Analyzing Change of Discomfort Index for Transpiration of Street Tree
Abstract
Thermal environment of cities are worsening due to severe urban heat island caused by climate change and urbanization. The cooling effect of street tree is regarded as an effective way to ameliorate the urban heat environment. The effect is a result of shadow formation and transpiration. Tree's transpiration has direct effects on not only temperature reduction, but also the humidity by releasing vapor. In the summer, increased humidity can aggravate thermal environments with in high temperature and humidity, such as South Korea. However, in previous studies, the effect of tree transpiration on improving the thermal environment, through reduction of temperature and increase of humidity, has not been adequately addressed. Therefore, this study aims to identify how the transpiration affects the discomfort index by analyzing the comprehensive impact of transpiration on air temperature and relative humidity. The changes in the amount of transpiration, air temperature, and relative humidity were estimated for the study area - a population dense urban street in Seoul, South Korea, on in dry day in July and August at 2 p.m.. July and August are the months when the discomfort index is the highest in Korea, especially at 2 p.m. The transpiration of trees was calculated based on hydrological models and verified using sap flow density measured by Granier-type heat dissipation sensors. On average, the transpiration of the street tree decreased the temperature by 0.3℃ and increased the relative humidity 2.6% in an hour. These changes in temperature and humidity, resulted in an increase in the discomfort index on most days. On days when discomfort index value was above 80, increase in discomfort was always the case. However, transpiration's effect on the variation of the discomfort index itself was relatively small due to the offsetting effects in comparison to the significant change in temperature and humidity independently. This results imply the transpiration not always improve the thermal environment. It is therefore necessary to select the type of green and planting location to maximize the cooling effect.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A21P2833Y
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 1632 Land cover change;
- GLOBAL CHANGE;
- 1637 Regional climate change;
- GLOBAL CHANGE