5:15 PM - 6:45 PM
[AHW25-P01] Subsurface warming progress in the Tokyo metropolitan area, Japan: subsurface thermal environment changes due to effects of urbanization and groundwater development
Keywords:subsurface temperature, groundwater flow, subsurface warming, urbanization, groundwater development, Tokyo metropolitan area
The Tokyo metropolitan area is one of the most populous megacities in the world, and the temperature increase in Tokyo, the center of the metropolitan area, is remarkable among other megacities in the world. The effects of such urbanization-related changes in the environment and groundwater development, the 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 Saitama Prefecture and 2013 in Tokyo Metropolis were used. Additionally, the continuous and elaborate changes in subsurface temperature and the differences in temperature variation with depth and stage of urbanization (urban center, surrounding urban center, and suburban area) were investigated. Based on these observations, we report the progress of subsurface warming phenomena beneath the Tokyo metropolitan area and the characteristics of regional differences.
Based on the results of subsurface temperature monitoring, subsurface warming was confirmed to have occurred continuously over a wide area, from the urban center to the suburban area. However, the warming rate varied depending on location and depth. The warming rate fluctuated with short- and long-term complexity in some observational wells located mainly in groundwater development areas. In the Musashino upland and Arakawa alluvial fan, which belong to the suburban area, the warming rate at 30–50m deep shows fluctuations with shorter cycles than the daily or annual variation. These temporal changes in the subsurface temperature are different from the characteristics of changes in air and soil temperatures, and it is difficult to conclude that warming was formed merely by heat conduction from the surface temperature change. Complex subsurface temperature changes were observed around areas where significant changes in the hydraulic heads occurred. The secular change in heat advection associated with artificially induced changes in groundwater flow is considered to be one of the main causes of long-term changes in subsurface temperatures.
On the other hand, although no daily or annual periodic variations are observed in the urban center and its surrounding, the long-term warming trend since 2000 has not been constant, and the temperature increase has been larger in the 2010s than in the 2000s. The warming rate of 0.01–0.02 K/year was found to occur at a depth of 30 m in the observation wells in the southeastern area of Saitama Prefecture and the eastern area of Tokyo Metropolis. This warming ratio was several times higher than that in suburban areas. In these areas, the hydraulic head, which was greatly reduced by groundwater pumping in the past, has recovered owing to pumping restrictions since the 1950s, and groundwater levels have been relatively stable in recent years. The depth difference in the hydraulic head decreased, and the effects of heat advection associated with vertical groundwater flow became relatively small with the recovery of the hydraulic heads. Therefore, the effects of surface warming in shallow areas may have increased. In some urban areas, the warming rate of subsurface temperatures at certain depths was higher than that at shallower depths with short-period fluctuations, suggesting the effects of waste heat from underground structures. The subsurface thermal environment records the history of surface and subsurface environmental changes, and the analysis of subsurface temperature changes at each observation well, together with urbanization and groundwater flow, is expected to clarify the formation mechanism of subsurface warming in the Tokyo metropolitan area.
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.
Based on the results of subsurface temperature monitoring, subsurface warming was confirmed to have occurred continuously over a wide area, from the urban center to the suburban area. However, the warming rate varied depending on location and depth. The warming rate fluctuated with short- and long-term complexity in some observational wells located mainly in groundwater development areas. In the Musashino upland and Arakawa alluvial fan, which belong to the suburban area, the warming rate at 30–50m deep shows fluctuations with shorter cycles than the daily or annual variation. These temporal changes in the subsurface temperature are different from the characteristics of changes in air and soil temperatures, and it is difficult to conclude that warming was formed merely by heat conduction from the surface temperature change. Complex subsurface temperature changes were observed around areas where significant changes in the hydraulic heads occurred. The secular change in heat advection associated with artificially induced changes in groundwater flow is considered to be one of the main causes of long-term changes in subsurface temperatures.
On the other hand, although no daily or annual periodic variations are observed in the urban center and its surrounding, the long-term warming trend since 2000 has not been constant, and the temperature increase has been larger in the 2010s than in the 2000s. The warming rate of 0.01–0.02 K/year was found to occur at a depth of 30 m in the observation wells in the southeastern area of Saitama Prefecture and the eastern area of Tokyo Metropolis. This warming ratio was several times higher than that in suburban areas. In these areas, the hydraulic head, which was greatly reduced by groundwater pumping in the past, has recovered owing to pumping restrictions since the 1950s, and groundwater levels have been relatively stable in recent years. The depth difference in the hydraulic head decreased, and the effects of heat advection associated with vertical groundwater flow became relatively small with the recovery of the hydraulic heads. Therefore, the effects of surface warming in shallow areas may have increased. In some urban areas, the warming rate of subsurface temperatures at certain depths was higher than that at shallower depths with short-period fluctuations, suggesting the effects of waste heat from underground structures. The subsurface thermal environment records the history of surface and subsurface environmental changes, and the analysis of subsurface temperature changes at each observation well, together with urbanization and groundwater flow, is expected to clarify the formation mechanism of subsurface warming in the Tokyo metropolitan area.
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.