10:45 AM - 12:15 PM
[AHW24-P01] Evaluation of subsurface warming in the Tokyo metropolitan area, Japan: Governing factors of subsurface thermal environment changes in groundwater development areas
Keywords:subsurface temperature, groundwater flow, subsurface warming, observation well, urbanization, Tokyo metropolitan area
Effects of urbanization-related changes in the environment and regional groundwater flow, on the subsurface thermal environment of the Tokyo metropolitan area in Japan, were evaluated. This was conducted by examining subsurface temperatures and their three-dimensional distribution as well as long-term changes. For this purpose, multiple observations of temperature-depth profiles from the year 2000 and subsurface temperature monitoring in observation wells from 2007 in Saitama Prefecture and 2013 in Tokyo Metropolis were used.
Subsurface warming was found at shallow depths throughout the study area in our previous study (Miyakoshi et al., 2010; 2017). Subsurface temperatures beneath the city center, such as those in central Tokyo, were particularly high, not only in the shallow parts but also in the deep parts. In contrast, relatively low temperatures were found beneath suburban areas. A comparison of past subsurface temperature data (2003–2005) and present subsurface temperature data (2013–2020) shows that subsurface warming has occurred in the shallow parts in the last 10 to 15 years. The increases in subsurface temperatures in the city center were found to be larger than those in the suburban area. The temporal differences in temperatures indicate an increasing trend of subsurface temperatures in both the areas. Additionally, present data indicate that subsurface warming occurred in deeper parts. This result suggests that the distribution of subsurface warming is expanding toward the deeper parts.
Based on the results of subsurface temperature monitoring, subsurface warming was confirmed to have occurred continuously. The warming rate varied depending on the location and depth. An almost-constant 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 (Miyakoshi et al., 2021). These observations were from the alluvial lowland, and the warming was driven mainly by heat conduction due to warming of the surface from urbanization.
In the Musashino upland(around Miyoshi area, Saitama Prefecture), and the Arakawa alluvial fan (around Kumagaya area, Saitama Prefecture), continuous subsurface warming, similar to that in lowland, was found. However, the warming rate fluctuated with time in some observation wells. Additionally, significant temperature changes were observed at depths greater than 100 m. Comparing the Musashino upland and the Arakawa alluvial fan, the period, magnitude, and depth of these changes were different. These temporal changes in subsurface temperature are different from the characteristic of changes in air and soil temperatures, and it is difficult to consider that the formation was formed merely by the effects of changes in surface temperature. Complex subsurface temperature changes were observed around areas where significant changes in hydraulic heads had occurred. It is considered that artificially induced change in groundwater flow is one of the main reasons for long-term changes in subsurface temperatures. Furthermore, the differences in the magnitude of subsurface temperature changes in the two regions reflect the differences in hydrogeological conditions and groundwater development. The mechanism of subsurface warming is expected to be evaluated by a combined analysis of changes in geological conditions, groundwater flow, and subsurface temperatures.
The part of this study was supported by JSPS KAKENHI Grant Number 19K12364, 22K12410 and 22K05012. The research in Saitama Prefecture was conducted as a part of AIST- Akita University -Saitama Prefecture joint research.
Subsurface warming was found at shallow depths throughout the study area in our previous study (Miyakoshi et al., 2010; 2017). Subsurface temperatures beneath the city center, such as those in central Tokyo, were particularly high, not only in the shallow parts but also in the deep parts. In contrast, relatively low temperatures were found beneath suburban areas. A comparison of past subsurface temperature data (2003–2005) and present subsurface temperature data (2013–2020) shows that subsurface warming has occurred in the shallow parts in the last 10 to 15 years. The increases in subsurface temperatures in the city center were found to be larger than those in the suburban area. The temporal differences in temperatures indicate an increasing trend of subsurface temperatures in both the areas. Additionally, present data indicate that subsurface warming occurred in deeper parts. This result suggests that the distribution of subsurface warming is expanding toward the deeper parts.
Based on the results of subsurface temperature monitoring, subsurface warming was confirmed to have occurred continuously. The warming rate varied depending on the location and depth. An almost-constant 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 (Miyakoshi et al., 2021). These observations were from the alluvial lowland, and the warming was driven mainly by heat conduction due to warming of the surface from urbanization.
In the Musashino upland(around Miyoshi area, Saitama Prefecture), and the Arakawa alluvial fan (around Kumagaya area, Saitama Prefecture), continuous subsurface warming, similar to that in lowland, was found. However, the warming rate fluctuated with time in some observation wells. Additionally, significant temperature changes were observed at depths greater than 100 m. Comparing the Musashino upland and the Arakawa alluvial fan, the period, magnitude, and depth of these changes were different. These temporal changes in subsurface temperature are different from the characteristic of changes in air and soil temperatures, and it is difficult to consider that the formation was formed merely by the effects of changes in surface temperature. Complex subsurface temperature changes were observed around areas where significant changes in hydraulic heads had occurred. It is considered that artificially induced change in groundwater flow is one of the main reasons for long-term changes in subsurface temperatures. Furthermore, the differences in the magnitude of subsurface temperature changes in the two regions reflect the differences in hydrogeological conditions and groundwater development. The mechanism of subsurface warming is expected to be evaluated by a combined analysis of changes in geological conditions, groundwater flow, and subsurface temperatures.
The part of this study was supported by JSPS KAKENHI Grant Number 19K12364, 22K12410 and 22K05012. The research in Saitama Prefecture was conducted as a part of AIST- Akita University -Saitama Prefecture joint research.