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Air temperature rise in Tokyo is distinctively large after 1980. Reflecting this feature, water temperature at Masugatanoike, a spring in Tokyo that stands a foot of terrace scarp that the height is 14 m, rose after the end of 1980s. The water temperature began to rise 4–7 years after the air temperature rose, showing a statistically significant correlation at the 5% level. This warming water must be explained by downward thermal conduction because there are no hot springs, subways, or major sewers around Masugatanoike spring. In this study, we first solved the surface energy balance in Tokyo from 1951 to 2009 to estimate the daily mean ground surface temperature. Next, we calculated the thermal conduction from the ground surface to a depth of 16 m. The initial condition was optimized so that root mean square error of the observed and calculated temperatures at a depth of 14 m from 1976 to 2009 was minimized. We found that 4–8 years were necessary until the effects of ground surface temperature reached a depth of 14 m after 1975. Namely, we demonstrated that downward thermal conduction is responsible for the above-mentioned lag relation, which was found in the observation.

hydrological research letters

Diagnostic study on warming mechanism of spring water temperature based on field observations and numerical simulation: a case study of Masugatanoike spring, Tokyo, Japan