We appreciate this reviewer's insightful comments on our manuscript "A Simulation Approach to Characterizing Sub-Glacial Hydrology". These will be very helpful in improving the final product.

In response to the Major Issues raised by the reviewer, we submit the following responses:

• Regarding the range of material properties used in the study: 
  • As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
  • The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
  • We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
• Regarding the range of bed roughness explored:
  • Peters 2005 presented a calculation for theoretical scattering loss due to sub-glacial roughness. In their calculations, small scale RMS roughness up to  sigma_bed / lambda = 0.5 was considered, based on observations of RMS roughness calculated from IPR data over Ice Stream C. As the reviewer points out, Peters calculated a scattering loss of 21 dB if sigma_bed / lambda = 0.5.
  • However, the Peters paper may have ignored the length scales over which roughness is measured, which is an important consideration. The theoretical scattering loss calculated in Peters assumes correlation length is very small compared to the first Fresnel zone (~100m in our case). However, the range of actual IPR derived roughness observed in Peters beneath Ice Stream C were compiled over a 1km distance, obviously violating this assumption. Peters appears to assume the range of RMS roughness seen with this 1km correlation length translates directly to their calculated theoretical roughness / scattering curve. This logic will result in incorrect understanding of the relationship between scattering and roughness.
  • Our method attempts to improve upon this by considering roughness at a meaningful length scale for scattering (correlation length <<100m). We explored a range of roughness up to sigma_bed / lambda = 0.2, with correlation length as small as possible relative to the 1^st Fresnel zone. Actual roughness at this length scale is not directly observable by conventional IPR, and therefore the true range of realistic values is unknown. Previous studies such as Peters 2005 or Bingham and Siegert 2009 measure roughness over longer distances, which may be relevant for modeling applications but is insufficient for our purposes. Our range may be smaller than the theoretical range explored in Peters 2005, however we attempt to justify our choice through direct observations of glacier bed roughness matched to our smallest possible correlation length.
  • Further, we note that in our real-world IPR simulations, roughness on long length scales (>~10²m) is captured explicitly through our simulation methods. We build a digital elevation model (DEM) derived from IPR ice geometry observations with approximately 5-8m spacing.
  • An entire study could be dedicated to sub-glacial roughness and the impact on radar returns. Ideally this will incorporate a range of RMS roughness and correlation lengths on radar scattering. However, such a study is well beyond the scope of this paper.
  • We acknowledge that more work should be performed in this realm, which we are willing state explicitly in the manuscript. We will alter language in the introduction, conclusion, and abstract to be clear about scope of the investigation, so it is clear why a more detailed roughness analysis is left to future work.  
• Regarding the omission of simulated specularity content:
  • The reviewer is correct in their observation that a focused simulation product could, in theory, be used to generate and analyze specularity in the same manner as we have demonstrated for reflectivity. Unfortunately, we acknowledge this as a current limitation of the methodology as presented. Schroeder 2015 suggested aperture lengths of up to 2km to generate specularity content for IPR data from Thwaites Glacier. This is reasonable when focusing real IPR data products, however it is unrealistic when focusing our simulated radargrams.
  • As we discuss in the Methodology section, the simulator is bound by a simulation radius “R”, which limits the domain of facets from which to consider transmitted and reflected waves. Increasing R exponentially raises computational cost. Conversely, R must be greater than the aperture length to generate a valid focused product. Even with access to high performance computing resources, simulations with R>2km are unrealistic.
  • Due to the above, analysis was necessarily limited to reflectivity. The study seeks to remove some of the ambiguity the reviewer expresses around interpreting this metric. We further limit our interpretation of the reflectivity signal to the transect in question, as a proof of concept for the simulation method.  Any attempt to extrapolate our findings to catchment-wide conclusions based on a single transect would be erroneous at this time. Although well outside the scope of this study, it is absolutely an area of future interest.
  • We suggest a revision to the discussion section, addressing the above concern and why the study is currently confined to reflectivity alone.
• Regarding limiting the study's scope to Röthlisberger channels and flat canals, coupled with conclusion language such as “We ultimately conclude the bright reflector from our IPR flight line is more likely a broad area of wide distributed water, such as a series of flat canals or sub-glacial lake” 
  • We concur with the reviewer’s assessment on this point. Similar to this reviewer’s feedback regarding material properties, we have not explored and eliminated all possible geometries. We can be more precise with language regarding what hypotheses our work has excluded, what hypotheses may be compatible with the data, and what remains unexplored.
  • As such, our language concluding the structure of the water may be too strongly worded. We will soften this language as the reviewer suggests, stating that the bright echo a) is not consistent with a Röthlisberger channel, b) is consistent with a broad specular water body as we have suggested, and c) other configurations (such as Nye channels, etc) and bed materials (tills, frozen bedrock, clays) remain unexplored. Therefore, it is possible other combinations also match the reflectivity profile in THW2/UBH0c/X243a.

