Effect of structural modification on heat transfer through man-made soils in urban green areas

This study examined the characteristics of heat and water transfer in structurally modified urban soils. To satisfy our goals, we measured the temperature and moisture content of anthropogenic soils to a depth of 50cm. Field observations was carried out for three sets (each of two pedons) of soils in the Tokyo Metropolitan area. Each pedon had the same turf coverage but different profile modifications in the green areas. Soil temperature, soil moisture, and precipitation data were collected during the summer (July-Sept) and winter (Oct-Feb) every 10min. From the results, we calculated the thermal diffusivity and thermal conductivity in each pedon. Soil temperature showed a clear daily variation down to 30cm depth. Temperature transmission to deeper layers was faster in pedon having stronger soil compaction and more artificial fragments than in pedons with weaker soil compaction and fewer concrete fragments. This finding suggests that strongly compacted soil has a relatively high thermal conductivity, and easily transfers heat to deeper soil. In pedons composed of soft, organic-rich, and clay-rich soil, water retention impedes the increase in soil temperature during daytime, whereas nighttime cooling is prevented by the lower heat transmission due to the larger porosity. Throughout the observations, the water content ranged from 0.1 to 0.45m(3)m(-3). The thermal diffusivity was obtained as 1.2-5.0x10(-3)cm(2)s(-1) in pedons without artifacts, but was higher (2.5-7.3x10(-3)cm(2)s(-1)) in all pedons containing larger volumes of concrete artifacts. Although the directions of heat flow by time within the profiles having lithological disturbance was not much different with that of natural soils, the observation data revealed that heat flow per time differed by structural properties of the profiles. Furthermore, thermal properties such as thermal diffusivity and thermal conductivity of the soils characterized with lithological disturbance were significantly higher than those of natural soils and it was notable that they were not influenced by either volume fraction of water or air in the soil. The fact suggested that the anthropogenic soils containing a large amount of modifiers and concrete artifacts have small capacity of water retention due to specific macro pores. Artificial materials and compaction regulate the drainage and water retention of the soil, although the water transfer behavior in the studied pedons could be rather complicated.