A continuous dynamic feature of the distribution of soil temperature and horizontal heat flux next to external walls in different orientations of construction sites in the autumn of Beijing, China

Abstract

Soil is an important carrier of the urban ecosystem. Many eco-systematic processes take place in the soil surface layer and are controlled directly by soil temperature. However, accelerated global urbanization and urban construction lead to urban heat islands, causing higher air temperatures in urban areas than those in rural areas. For the same reason, soil temperature is higher in urban areas. Higher soil temperatures may cause changes in eco-systematic processes. Therefore, studying and understanding how soil temperature changes in urban areas is necessary. In other words, the heat impact from urban construction, including its process and extent, on soil needs further researches. In this study, the experimental transect was arranged in the ecotone between construction and green space. The temperature of the surface soil layer was investigated and recorded to analyze urban construction horizontal heat impact process and quantity on the soil of green space in urban areas during autumn within Beijing city, China. Several acquired results are as follows: in autumn, downward-trend distributions of soil temperature were found along the construction-soil micro gradient transect next to the south, north, east and west side external walls. The south, north, east and west side external walls' maximum horizontal heat impact scopes are 0.15, 0.0167, 0.1 and 0.05 m, respectively, on a diurnal scale. The daily horizontal heat impact process was different along the experimental transect, changing with orientation of the building external wall. According to continuous investigation, a formulation of horizontal heat flux (Gh, between buildings and soil) and ΔT (difference between soil temperature 0 and 0.05 m from the building baseline) was generated: Gh = 31.77ΔT + 8.11 (P < 0.0001), where R2 was 0.976. In addition, no significant difference (P > 0.05) was found between the value of simulation and measurement via a two-tailed T test. Based on this formulation, a continuous dynamic-feature for heat flux was calculated. On a diurnal scale, the mean horizontal heat flux at different orientations of external walls for both sunny and cloudy days are as follows: 44.13 and 7.07 W/m2 for south, 28.15 and 9.22 W/m2 for north, 21.44 and 9.66 W/m2 for east and 24.42 and 13.54 W/m2 for west. Downward gradient distributions of mean horizontal heat flux of autumn were calculated; for south, north, east and west side external walls, the maximum was 29.94, 25.44, 24.87 and 23.50 W/m2 and the minimum was 7.31, 6.46, 7.45 and 4.81 W/m2. Therefore, construction sites were considered to be a heat source for soil and provided soil with thermal energy, leading to a higher soil temperature around construction sites in urban areas during autumn. This study focused on urban construction horizontal heat impacts on the soil of adjacent green space in conditions of different weather in autumn and helped us to understand the quantitative processes of horizontal heat impact. In addition, the study will benefit research examining the ecological relationships of urban heat islands and ecosystem processes.