Multi-scale Investigation of Principal Heat Transport Processes in Subsurface Urban Heat Islands

• Applicant: Professor Dr. Philipp Blum
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: from 2012 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 230799312

• Final Report Year: 2017

Final Report Abstract

The main objective of this project was to investigate the principal heat transport processes in urban aquifers using multiple scales and the link between satellite derived land surface temperatures (LST) and groundwater temperatures (GWTs). The first part of this project work discusses shallow GWTs unaltered by human influence. 2,548 GWT measurement points in 29 countries are compiled, revealing characteristic trends in the offset between shallow groundwater temperatures and land surface temperatures. Evapotranspiration and snow govern this offset globally through latent heat flow and insulation, respectively. Considering these two processes only, global shallow groundwater temperatures can be estimated with a root mean square error (RMSE) of 1.4 K and a first prognosis of future subsurface climate conditions can be made. The second part of this study explores the anthropogenic impact on temperatures on a country wide scale. At the example of three temperature datasets in Germany (measured surface air temperature, measured GWT, and satellite-derived LST) the so-called anthropogenic heat intensity (AHI) is introduced. Taking nighttime lights as an indicator of rural areas, it provides the difference between local temperatures and median rural background temperatures. This concept is analogous to the established urban heat island intensity, but applicable independent of land use and location and useful in the characterization of 3D spatial variability of urban heat islands. In Germany, results indicate that groundwater temperatures are more vulnerable to human activity than above-ground temperatures. Third, subsurface urban heat islands (SUHI) are investigated more thoroughly. The objective of this part of the study is to understand the dynamics of dominant heat flux processes into the subsurface beneath two German cities of Karlsruhe and Cologne. Thus, statistical and spatial analytical heat flux models were developed for both cities. The models include the spatial representation of various sources of the anthropogenic heat fluxes into the subsurface (AHFs): (1) elevated ground surface temperatures, (2) basements, (3) sewage systems, (4) sewage leakage, (5) subway tunnels, and (6) district heating networks. The results show that district heating networks induce the largest AHFs with values greater than 60 W/m^2 and one order of magnitude higher than fluxes from any other sources. A covariance analysis indicates that the spatial distribution of the total flux depends mainly on the thermal gradient in the unsaturated zone. On a citywide scale, basements and elevated ground surface temperatures are the dominant sources of heat flow. Overall, 2.1 PJ/a and 1.0 PJ/a of heat are accumulated on average in Karlsruhe and the western part of Cologne, respectively. Extracting this anthropogenically originated energy could sustainably supply significant parts of the urban heating demand. Furthermore, using this heat could also keep groundwater temperatures from rising further. In the last part of this study the spatial properties of SUHI and surface UHI are compared in several German cities and correlations of up to 80 % are found. The best correlation is found in older, mature cities such as Cologne and Berlin. However, in 95 % of the analyzed areas, groundwater temperatures are higher than land surface temperatures due to additional subsurface heat sources such as buildings and their basements. Local groundwater hot spots under city centers and under industrial areas are not revealed by satellite-derived land surface temperatures. Hence, an estimation method is proposed that relates groundwater temperatures to mean annual landsurface temperatures, building density, and elevated basement temperatures. Using this method, regional GWTs in Karlsruhe, Cologne and Berlin are accurately estimated with a root mean square error (RMSE) of 1.4 K. If the previously established influence of evapotranspiration and snow are also taken into account, this RMSE can further be reduced to 0.9 K.

Publications

• Spatial resolution of anthropogenic heat fluxes into urban aquifers. Science of the Total Environment, 524-525, 427 (2015)
  S. A. Benz, P. Bayer, K. Menberg, S. Jung, P. Blum
  
• Human impact on groundwater temperatures. Dissertation, Karlsruher Institut für Technologie (KIT), Fakultät für Bauingenieur-, Geo- und Umweltwissenschaften (BGU) (2016)
  S.A. Benz
  
• Linking Surface Urban Heat Islands with Groundwater Temperatures. Environmental Science & Technology, 50 (1), 70 (2016)
  S. A. Benz, P. Bayer, F. M. Goettsche, F. S. Olesen, P. Blum
  
• Global panems of shallow groundwater temperatures. Environmental Research Letters, 12, 034005 (2017)
  S. A. Benz, P. Bayer, P. Blum
  
• Identifying anthropogenic anomalies in air, surface and groundwater temperatures in Germany. Science of the Total Environment, 584-585, 145-153, (2017)
  S. A. Benz, P. Bayer, P. Blum