The subsurface urban heat island of Basel City: more than a decade of spatiotemporal high-resolution monitoring and modelling

Managing urban groundwater resources is crucial not only for water quantity questions, but also to safeguard water quality. While evaluating hydrochemical parameters is part of common monitoring strategies, given the ongoing trend of geothermal energy usage, the evaluation of thermal regimes has gained increasing interest. This study presents an analysis of groundwater temperature (GWT) datasets from conventional monitoring networks and from seven high-resolution multi-level monitoring systems in Basel City, Switzerland. With GWT hotspots of up to 20°C, the monitoring data clearly showed the transient development of a subsurface urban heat island (SUHI). An existing suite of three-dimensional thermal hydraulic (3D-TH) models for four distinct groundwater bodies (GWB) was updated to enable SUHI analysis. For this, GWT, groundwater heads, river temperatures, river stages and groundwater user data were merged and introduced into the 3D-TH, enabling a 6-year calibration plus a 5-year validation period. The updated models provide insights into the long-term groundwater and heat flux dynamics across Basel's GWB. The findings underscore the flexibility of monitoring and modelling in evaluating urban groundwater systems, promoting a sustainable use and management of shallow geothermal energy and formulating SUHI mitigation strategies.This article is part of the theme issue 'Urban heat spreading above and below ground'.