Thank you to the authors for this wonderful effort! I really enjoyed reading this paper. I think that there is a lot that can be done here toward a better understanding of Martian hydrology, and I am glad to see this initial work moving things in the right direction. I deeply appreciate the thoughtful and beautiful figures produced by the authors and their exploration of how the distribution of water on Mars differs with different GELs. I have several questions and comments , mostly centering around the implementation of the hydrology model, pre-processing of the topography, and potential for validation of the results. I have attached my detailed comments in a PDF. I look forward to the authors' response and to seeing the final iteration of this paper. 

Kerry Callaghan

This manuscript presents the first global, kilometer-scale, quantitative assessment of surface water distribution on early Mars. This assessment was made using a newly developed hydrological equilibrium model, which was constrained by high-resolution DEM data. This approach is both original and technically impressive, as well as being scientifically important. Notably, the systematic exploration of the total water inventory and the subsequent shift from a nearly uniform distribution of water to a dominant northern ocean is a pivotal finding that will resonate widely within the communities of Mars climate, hydrology, and geomorphology.

The model framework is well designed, computationally efficient and clearly described. I particularly value the authors’ intention to couple this hydrological model with a GCM in future work, as this makes the present study a valuable foundation for the next generation of coupled climate–hydrology simulations.

Overall, I consider this manuscript to be strong and publishable following revision. The comments below aim to clarify the physical interpretation of the results and increase the robustness and impact of the conclusions, rather than questioning the validity of the modelling framework itself.

Major comments

1. The assumption of spatially uniform precipitation and the interpretation of equilibrium states.

In the current version, it is assumed that precipitation and evaporation are spatially uniform, and the system is iterated until a steady state is reached. Under these conditions, the model robustly converges to a unique equilibrium water distribution for a given GEL, regardless of the initial conditions. This is an important and interesting result.

However, assuming homogeneous precipitation is a very strong symmetry constraint and likely plays a central role in the uniqueness and stability of the equilibrium solutions. Numerous GCM studies of early Mars have shown that rainfall (or snowfall) patterns depend strongly on obliquity, topography and atmospheric composition, and are generally far from uniform.

I therefore suggest clarifying the robustness of the conclusions with respect to this assumption and partially testing it if possible.

For example:

• Could the authors include one or two idealized, non-uniform precipitation patterns (e.g. latitudinally varying precipitation or enhanced precipitation in mid-to-high latitudes) to test whether the qualitative, GEL-dependent transitions remain valid?
• Alternatively, prescribing a fixed precipitation pattern derived from existing early Mars GCM studies (even in a simplified, time-averaged form) would be informative.

Such sensitivity experiments would not require full GCM coupling, but would greatly strengthen the interpretation of the results by clarifying whether the derived equilibrium represent:

-  purely topography-controlled end-member states, or

-  dynamically robust equilibrium under more realistic climatic forcings.

2. The climatic context of the inferred equilibrium.

I suggest explicitly stating in the manuscript that the present simulations should be interpreted as an idealized reference case, in which precipitation and evaporation are spatially uniform.

In this framework, the derived steady states represent water distributions that are topographically reachable under globally distributed water input, rather than predictions for a specific early-Mars climate scenario.

It would be very helpful to briefly discuss, at a conceptual level, how the results might differ under other plausible climatic regimes for early Mars, for example:

• high-obliquity climates with enhanced low to mid- to latitude precipitation or snowfall,
• climates dominated by ice-edge melting or spatially localized water input, or
• episodic or hemispherically asymmetric precipitation patterns suggested by previous GCM studies.

Such a clarification would help readers avoid over-interpreting the equilibrium distributions as direct reconstructions of early-Mars climate and would naturally motivate future coupling of this hydrological model with a GCM.

Minor comments

1. Notation of depression IDs

Both italic and non-italic forms of the “ID” appear throughout the manuscript. Please clarify whether this distinction has a specific meaning or is a typographical inconsistency.

2. Line ~218: expression for excess volume

The expression (V^excess=V^excess-V^avail) reflects a programming operation rather than a mathematical definition. For clarity, I suggest rephrasing this line in a mathematically explicit form.

3. Line ~228: order of water transfer in the hierarchy

The algorithm prioritizes transferring excess water to sibling or downstream depressions before considering higher-level (parent) depressions. Could the authors please explain why this order is preferred, either physically or algorithmically, rather than transferring excess volume first to deeper hierarchical levels?

4. Relation between crater age and storage capacity (Line ~380)

The discussion correctly notes that younger terrains tend to have fewer craters and thus lower storage capacity, whereas older terrains exhibit greater water retention. An additional latitude–longitude cross-section or map explicitly demonstrating this relationship (e.g. crater density versus storage capacity) would clarify and enhance the visual appeal of this argument.

5. Timescale to reach equilibrium

Although the paper focuses on steady states, briefly discussing the timescales required to reach equilibrium under typical evaporation rates would help to connect the results to geological constraints on lake lifetimes and valley network formation.

6. Role of subsurface flow

The absence of groundwater and infiltration is clearly acknowledged. It would be helpful to briefly discuss the expected direction of the bias introduced by this assumption, for example over- or underestimation of surface water retention in specific regions.

This study marks a significant advance in our ability to quantify potential surface water distributions on early Mars with high spatial resolution. The modelling framework is robust and original, and the findings are highly valuable.

I recommend revision prior to acceptance, mainly to:

- better contextualize the results climatically.

- assess (or more clearly define) the role of the homogeneous precipitation assumption.

Addressing these points will significantly increase the clarity, robustness and long-term impact of the manuscript.