Spatio-temporal distribution of subsurface urban heat islands – Insights from shallow groundwater temperature monitoring in Vienna

In numerous cities worldwide, a rise in surface temperatures had been observed, contributing to the so-called "urban heat island effect". This effect leads to extended and hotter periods of warm weather within urban areas not only above but also below ground. The heat in the subsurface can be used for shallow geothermal energy, but it requires knowledge of spatial and temporal variations in groundwater temperature for efficient and environmentally friendly utilization of groundwater for heating and cooling. In the course of the 'Heat below the City' project, we have compiled spatial high-resolution data and developed groundwater temperature maps for the city of Vienna targeting the coldest and warmest annual conditions. Borehole temperature profiles were recorded in October 2021 and April 2022. This enabled the identification of distinct urban heat islands. Additionally, available long-term data (2001-2020) was used to conduct annual temperature trend analyses and extreme value assessments to evaluate temperature changes over time. In Vienna, an average annual temperature increase, considering all significant trends, of 0.9 ± 0.1 K/decade was observed for air, soil and shallow groundwater between 2001 and 2021. However, the increase is non-linear and, over the last decade, the change has accelerated with an increase of 1.4 ±0.2 K/decade (only significant trends taken into account). The current annual mean temperature is 14.1 °C (2021/ 2022) with individual warmer urban heat islands and locally heated locations of up to 30.6°C. Trends in extreme temperatures (represented by the lower/upper 10^th percentile air, soil and groundwater temperature in quantile regression) generally show the strongest increase in the lower 10^th percentile temperatures for all air and soil temperatures. But this varies site-specifically in shallow groundwater, where urban infrastructure and the interaction between surface and groundwater, in addition to climate change, influence groundwater warming. Potentially, those urban heat islands with increasing trends in groundwater temperatures have great potential for heat utilization, but should not be used for extraction of cold. These findings emphasize the importance of spatial and temporal high-resolution data and highlight the necessity for site-specific aquifer characterization for a sustainable use of shallow geothermal energy for heating and cooling.