The impacts of land cover types on urban outdoor thermal environment: the case of Beijing, China
© Yan and Dong; licensee BioMed Central. 2015
Received: 23 February 2014
Accepted: 4 May 2015
Published: 14 May 2015
This study investigated the microclimatic behavior of different land cover types in urban parks and, the correlation between air temperature and land cover composition to understand how land cover affects outdoor thermal environment during hot summer.
To address this issue, air temperatures were measured on four different land cover types at four observation sites inside an urban park in Beijing, China, meanwhile, the land cover composition of each site was quantified with CAD, by drawing corresponding areas on the aerial photographs.
The results showed that the average air temperature difference among four land cover types was large during the day and small during the night. At noon, the average air temperature differed significantly among four land cover types, whereas on night, there was no significant difference among different land cover types. Results of the linear regression indicated that during daytime, there was a strong negative correlation between air temperature and percent tree cover; while at nighttime, a significant negative correlation was observed between air temperature and percent lawn cover. It was shown that as the percent tree cover increased by 10 %, the air temperature decreased by 0.26 °C during daytime, while as the percent lawn cover increased by 10 %, the air temperature decreased by 0.56 °C during nighttime.
Results of this study help to clarify the effects of land cover on urban outdoor thermal environment, and can provide assistance to urban planner and designer for improving green space planning and design in the future.
In the last decades, a great concentration of people around urban areas took place worldwide. The urbanization process, with its fast population increase, creates changes in the urban climate . A distinct feature of urban climate is the urban heat island (UHI) effect, in which the urban air temperature is higher than the air temperature of the surrounding rural or suburban areas . UHI has significant negative effects on the buildings energy consumption, outdoor air quality, living environment, and habitability of cities [3, 4]. With increasing urbanization, the urban heat island will affect a larger number of urban residents. Therefore, there is a pressing need for urban researchers to evaluate strategies that may mitigate against further increases in temperatures in urban areas.
Among all cooling measures, planting of vegetation in urban areas is one of the simplest and most effective strategies to mitigate the UHI effect [5, 6]. Urban green areas can ameliorate UHI effect by the combined impact of shading and evapotranspiration. A series of field measurement studies show that vegetated areas are likely to be cooler than their surrounding urban built environment [7–10]. This vegetated cool patch within the warmer built-up environment is referred to as a “park cool island” (PCI). In a study, by Jansson et al. , temperatures of an urban vegetated park and its surroundings in central Stockholm were measured during three summer days, the results indicated that the PCI intensity was in the range of 0.5–0.8 °C during the day and reached a maximum of 2 °C at sunset. In Mexico City, Jauregui  found that the air temperature of a large irrigated park was 3–4 °C cooler than its built-up surroundings. Similar results were also obtained from a study of two urban parks in Singapore . The cooling effect of urban parks in cities has been confirmed by the above studies. However, the physical constitution of the parks, such as the type of pavements, the amount of tree and grass cover and the floristic structure of vegetation can be expected to affect the cooling effect . Cao et al. , based on remote sensing data, studied the effects of park characteristics on the formation of PCI in Nagoya, and found that PCI intensity is clearly related to the area of tree and shrub inside the park, and that grass has negative impact on PCI formation. Comparing the temperatures of 61 parks during the summer at noon in Taipei City, Chang et al.  found that parks with more than 50 % paved coverage and little tree and shrub cover were on average warmer than their surroundings. The land cover composition of a park has been shown to positively correlate with the magnitude of differences in air temperature.
From these findings, it can be inferred that park characteristics, especially in regard to the type of land cover and the proportion of different land cover types may have a significant effect on outdoor air temperature [17, 18]. Therefore, knowledge of the relationship between air temperature and land cover is critical in improving landscape design strategies to ameliorate urban thermal environment. However, the quantifiable effect and statistical relationship between air temperature and land cover in a park have not been established and thus limit the design of optimal parks with remarkable PCI effect. In the present study, we investigated the thermal performance of different land cover types, and how the feature of land cover composition influences local air temperature in the city of Beijing, China. The work focused on humidity and hot summer days, when the green areas are more frequently used by the citizens in the study area.
to examine diurnal variations in air temperature of different land cover types;
to compare the air temperature difference among various land cover types;
to quantify the relationship between air temperature and land cover composition during daytime and nighttime in summer.
Study area and site description
Beijing is located in the northern part of the North China Plain. It is the second largest city in China with a total population of 19.6 million by the end of 2010. It has a monsoon influenced humid continental climate characterized by hot and humid summers and generally cold, windy and dry winters. According to the climatological normals (1971–2000), January is the coldest month with an average temperature of −3.7 °C, while July is the hottest month with an average temperature of 26.2 °C. The main wind direction is from southeast to northeast in summer and in reverse during winter. Since 1978, Beijing’s urban population and yearly construction area have been gradually increasing, leading to significant modifications in the underlying surface properties and notable increase in the intensity of the UHI effect.
Land cover percentages for each observation site (%)
Air temperature measurements
Measurements were carried out simultaneously at four observation sites on 18th, 27th, 28th, July of 2010. Atmospheric conditions on the measuring days were fairly clear and windless. During the observations, air temperature was measured once every two hours from 8:00 to 20:00 h. The major instruments used in the measurement were Testo humidity temperature meters. The measurements were taken at 1.5 m above the ground, which is approximately the level where a person of mean height breathes in. At each measurement point, we waited until the wind speed slowed below 2 m/s and wait for an additional few minutes for the temperature measurement to stabilize before taking the air temperature data.
In the data analysis, 14:00 h and 20:00 h were selected to represent the daytime and nighttime, respectively. Statistical analyses were performed to test the influence of land cover on air temperature using one-way ANOVA, and multiple comparisons were made using the Turkey HSD method. Simple correlation analysis was also performed to study the relations between local air temperature and land cover patterns of each observation site during daytime and nighttime. Statistical analysis was performed by running the SPSS/PC + software package (SPSS, Inc.) on a personal computer. P values of less than 0.05 were regarded as statistically significant.
Results and discussion
Temperature difference among land cover types
Relationship between local air temperature and land cover pattern
The air temperature difference among four land cover types was more significant during the day than during the night. At noon, the average air temperature differed significantly among four land cover types, which is lower on trees area than on paved area, lawn area, or water area; whereas on night, there was no significant difference among four land cover types.
It was also found that the average temperature of each observation site was closely related to the land cover composition. The site with large proportions of paved cover and little vegetation had a very high daytime air temperature, while the site with large proportions of tree cover had a low daytime air temperature. At night, however, an opposite trend was observed.
The effect of land cover composition on air temperature demonstrated a diurnal variation and was also dependent on land cover type. We found that site difference in average air temperature at 14:00 h was better predicted by percent tree cover, while average air temperature at 20:00 h was better predicted by percent lawn cover. It was shown that with a 10 % increase in the percent tree cover, the air temperature decreased by 0.26 °C during daytime in summer, by contrast, as the percent lawn cover increased by 10 %, the air temperature decreased by 0.56 °C during nighttime in summer.
This study extended our scientific understanding of the effects of land cover, especial the land cover composition on air temperature in summer. In addition, the findings have important implications for urban green space planning and design. In order to ameliorate the outdoor thermal environment, planners and designers must consider the composition and configuration of the land cover. However, it should be noted that this study was conducted only for one urban green space during a single summer season. Therefore, future work should expand this research to consider more land cover in different metropolitan areas, and expanding the analyses to also include different seasons.
We sincerely thank two anonymous reviewers for their constructive comments and suggestions. This research was supported by the Research Foundation of Talented Scholars of Zhejiang A & F University (NO. 2014FR063).
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