge of urban forestry and urban hydrology.
Highlights
It has been widely recognized that urban vegetation plays an important role in mitigating the urban heat island effect under the dual impact of global warming and rapid urbanization, and this process is directly related to urban ET [1,2,3]
The results showed that the daily average ET rate of an urban lawn (ETlawn) and Ttree were 2.50 and 1.40 mm d−1, respectively, throughout the whole study period
Throughout the study period, the daily ETlawn was mainly decided by restricted by energy (Rs) (r = 0.560, p < 0.001) and vapor pressure deficit (VPD) (r = 0.505, p < 0.001), while the daily
Summary
It has been widely recognized that urban vegetation plays an important role in mitigating the urban heat island effect under the dual impact of global warming and rapid urbanization, and this process is directly related to urban ET [1,2,3]. Urban ET refers to the evaporation of water from different underlayers to the atmosphere in urban areas, which is the link between the urban water cycle and energy cycle. It can cool the surroundings by consuming a large proportion of available energy and reduce urban runoff by larger water holding capacity, which cannot be ignored in the construction of sponge cities [4]. Vegetation coverage in many urban areas may be higher than that in adjacent suburbs
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