<p>In recent years, the climate change caused by increasing greenhouse gas emissions has become a worldwide challenge leading to environmental problems. As fossil energy consumption continues to rise annually, the importance of renewable energy has grown significantly. Shallow geothermal systems are an environmentally friendly energy source that is local and renewable and emits few greenhouse gases. Nevertheless, in view of their implementation close to the surface, they make them more affected by atmospheric variations and soil moisture movement. The primary objective of this study is to assess the impact of seasonal hydrothermal fluctuations on the thermal performance of shallow horizontal air–ground heat exchangers applied for heating and cooling indoor spaces. The research highlights the importance of considering the atmosphere–soil interaction in order to accurately predict ground temperature. Furthermore, the developed numerical framework involves the combined effect of the thermal and hydraulic properties of partially saturated soils. The performance of horizontal air–ground heat exchangers is shown to be strongly affected by heat exchanger radius and fluid flow velocity in this study. Moreover, the results reveal that neglecting the ground–atmosphere interaction leads to an overestimation of the fluid outlet temperature in the winter and an underestimate in the summer. The maximum temperature difference is around 2&#xa0;°C in the summer and 1.4&#xa0;°C in winter. These variations are function of installation depth. Additionally, higher fluid velocity reduces the heat exchange time between the working fluid and the unsaturated soil.</p>