The manuscript by Roberge et al. presents a new implementation in Landlab that allows the concentration of a tracer in moving sediments to be traced. This development is exciting, and I fully share the initial motivation behind this study. We do indeed need this type of model to link field measurements of provenance, for example, with the corresponding landscape evolution. 

The implementation of the model is clearly presented, and all the equations needed to understand and reproduce this implementation are provided. The examples are illustrative and give a good idea of the potential applications of this model. I particularly like the example of the ‘diffusion’ of colour bands in the landscape, which can correspond to different types of rock. This example could also illustrate another application of the model that does not seem to have been highlighted, namely the study of erosion laws. The link between the dispersion of a tracer and different erosion laws in a specific case could help to justify or calibrate these laws, which remain uncertain in landscape evolution models.

That said, I think there is a simple experiment missing that would demonstrate the validity of the model, which I proposed in my 2016 paper: placing tracers on a pixel at the top of an inclined plane with a constant slope and only diffusion (and no uplift). In this case, we have a simple analytical solution that links the evolution of the spatial standard deviation of the concentration with the diffusion coefficient and time (Einstein's formula). By comparing the theoretical predictions with the Landlab results, you could show that the model gives consistent results, and perhaps independent of the model's time step and space step. It would be useful to discuss the dependence of the new module on these two parameters.

Specific comments

Line 27 «few LEMs account for the storage, fate, and transport of other sediment properties” : which one ?

Line 29-30 I agree, this was my motivation for the grain tracers in  Cidre in the 2016 paper. 

Line 205 In the equation how Cxpi^t+1 is known ? and in the line bellow it is written that the “remaining unknown is 𝐶𝑋𝑠𝑡+1 on both sides of the equation” but I do not see it. Is there a tipo?

Line 250. Could you explain just a bit more how to obtain this equation?

Summary

This manuscript demonstrates the use cases of the newly added numerical component called ConcentrationTracker in the Landlab landscape evolution model. Specifically, the authors present two different types of tracer concentration trackers: one for hillslope processes and one for fluvial erosion and transport. Through several examples, they show how the model performs when tracking the transport of red rock particles throughout a landscape.

Special attention should be paid to the two key assumptions used in the model:

1. Over long timescales, changes in suspended sediment concentrations become negligible.
2. Homogeneous mixing occurs at every node.

General comments

The manuscript represents a substantial contribution to Earth surface modeling science through the implementation of two concentration tracker components in the widely used Landlab model. The scientific approach, based on mass conservation in both hillslope and fluvial erosion and transport contexts, is rigorous and supported by illustrative tests. I believe the concentration trackers will help bridge existing gaps related to overlooked storage or transport lag-time effects in sediment provenance studies.

The tests are clearly explained and easily reproducible, and the model components are fully integrated within Landlab, allowing users from different disciplines to take full advantage of this new tool. The manuscript is well structured and easy to read and understand.

However, I think the figures could be improved (see specific suggestions below). Overall, I believe this manuscript does an excellent job of explaining this new tool.

Line-by-line comments

• Line 72–73: I would reformulate this as: “with a single-surface detachment-limited approach similar to…”
• Line 100: For “we take a more generalized approach,” I suggest explicitly stating what you add:

1. Any user-defined tracer can be used, rather than only very specific ones.
2. Landslide deposits and other processes are also included via geomorphologic units in Landlab.
   
   This helps the reader immediately see the added value of this tool.
3. Lines 130–140: I think the article would benefit greatly from a figure explaining these concepts. Table 1 could be integrated into that figure (I made a quick sketch of what this could look like in the attachment).
4. Line 135: Use “neighbouring” instead of “neighboring.” I have not checked the entire article, but there may be other inconsistencies between American and British spelling throughout.
5. Figure 1:
   
   I think a few additions could make this figure clearer (see attachment):

1. The incoming and outgoing arrows should be horizontal and enter/exit at the centers of the box sides.
2. Add dotted-line boxes to illustrate the 2D grid.
3. Indicate the area “a.”
4. Perhaps also indicate the weathering process visually.

• Figure 2:
  
  Similarly, some minor adjustments could greatly improve readability (see attached suggestion).
• Line 243: I suggest rewriting this as:
  
  “Over large timescales, the relative change in sediment concentration for a given water column becomes negligible. This implies that our concentration trackers can only be used over large timescales.”
  
  It is important for future users to understand the timescales for which these concentration trackers are valid.
• Line 307: It would be useful to provide estimates of the timescales over which the assumption of homogeneous mixing holds, so that future users know when the concentration trackers can and cannot be applied. I suggest citing sources about homogenous mixing to back up this statement.
• Figure 3:
  
  I have several suggestions:

1. Rename the examples from “Example 1,” “2,” “3” to “High diffusivity,” “Steep hillslope,” and “High soil production ratio” to make the figure’s message clearer.
2. Draw boxes around A&D, B&E, and C&F, with the titles or example names above the boxes.
3. Panel G:

• Change the y-axis label to “Tracer concentration at x=0.”
• Use a different color scheme (e.g., #D81B60, #DC267F, #FFB000) so it is not confused with the time color scheme from Fig. 3d–f.
• If possible, indicate the sediment input at the 150 m marker.

• Figure 4:
  
  Apply similar changes as for Figure 3. You might rename the titles to “Slow settling speed” and “High settling speed.”
• Figure 5:
  
  I think panels A and B could be removed, as they do not add information essential for the reader (amusingly, the mountain shape looks a bit like a cat).
• Figure 6:
  
  Consider merging the panels into two plots:

1. Bedrock regions with drainage area. Legend: diamond = subcatchment outlet, star = main river outlet, dotted line = subcatchment boundary, color scale from white to dark blue representing drainage area, colour scale of bedrock colour.
2. Elevation model with hillshade overlay and river surface colors. Currently, the color distribution in different rivers is not visible in the north. Legend: elevation color scale and fraction of red color scale.

• Line 452: You could also add that multiple tracers can be tracked simultaneously.