Tsypin and colleagues have submitted a manuscript on evolving subsurface thermal regimes, with the key novel contribution being the role of changing recharge. They conclude that advection plays a role in general, but that changing advection (e.g. more cold-season recharge) cannot overwhelm the overall pattern of groundwater warming from diffusion. The study is generally well written and interesting.

A few comments, in no particular order:

1. Many times in this manuscript the authors use 'temperature' to refer (I think) to surface air temperature; such examples can often be found when referring to climate data. But in a manuscript focused on groundwater temperature, they should always be clear about where in the earth system domain they are referring to when they mention temperature.

2. The authors miss some quite related studies from the Netherlands on the role of groundwater flux on subsurface warming. For example Bense et al. (10.1029/2019WR026913) show how changes to groundwater flow regimes can be inferred from temperature profiles (a close topic to the present manuscript although this former study did not forecast into the future), and Bense et al. 2017 (10.1002/2017GL076015) showed that recharge influences where the inflection point occurs in evolving temperature profiles (see L542, where I think a conference abstract is cited for this point). Also, Figure 9 of the present study shows deep temperature shifts when the flow regime changes. This was also demonstrated by Bense et al. 2017 (10.1002/ 2017WR021496) who showed that using a linear temperature-depth profile as an initial condition results in deep shifts in temperature with time because that initial condition doesn't have the same groundwater flow regime as the forward modeling (e.g., Taniguchi et al. 1999 solution). Side note: I think the authors could do a better job of explaining that groundwater flow regime shifts are what is driving the temperature changes at depth in some of their results (which could not happen under heat diffusion alone under the timescales considered here).

3.  Figure 1b - it is stated that this is a 3D view of the model mesh. However, neither the mesh nor the model parameters are indicated (from what I can see). This is just a geological model (or perhaps geometric model of the model domain). Much is made of the geology in this paper (rightfully so), but I think it would be useful to just indicate the parameters on this model domain (at least k and Ss - or put in a clear table). If a table, the thermal properties for each layer would be useful to list too (in the main text).

4. The recharge modeling is impressive - it might be good to be more explicit that all of these recharge realizations do not translate into equivalent model runs for groundwater temperature. I only figured that out later, but I admit to reading this in a moving car with children yelling in my ear etc. The authors calibrate this to streamflow. How does the model performance compare for just baseflow? That might be more comforting to know, since that is a better indicator of whether the recharge and discharge processes are being captured reasonably.

5.  Eq. 1 - units don't work - is Recharge in units of 1/s (like a volume flux per unit volume)?

6. This is a big numerical model domain with many elements. Was this solved with high-performance computing or anything - or just run on a 'regular' computer? My simulations with this many nodes took weeks to run, but that was 10 years ago.

7. Boundary conditions questions: a) is there any vertical hydraulic gradient on the vertical boundary conditions at the sides? b) Does the model allow for exfiltration at the surface if the water table rises too high? c) I'm surprised the surface air temp and ground surface temperature are so similar. Is there no snow here? The mean annual air temp of 9C suggests there would be.

8. Using mesh size for characteristic length for Peclet number is arbitrary (but maybe common) - how do the authors know what the threshold is for this Pe formulation for which advection matters? Later on they present Pe values and use these to highlight the role of advection, but this is not a formulation for which Pe =1 implies that advection = conduction (like say the Bredehoeft and Papadopulos 1965 formulation does - if I remember right). So what is the threshold for when advection matters? Maybe that is hard to say.

9. The recharge change runs in the groundwater model are interesting (see Table 1), but I'm still not clear where some of these came from (esp. the discrete synthetic scenario). I guess at least one is more of a sensitivity run rather than an expected scenario? Also, it would be helpful if Table 1 were more precise about timelines rather than "present" and "late century". What years are the before/after runs here for?

10. Model calibration - consider presenting the RMSE normalized to head range. I think that would do a better job of showing the fit. 6.6 m sounds high, but the head is highly variable, so it is not a bad fit at all.

11.  Where do groundwater temp measurements come from for calibration? Case borehole temp profiles? Pumped groundwater with temp recorded? Open boreholes? Large diameter? Are convection or seasonal bias concerns?

12.  Figure 7 - it would be interesting to also see changes in discharge presented somehow. The authors talk about changes in recharge (input) and head (storage), but what about output (relevant for some of the reasons groundwater temperature is relevant)

13.  L386 "Profile A, located on a glacial plateau, has a shallow seasonal envelope, entirely above the water table, suggesting no advective transfer of the seasonal thermal signal into the saturated zone" - I don't think you have to be saturated for heat advection to matter. It depends on the water flux, not the saturation per se. if you have recharge through the vadose zone, advection can still theoretically matter.