In response to the Minor Issues raised, we submit the following responses:

• Abstract Line 1: Depending on its configuration water can either enhance or reduce sliding and/or retreat.  
  •  Thank you. We agree with this suggested change.
• Abstract Line 3: Given the recent paper by Schlegel et al. ( https://doi.org/10.1017/aog.2023.2) you may want to consider the use of IPR here.
  • We thank the reviewer for bringing this to our attention. We will change our nomenclature from IPR to airborne RES, consistent with Schlegel et al.

• Table 1: 1.71 MW seems like an extremely high value.  It’s unusual to report a post-processing number for transmit power and presenting it in this way could confuse readers significantly.
  • The reviewer’s concern on this point is reasonable. We have coherently pre-summed the radar power for simulation expediency. Therefore, the power in our simulations is significantly higher than MARFA’s 8kW native power. We want to be clear this is NOT the same as post-processing, which may sum power incoherently. Summation happens during data processing after an actual survey. 
  • For additional clarity, we suggest putting native MARFA values in parentheses next to the simulator parameters. In the table description, we can state that native instrument values are in parentheses where they deviate from the simulation parameters, and the reason for this difference is because the simulator approximates pre-summed power and PRF for computational efficiency.

We would like to thank this reviewer for their comments on our manuscript "A Simulation Approach to Sub-Glacial Hydrology". Their insight will be very helpful in improving the final product. 

In response to the reviewer's "Major Issues", we submit the following responses:

• Regarding the range of permittivity values for the bed material used in the study:
  • As the reviewer correctly points out, we did not explore many possible combinations of bed materials. This was intentional, as the scope of this work was to introduce the conceptual framework for using such a simulation technique in the study of sub-glacial hydrology, then demonstrate its potential usefulness in constraining hypotheses for a single feature seen in a recent IPR survey.
  • The emphasis of the work and most important conclusions involve the water geometries considered in the study. Altering the substrate dielectric constant will merely scale power. We acknowledge that definitive diagnosis of the subglacial environment beneath Thwaites may require additional assessment with a broader combination of materials. The nature of this technique may never eliminate all possible combinations of materials and geometry. However, it can be a valuable tool for narrowing the range of likely sub-glacial configurations.
  • We propose revisions to acknowledge the range of unexplored material properties more explicitly. This will include stating that we have not completely excluded all other possible hypotheses. Instead, we have found at least one possible match between the observed reflectivity and a hypothetical water structure.
• Regarding the broader applicability of the model beyond the Thwaites water system:
  • We appreciate and agree with the reviewer on this point. The choice of the Thwaites water system was a convenient example of how this method can be used, but of course the method could be applied more broadly to examine other regions and hypotheses. We propose additional language in the abstract, introduction and conclusions discussing the broader applicability of the technique, and also clarify that our Thwaites use case is just one example.

Regarding the minor comments raised by this reviewer:

• Line 25: Better to add more recent citations.
  • We agree to add updated citations for the relationship between hydrology, shear stress, and ice rheology as suggested.
• Line 32: Missing citations
  • We agree to add citations establishing IPR as a technique in the study of hydrology, as the reviewer suggests.
• In the introduction section where the objectives are outlined, it is important to include the potential applications of the model to other catchments, thereby emphasising its versatility beyond the specific case of Thwaites Glacier.
  • This comment appears similar to the reviewer’s second concern under “Major Issues”. We believe our proposed resolution addresses both points.
• On providing a dedicated "Study Area" section:
  • We appreciate this comment and agree to create a dedicated “Study Area” section to describe the target region of Thwaites glacier.
• On separating "Results" and "Discussion" to improve clarity:
  • We will address the reviewer’s concern by separating the Results and Discussion for clarity.
• On adding table and figure borders:
  • We acknowledge that authors and readers have differing stylistic preferences. However, we believe our figures are well aligned with the Cryosphere style guidance and prefer to keep them as is.
• With respect to adding a discussion of future research directions in the Conclusions:
  • We agree to add language as suggested by the reviewer.