Long-Term Evolution of Anthropogenic Heat Fluxes into a Subsurface Urban Heat Island

Anthropogenic alterations in urban areas influence the thermal environment causing elevated atmospheric and subsurface temperatures. The subsurface urban heat island effect is observed in several cities. Often shallow urban aquifers exist with thermal anomalies that spread laterally and vertically, resulting in the long-term accumulation of heat. In this study, we develop an analytical heat flux model to investigate possible drivers such as increased ground surface temperatures (GSTs) at artificial surfaces and heat losses from basements of buildings, sewage systems, subsurface district heating networks, and reinjection of thermal wastewater. By modeling the anthropogenic heat flux into the subsurface of the city of Karlsruhe, Germany, in 1977 and 2011, we evaluate long-term trends in the heat flux processes. It revealed that elevated GST and heat loss from basements are dominant factors in the heat anomalies. The average total urban heat flux into the shallow aquifer in Karlsruhe was found to be similar to 759 +/- 89 mW/m(2) in 1977 and 828 +/- 143 mW/m(2) in 2011, which represents an annual energy gain of around 1.0 x 10(15) J. However, the amount of thermal energy originating from the individual heat flux processes has changed significantly over the past three decades.