Linking Surface Urban Heat Islands with Groundwater Temperatures
In the cities, the temperatures are usually higher than the surrounding rural areas. This phenomenon is called Urban Heat Island (UHI) and is due to a variety of factors such as population density, surface sealing, vegetation levels, industry and transport. The interesting thing is that it exists both above and below the ground and it affects the atmosphere, surface and subsurface of the cities.
Researchers from the Karlsruhe Institute of Technology (KIT) and ETH Zurich have developed a method of estimating groundwater temperature under cities from surface temperatures and building densities measured by satellites. Up to now, satellites have been used to measure heat on the surface, but the relationship and interaction between ground and underground temperatures had not been examined before. The report was published in the ‘Environmental Science & Technology’ journal in November 2015.
Photo courtesy of ACS Publications
Scientists compared surface and underground temperatures in 4 German cities: Berlin, Munich, Cologne and Karlsruhe. In 95% of the areas studied, groundwater temperatures were higher than those on the surface. This was explained due to additional underground anthropogenic heat sources such as cellars of buildings and sewers. This higher temperature of groundwater could be used for heating in winter and cooling in summer, via geothermal or groundwater heat pumps.
Using satellite-derived surface temperatures and interpolated groundwater temperature measurements, they compared surface and underground heat islands and found a spatial correlation of up to 80%. The strongest correlation was found in older, mature cities (Berlin and Cologne), meaning the older the city is, the more pronounced is its underground heat. This percentage shows that satellite measurements of surface temperature alone are not sufficient to estimate temperature of groundwater. For this reason, scientists also considered population density and cellar temperature, resulting to an estimation of regional groundwater temperatures with a mean absolute of 0.9 Kelvin.
Once established, the method will be an inexpensive and time-effective way to estimate the geothermal energy stored underneath our cities, as there will be no need of complex groundwater temperature measurements and interpolations.
More research is required, but it seems that it will be easy to detect and use energy from close-to-surface groundwater in the near future.