5:15 PM - 7:15 PM
[AHW31-P01] Evaluation of subsurface warming in the Tokyo metropolitan area, Japan: governing factors and characteristics of temperature change in low permeable layers
Keywords:subsurface temperature, groundwater flow, subsurface warming, urbanization, underground infrastructure, Tokyo metropolitan area
The Tokyo metropolitan area is one of the most populous megacities worldwide, and the temperature increase in Tokyo, the center of the metropolitan area, is remarkable among other global megacities. The effects of urbanization-related changes in the environment and groundwater development, progression of global warming, and climate change on the subsurface thermal environment of the Tokyo metropolitan area in Japan were evaluated. This was conducted by examining the subsurface temperatures and their three-dimensional distributions as well as long-term changes. For this purpose, multiple observations of temperature-depth profiles from 2000 and subsurface temperature monitoring in observation wells from 2007 in the Saitama Prefecture and 2013 in the Tokyo Metropolis were used. Additionally, the continuous and elaborate changes in subsurface temperatures and the differences in temperature variations with depth and stage of urbanization (urban center, surrounding urban center, and suburban area) were investigated.
The results revealed a continuous increase in subsurface temperatures, particularly in shallow subsurface layers, in many observation wells. There were clear regional differences and temporal changes in the distribution of subsurface temperatures, with urban centers showing relatively higher temperatures and higher rates of temperature increase than the suburban areas. Subsurface warming in the Tokyo metropolitan area is caused by a combination of land-use changes in various regions, surface and shallow subsurface temperature increases associated with urbanization, groundwater flow on a regional scale reflecting groundwater development, and global warming (Miyakoshi et al., 2019). To evaluate the complex subsurface thermal environment in urban areas, this study focused on subsurface warming within low-permeability layers and reported its characteristics. As the effects of heat advection owing to groundwater flow are expected to be relatively small in low-permeability layers, these layers are suitable for assessing the effects of heat conduction from surface heat sources and underground structures.
At two sites within the Arakawa Lowland, the subsurface temperature profiles showed the lowest temperatures at depths of 30–50 m, with a pronounced temperature increase in the shallow alluvial clay layer above this depth. The temperature increases at a depth of 20 m between 2003 and 2024 exhibited regional differences: approximately 0.4℃ at the observation well on the Tokyo Bay side and approximately 0.7℃ at the well on the inland side. The long-term trend in the subsurface temperature during this period showed a secular change. Although the primary cause of the temperature increase in the alluvial clay layer can be considered to be near-surface heat, regional differences in subsurface warming were attributed to differences in the degree of urbanization and the duration of its progression.
However, the two observation wells located upland of the city center and reclaimed land from Tokyo Bay showed peculiar subsurface temperature profiles, with the highest temperature at 20 or 40 m, rather than near the surface. These subsurface temperature increases were several to 10 times greater than the subsurface warming because of the surface heat effects. Waste heat from the underground infrastructure around the observation wells was the primary cause of the temperature increase, and the changes in the subsurface warming trend were considered to be related to the installation and usage status of these underground structures. These results suggest that subsurface warming in urban areas can be significantly affected not only by surface temperature increases due to urbanization and global warming but also by subsurface development. Further insights into the mechanisms of urban subsurface warming can be obtained by evaluating the relative contributions of these factors.
The research in Saitama Prefecture was conducted as a part of AIST- Akita University -Saitama Prefecture joint research. The part of this study was supported by JSPS KAKENHI Grant Number 22K12410 and 22K05012.
The results revealed a continuous increase in subsurface temperatures, particularly in shallow subsurface layers, in many observation wells. There were clear regional differences and temporal changes in the distribution of subsurface temperatures, with urban centers showing relatively higher temperatures and higher rates of temperature increase than the suburban areas. Subsurface warming in the Tokyo metropolitan area is caused by a combination of land-use changes in various regions, surface and shallow subsurface temperature increases associated with urbanization, groundwater flow on a regional scale reflecting groundwater development, and global warming (Miyakoshi et al., 2019). To evaluate the complex subsurface thermal environment in urban areas, this study focused on subsurface warming within low-permeability layers and reported its characteristics. As the effects of heat advection owing to groundwater flow are expected to be relatively small in low-permeability layers, these layers are suitable for assessing the effects of heat conduction from surface heat sources and underground structures.
At two sites within the Arakawa Lowland, the subsurface temperature profiles showed the lowest temperatures at depths of 30–50 m, with a pronounced temperature increase in the shallow alluvial clay layer above this depth. The temperature increases at a depth of 20 m between 2003 and 2024 exhibited regional differences: approximately 0.4℃ at the observation well on the Tokyo Bay side and approximately 0.7℃ at the well on the inland side. The long-term trend in the subsurface temperature during this period showed a secular change. Although the primary cause of the temperature increase in the alluvial clay layer can be considered to be near-surface heat, regional differences in subsurface warming were attributed to differences in the degree of urbanization and the duration of its progression.
However, the two observation wells located upland of the city center and reclaimed land from Tokyo Bay showed peculiar subsurface temperature profiles, with the highest temperature at 20 or 40 m, rather than near the surface. These subsurface temperature increases were several to 10 times greater than the subsurface warming because of the surface heat effects. Waste heat from the underground infrastructure around the observation wells was the primary cause of the temperature increase, and the changes in the subsurface warming trend were considered to be related to the installation and usage status of these underground structures. These results suggest that subsurface warming in urban areas can be significantly affected not only by surface temperature increases due to urbanization and global warming but also by subsurface development. Further insights into the mechanisms of urban subsurface warming can be obtained by evaluating the relative contributions of these factors.
The research in Saitama Prefecture was conducted as a part of AIST- Akita University -Saitama Prefecture joint research. The part of this study was supported by JSPS KAKENHI Grant Number 22K12410 and 22K05012.