Groundwater temperature evolution in the subsurface urban heat island of Cologne, Germany

Abstract

Long‐term heating of shallow urban aquifers is observed worldwide. Our measurements in the city of Cologne, Germany revealed that the groundwater temperatures found in the city centre are more than 5 K higher than the undisturbed background. To explore the role of groundwater flow for the development of subsurface urban heat islands, a numerical flow and heat transport model is set up, which describes the hydraulic conditions of Cologne and simulates the transient evolution of thermal anomalies in the urban ground. A main focus is on the influence of horizontal groundwater flow, groundwater recharge and trends in local ground warming. To examine heat transport in groundwater, a scenario consisting of a local hot spot with a length of 1 km of long‐term ground heating was set up in the centre of the city. Groundwater temperature‐depth profiles at upstream, central and downstream locations of this hot spot are inspected. The simulation results indicate that the main thermal transport mechanisms are long‐term vertical conductive heat input, horizontal advection and transverse dispersion. Groundwater recharge rates in the city are low (<100 mm a−1) and thus do not significantly contribute to heat transport into the urban aquifer. With groundwater flow, local vertical temperature profiles become very complex and are hard to interpret, if local flow conditions and heat sources are not thoroughly known. Copyright © 2014 John Wiley & Sons, Ltd.

Groundwater temperature anomalies in central Europe

“…Heat sources of AHI max For 16 out of the 45 hot spots of the class artificial, we were able to identify potential heat sources summarised into seven classes (table 1). It is important to note that other underground heat sources such as industrial cooling, geothermal applications or sewage pipes are likely [22,39,40,61], but could not be detected with the here proposed method relying on satellite imagery and local knowledge. The highest temperature anomaly is associated with a hot spring in Austria.…”

“…Changes in land use and advancing urbanisation in particular, directly influence groundwater recharge, level and temperature [34,35]. Increased surface temperatures due to artificial, sealed surfaces and underground structures raise the GWT beneath cities leading to so-called subsurface urban heat islands (SUHI) [36][37][38][39]. These SUHIs are often quantified by measuring the urban heat island intensity, which is defined as the difference between GWT in the urban area and in the rural background.…”

Meeting the demand: geothermal heat supply rates for an urban quarter in Germany

“…Also, higher ground and groundwater temperatures positively affect the heat extraction rate. The studies by Benz et al (2015), Zhu et al (2015) and Menberg et al (2013) show that groundwater temperatures beneath cities are up to 7 K higher compared to rural areas. Due to this so-called subsurface heat island effect, the exploitation rate can be raised from 13 to 33% (Rivera et al 2017).…”

Estimation of Groundwater Temperatures in Paris, France

“…In most studied cases, temperature in urban ground and groundwater is elevated, which manifests in a large-scale heat carpet underneath a city. This so-called subsurface urban heat island (SUHI) is highly case specific, often with the highest temperatures beneath city centers and strong local spatial variations [7,[22][23][24][25][26][27][28]. Ferguson and Woodbury [29] demonstrated that elevated groundwater temperatures below cities are mainly caused by heat losses from buildings.…”

“…Various studies have reported SUHIs below large cities worldwide, for example, in Moscow [24], London [37], Cologne [7,28,38], Osaka [27,39,40], and Ankara [41]. Within these cities, temperatures are measured in a limited number of boreholes or groundwater wells and are commonly several degrees higher than in surrounding rural soil and pristine groundwater bodies.…”