Impact of simplifications on numerical modelling of the shallow subsurface at city-scale and implications for shallow geothermal potential

Anthropogenic infrastructures in the shallow subsurface, such as heated basements, tunnels or geothermal systems, are known to increase ground temperatures, particularly urban areas. Numerical modelling helps inform on extent of thermal influence structures, and its potential uses. Realistic subsurface is often computationally costly requires large amounts data which not readily available, necessitating use simplifications. This work presents a case-study city centre Cardiff, UK, for high resolution compares results when three key components (namely elevation, hydraulic gradient distribution basement geometry) implemented either ‘realistically’, i.e. with data, ‘simplified’, utilising commonly accepted assumptions. Results presented at point (local) scale domain (aggregate) investigate impacts simplifications have model outputs different purposes. Comparison measured individual locations shows that accuracy temperature from numerical models largely insensitive simplification implemented, while changes geometry affect mean predicted by much 3.5 °C. At scale, temperatures within first 20 m show notable (approximately 1 °C volume-averaged 0.5 surface-averaged), average heat flux over about 0.06 W/m2 depth. These increased result beneficial conditions utilisation, producing drilling cost savings around £1700 per typical household system 9% energy potential. Simplifications (to lesser degree) hydraulics can an overestimation these therefore over-predict potential, elevation showed little impact.