This manuscript is about the dissolution of calcite in porous media and the resulting self-organization of preferential flow paths. The results are based on numerical simulations and are analyzed with the help of thermodynamic variables.

Although the basis of the model seems to be similar to what has been assumed in models of karst evolution for decades, I found this study  very interesting. The are numerous points to think about. On the other hand, I find it a bit too difficult. Each step is not really difficult  in itself, but in sum it is very challenging not to get lost. When reading it, I had to go back quite often to find out whether something is indeed not clear or whether I just missed it. So I am a bit afraid that scientists who did not have to hold on for writing a review will likely give up.

So let me just make some suggestions, hoping that they may be helpful for clarifying some aspects.

Quite in the beginning, I wondered about the porosity and permeability (Eqs. 7 and 8). If I got it correctly, you assume constant initial permeability and introduce spatial variability be assuming a variable factor in the the Kozeny-Carman relation. This looks unconventional.  The conventional approach would be the same factor, but variable initial porosity. I guess that you used this approach in order to define the pore volume time. However, I think that it also has an effect on how the development of the permeability. In your approach, a given change in porosity (from the constant initial value) introduces the same relative change in permeability. In the more conventional approach, the relative change would be smaller for cells that already have a high permeability. So I would guess that the formation of preferential flow paths in stronger in your approach than in the conventional version. This makes we wonder whether it has an effect on the results. Maybe you can clarify this aspect.

May main concern is, however, that the entropies are somewhat abstract properties. It all makes sense, but I am still left with the feeling  that I do not understand fully what happens in the system. One point is why H+ is associated to the preferential flow paths more closely than H2CO3.

In this context, I would find it very helpful to separate the propagation of the reaction front from the changes in the flow pattern. So considering the dynamics of the reaction without taking into account the effect on the permeability. Then it would be easier to understand what exactly the effect of the changes in permeability is. Maybe the dynamics of the reaction is much faster and it is already clear, but I am not sure. Perhaps it would also be helpful to complement the 2D pictures of the concentrations in Fig. 3 by longitudinal profiles (integrated along  the y-axis) in order to see whether the species are just concentrated more or less along the preferential flow paths or also follow strongly  different distributions along the x-axis.

In summary: really interesting stuff, nothing wrong as far as I can see, but the paper would benefit from a more basic and clearer explanation of the model's behavior.

Best regards,

Stefan Hergarten

Please see the attach.