Integrating heat extraction into the General Lake Model (GLM v3.9.106)

Abstract. The growing global demand for heating and cooling challenges decarbonization efforts, which requires a shift to renewable solutions. Inland waters, especially lakes and reservoirs, account for a significant portion of global land heat uptake, offering potential to help meet these demands. When used for heating, the resulting cooled lake could benefit from buffering climate-induced warming. However, existing lake models lack dedicated capabilities to simulate thermal energy extraction limiting the assessment of its environmental impacts on aquatic ecosystems. Here, we present a new thermal use module integrated into the General Lake Model (GLM) framework. The module features: (1) depth-flexible water extraction and injection at static or dynamic elevations; (2) user-defined operational parameters for flow rates, temperature spread, and heat flux; and (3) coupling with the Aquatic Ecodynamics (AED) library, enabling simulations of biogeochemical responses to thermal alterations. Heat transfer calculations are integrated with GLM's existing hydrodynamic and thermodynamic algorithms and maintain energy and mass conservation. We tested the module using a heat-extraction scenario (-2 K) for a drinking water reservoir in Germany with 19 years of monitoring data. The open-source module provides a new tool for both energy and lake management by enabling the evaluation of the thermal and ecological impacts of heat extraction.