the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jul 2026
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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3787', Anonymous Referee #1, 24 Aug 2026
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RC2: 'Comment on egusphere-2026-3787', Anonymous Referee #2, 07 Sep 2026
Generally, I think this is a good addition to the General Lake Model and will provide a useful tool to think about the effects of heat pump applications as we see increased shift to the carbon-neutral heating/cooling systems. Overall, I think the paper could be strengthened in places to fully considered the rationale for the development, in the context of both the research questions that have and could be answered as well as why a specific module (within GLM) is needed over and above the current modelling tools and implementations that are existing. I also think that the presentation of the results could be tidied up to better display the validation of the model and effect of the heat pump by combining some of the tables and presenting the model output more clearly.
Introduction:
- The purpose of second paragraph (Lines 27-44) of the introduction is not very clear. I would suggest splitting into two. Then the second part about why the current configuration of the inflow/outflows that are current in GLM are not suitable for heat extraction studies should be put in a separate paragraph but I think this is relevant here. Some limited review of the literature in the introduction, e.g.:
Kirillin, G., Shatwell, T., & Kasprzak, P. (2013). https://doi.org/10.1016/j.jhydrol.2013.05.023
Fink, G., Schmid, M., & Wüest, A. (2014). https://doi.org/10.1002/2014WR015509
Gaudard, A., Wüest, A., & Schmid, M. (2019). https://doi.org/10.1016/j.renene.2018.10.095
Chen, X., Zhang, G., Peng, J., Lin, X., & Liu, T. (2006). https://doi.org/10.1016/j.applthermaleng.2006.03.009
Råman Vinnå, L., Wüest, A., & Bouffard, D. (2017). https://doi.org/10.1002/2016WR019686
- The first part of this paragraph is also important to explain the rationale of this model development but needs more context. I would expect the text to refer to existing research on the impact of anthropogenic heating/cooling from both empirical and simulation studies to show the reader why we need this development.
Methods:
- A bit more background on the current inflow configuration options would help the reader understand why and how this development is occurring. For example, the reference to the “submerged inflow option” is not that clear (Line 72). See my comment in the introduction as well.
- Use consistent terminology. Sometimes you refers to a “heat flux” (e.g. L105) and sometimes to “energy transfer rate” (e.g. L90).
- Can you map the text more specifically on to Figure 2 using numbers for example. I found it difficult to see the link between the text and the Figure. This is a useful but complicated figure and I think a more detailed figure caption would be helpful. For example, is there a relevance to the colours/shapes of the boxes? Can it be made clear what is new and what is existing?
- Is the “capture” referred to here (L130 – 140) related to the extraction phase?
- Which are the “core files”? (Figure 2 caption)
- HP-ON/HP-OFF needs defining on first use and remain consistent.
- In table 3 could you include the parameter names (as per the .nml) used in he configuration of the study case.
Model evaluation:
- It’s not clear to me what is the data used as the heat pump in the HP-ON simulation? – L189: “In the absence of data of a lake with an operating heat pump, we used 19 years (1996 to 2014) of observational data from a well-characterized German reservoir (Große Dhünn).” I’m a little confused how the two set ups (HP-ON and HP-OFF are different).
- For the temperature responses (L211-226), I guess this is a different configuration with the evaporation/precipitation turned on? Is this a different model set up to that used to validate the water and energy balance?
- I would suggest removing some of the numerous results tables. Many of these are repeated in the text and do not provide significantly more information.
- Figure 3 may be better presented with the differences plotted to more clearly show these changes. In addition it would be helpful to have height of the extraction/injection height for reference, or in the caption.
- L230 is difficult to read. I suggestion: “At an elevation of 40 m above bottom (Figure 4a), the mean temperature difference is 0.02 K and at 5m above the bottom is -0.79 K, indicating the limited heat pump influence close to the surface and close to the bottom.”
- Figure 4 data are all in Celsius, but the results are reported in K in the text. I would add a second axis to allow reader to see both. Or report only the differences between HP on and off.
- Can you say anything about the seasonality or temporal variation in the responses? When are the largest differences observed between the two simulations?
- Table 8 is not necessary for the main text – move to SI or just retain the text
- L265: this paragraph topic sentence is not clear. What are “It” and “what”.
- Can you say anything about how the effects shown here are similar/not to other heat pump/heat addition studies- e.g. Fink et al 2014.
- L275 – not sure what the purpose of this paragraph is as it relates to the proposed new GLM module. Is this explaining why cannot just use the existing inflow/outflow dynamics for heat pumps? Or just generally about the configuration of inflows for the 1D GLM? If the former, perhaps it is better for the introduction – see my comment above. If the latter, I’m not sure it is that relevant and parts could be integrated with the paragraph above (L265-275).
- Again, I think some brief discussion on how this work relates to the work of Fink et al., 2014 and Gaudard et al., 2019 more specifically.
Citation: https://doi.org/10.5194/egusphere-2026-3787-RC2 -
CC1: 'Comment on egusphere-2026-3787', Damien Bouffard, 09 Sep 2026
Dear authors,
It is great to see heat extraction capabilities now included in GLM
That said, I would suggest reconsidering the comparison to Simstrat in L66–70, as most of the listed advantages of GLM do not actually hold up: Simstrat is equally open-source under a permissive license (same licence) (i), is coupled to FABM which allows coupling to a variety of water quality models (ii), and is under active development and support (iv). Point (iii) stating that GLM has a larger user community is fair.
Damien on behalf of the Simstrat team
Citation: https://doi.org/10.5194/egusphere-2026-3787-CC1
Model code and software
GLM: Thermal Use Module to integrate heat extraction into the General Lake Model Taynara Fernandes, Matthew Hipsey, Laura Soares, Bertram Boehrer, and Karsten Rinke https://doi.org/10.5281/zenodo.21035097
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The manuscript is an excellent contribution to GMD as it highlights a new module for the popular GLM-AED model that provides scientists and managers with the option to simulate heat extraction from lakes and reservoirs, which, potentially, can be a low emission heating method. I very much enjoyed reading the draft and only offer a couple of minor comments to an overall well-described study.