Minor comments

L60-65 - I think the controversy over the role of advection is overstated here. In the Benz et al. study, the authors used a parsimonious approach in a global study and justified this using 2D numerical models of advection and conduction. They basically found that for typical hydrogeological systems and recharge rates, heat advection is not a primary driver of groundwater temperature change. However, they note that is not the case for some basins. Other studies cited here are in steeper topography or with much higher recharge, and so it makes sense that they found that advection mattered.

Figure 11 a - label missing for scale bar?

Conclusions _ 'depth limit of advection overprint' sounds a bit jargony for the conclusions

In general, these comments are all easy to address I think, and I look forward to seeing this article published.

I enjoyed reading the manuscript “Influence of groundwater recharge projections on climate-driven subsurface warming: insights from numerical modeling.” The manuscript presents a clear objective, employs a sound methodology to achieve it, and reports results that are well presented and supported by the discussion and conclusions. Most of my comments focus on improving clarity in certain sections.

• Line 167: Could you please explain why these seven GCMs were selected? Since many GCMs are available, I am curious whether these models have specific characteristics that make them particularly suitable for this study. Clarifying this would also address the question of why seven—why not five, ten, or the full ensemble?
• For completeness, could you report the area of the model domain? The mesh spans 170 × 150 km, but (as I understand it) the numerical model only simulates part of that rectangle. Are some cells within the bounding rectangle not active? A brief clarification would help.
• It would be helpful to include a short paragraph or a few sentences describing the computational cost of the simulations and the computing resources used (e.g., HPC system, number of cores, total runtime).
• Line 219: Why were thermal and hydraulic parameters calibrated for only two layers? A short justification would improve clarity.
• Line 227: What proportion (in percentage) of the model’s lateral boundaries coincide with rivers?
• Some darker points appear in Figure 5a. Do these points represent a specific subset of observations, or are they a rendering artifact? Additionally, could you add a measure of bias between modeled and observed heads and temperatures?
• In Figure 6 why were these three wells chosen? Would it not be more informative to select wells from three distinct regions with different expected drawdown responses (Fig. 7a)?
• Line 386: The text states that Profile A has a shallow seasonal envelope entirely above the water table, implying no advective transfer of the seasonal signal into the saturated zone. Isn’t this also the case for Profile D?
• Section 6.1 is clearly written, and Figure 13 is well designed. However, I’m a bit confused: GD3 is described as an area where “the Rupelian is locally eroded, allowing direct hydraulic connection between Quaternary aquifers and deeper formations”. In Figure 13, the region labeled GD3 appears to still contain Rupelian clay (gray unit), which does not reflect such a direct connection. Could you clarify this?
• Line 517: The sentence “The groundwater system is … reversible” may cause confusion. It is clear that your model does not simulate mechanical deformation of the subsurface (and this is okay), yet changes in groundwater storage can lead to changes in hydraulic properties, which may be irreversible depending on geomechanical conditions (Galloway & Burbey, 2011, not my paper, just a reference). I recommend clarifying the assumptions under which the groundwater system is considered reversible in your simulations or rephrasing the sentence to avoid misunderstanding.

Reference:

Galloway, D. L., & Burbey, T. J. (2011). Regional land subsidence accompanying groundwater extraction. Hydrogeology Journal, 19(8), 1459–1486.

The study by Tsypin et al. aims to quantify future groundwater warming under various climate change scenarios. Its key novelty lies in the explicit consideration of heat advection by groundwater flow, an aspect often overlooked in previous research.

The manuscript is well written and presents a compelling approach. Several comments should be addressed to further strengthen its impact.

General:

• It would be beneficial for the reader to make titles of different chapters more detailed.
• Figures and legends need more clarity.

Specific comments:

L57 Regarding the impact of GW dynamic on subsurface temperatures, you may also check Klepikova et al., ERL, 2025.

L67 As for the impact of advection on temperature profiles, a review by Kurylyk, 2018 should be cited.

L104 “representative of EU geology” sounds weird.

Fig.1 please provide scale reference on all subfigures.

Section 3: In order to improve the clarity I believe that general conceptual models should be described first.

L167-168 The main difference in between these scenarios should be already mentioned here.

L254 D of fluid?

Figure 3. Expand and detail the legend.

Figure 4. The clarity of this figure could be improved as well.

L343 Could you tell more about the T data (depth/profile or point?/range/precision)?

L370 Please explain “teleconnections”.

Figure 13. It is not really clear what do you want to highlight by horizontal arrows.

L535 This is confusing as the depth of penetration should be regulated by the GW flow rate and thermal properties of rocks.

L541-544 Rephrase this sentence to make it clearer (i.e. what is meant by “this response”?)

L544 differentiation of what?

L645 for which depth?