Evolution of the Subsurface Thermal Environment in Urban Areas: Studies in Large Cities in East Asia

Abstract

Temporal variation in the ground surface temperature (GST) propagates downward and disturbs the subsurface temperature structure. Analysis of subsurface temperature profiles gives information on the past GST history. For investigation of subsurface thermal environment evolution in urban areas, we reconstructed GST history from borehole temperature data obtained in large cities in East Asia. Most of the estimated GST histories show significant surface warming in the last century. In the Bangkok area, the amount of the GST increase is larger in the city center than in suburban and rural areas, corresponding to the degree of urbanization. The amount of heat accumulated in the subsurface due to surface warming, which can be calculated at the sites where GST histories were reconstructed, is a useful indicator of the subsurface thermal environment. We conducted monitoring of borehole temperature and soil temperature aiming to observe process of downward propagation of GST variation. Data obtained in some wells exhibit periodic or long-term temperature variations attributable to human activities. Soil temperatures measured within 1 m of the ground surface show prominent annual variation and provide information on the relation between GST and air temperature.