Patterns of Subsurface Urban Heat Island Intensity (SubSUHII)

The strength of subsurface urban heat islands (SubSUHII) hasn't been studied much, even though this could be a key indicator of how urban infrastructure and human activities affect groundwater temperatures. Moreover, understanding SubSUHII is crucial for assessing the impact of cities on underground temperature patterns and what this means for groundwater resources.

This study tackles two main issues. First, we analyse how to quantify SubSUHII reliably. Similar to the intensity of the atmospheric urban heat island, SubSUHII is described as the difference in temperature between the average annual groundwater in urban wells and in a rural setting. Particular attention is given to how urban boundaries should be defined, and how measurement depths and rural background groundwater temperatures (GWT) should be conceptually addressed in the subsurface, highlighting both the similarities and fundamental differences with atmospheric urban heat islands.

Secondly, we explore how SubSUHII spatial patterns can be compared across multiple cities using an updated global groundwater temperature dataset by Benz et al. (2024) in conjunction with several observed groundwater temperature datasets. While our data covers more than 40 cities in total, this pilot study particularly focuses on Karlsruhe (Germany), Turin (Italy) and Cardiff (UK) where data availability is highest. These cities cover a wide range of climatic, geological and urban contexts, allowing for a comparative analysis of SubSUHII under different conditions. At the urban level, we analyse variations in SubSUHII as a function of local climate zones (LCZ) and groundwater measurement depth. Finally, we investigate the role of urban morphology and infrastructure, including building types and population density, to identify recurrent patterns of SubSUHII across cities and countries.

By providing a consistent framework for calculating SubSUHII and identification of local and global patterns, this study helps to provide a more comprehensive understanding of urban thermal impacts in the subsurface.