Reconstruction of the thermal environment evolution from subsurface temperature distribution in Japan and Thailand

Temperature changes at the ground surface propagate into the underground and disturb the subsurface temperature structure. Analyzing disturbances in the subsurface temperature structure, we can reconstruct the past ground surface temperature (GST) change, which is closely related to the past surface air temperature change. This method can be applied to studies of thermal environment evolution in urban areas such as the development of "heat islands". We have been investigating GST histories in three areas, which are located in Japan and Thailand. The three areas are the northern part of Kanto area, Osaka area, and Bangkok area. Kanto area and Osaka area have the greatest and second greatest population in Japan, each other. Bangkok area has the greatest population in Thailand. In the northern part of Kanto area, we conducted measurements of temperature profiles in groundwater monitoring wells at 25 sites in 2009, 2010, and 2011. In Osaka area, temperature profiles were measured at 31 sites in 2011 as the project of the Sumitomo Foundation (M. Taniguchi). In Bangkok area, we measured temperature profiles at 45 sites in 2004, 2006, 2008, and 2010. We examined the shapes of the temperature profiles and selected ones that are not significantly disturbed by groundwater flow. Reconstruction of GST history for the last about 300 hundred years was made at two sites in the northern part of Kanto area, at six sites in Osaka area, and at six sites in Bangkok area. We used a multi-layer model that allows layers with different thermal properties, determining layer boundaries based on lithology of the formations around the wells. All of the reconstructed GST histories show surface warming in the last century. In the northern part of Kanto area, the amount of the temperature increase from 1700 to 2010 is about 2.5 K at both sites. In Osaka area, the amount of the temperature increase from 1700 to 2010 ranges from 2.5 K to 5.0 K and is larger in the city center and the southern part of Osaka than in the surrounding area. The southern part of Osaka is factory area. In the Bangkok area, the amount of the temperature increase from 1700 to 1990 ranges from 0.4 to 2.6 K and is larger in the city than in the area to the west of Bangkok and in the northern rural area. These tendencies may reflect difference in the degree of urbanization or human activities. Comparing with the all GST, temperature increase in the city center of Osaka is the largest of all the sites at present. Temperature increase in the surrounding area of the northern part of Kanto and that of Osaka is similar to each other. These results should be combined with other information on development of the two large cities to investigate the main cause of the surface warming, e.g., increase in the surface air temperature and land use change. We also estimated the amount of heat stored in the subsurface after 1900 based on the reconstructed GST histories. Then heat storage at the city center of Osaka is up to 550 MJ/m2. It is over twice as other sites. In this presentation, we also discuss with accuracy of GST analysis. Groundwater flow great impacts on the GST analysis. Therefore, we show the evaluation of the affection using numerical simulations.;