The manuscript titled “Runoff Evaluation in an Earth System Land Model for Permafrost Regions” proposes a new framework using ATS simulations to optimize runoff coefficients, offering a more physically based alternative to traditional parameterizations in Earth system land models. Furthermore, the study explores the potential transferability of the optimized runoff coefficients across different Arctic watersheds. This work presents methodological innovations that are valuable for improving runoff simulation in permafrost-affected regions and holds implications for enhancing water resource management in high-latitude environments.

The manuscript is generally well-written, and contributes meaningfully to the field. However, there are several issues that should be addressed to improve the clarity and precision of the manuscript. I therefore recommend the manuscript a minor revision, by considering some comments and questions posed below.

1. While the study proposes a broadly applicable framework, the current analysis is limited to only two watersheds in Alaska: the Sagavanirktok (Sag) River Basin and the Teller watershed. Given this limited spatial extent, the current title may overstate the geographic generalizability. A more precise title such as “Runoff evaluation of an Earth System land model in the permafrost region of Alaska” would better reflect the study’s current scope.

1. Please correct the unit formatting on Line 316: “km2” should be changed to “km²” (with superscript).

1. Figure 6 attempts to illustrate the effect of changes in soil physical properties and snow thermal conductivity on runoff simulations within the ELM model. However, the current visualization makes it difficult for readers to extract meaningful comparisons. Consider redesigning the figure to improve visual clarity. For example, using grouped bars or difference plots to highlight contrasts between scenarios.

1. In Figure 7a, the legend partially obstructs key elements of the plot. Since subplots 7a and 7b share the same legend, a common external legend (e.g., placed to the right of the panel or beneath both subplots) would declutter the figures and enhance readability.

This study evaluates hydrological runoff in two permafrost-affected Arctic watersheds, Teller and Sag River, using two different models: the high-resolution, physics-based 3D permafrost hydrological model ATS and the coarse-resolution 1D Earth system model E3SM-ELM.

The authors simulate 22 representative watersheds in the Sag River basin with the ATS model. The runoff calculated by ATS is then used to calibrate linear parameters of the ELM model. These parameters are calibrated both annually and seasonally. The results show that by weighting four discharge components that contribute to total runoff in ELM, the modeled runoff aligns more closely with ATS results. The main conclusion is that the linear parameters (c1, c2, c3, c4) derived for Sag River can be applied to Teller, suggesting potential scalability to other sites.

The transferability of these linear parameters across Arctic watersheds depends on which of the four discharge components dominate at different times of the year. Seasonal variability plays a key role: snow is a major factor during winter and spring, while rainfall becomes more important in summer. At Teller, a deeper active layer and talik development may enhance year-round subsurface hydrology. This suggests that parameter transferability may work reasonably well for colder Arctic regions but may be less applicable to sub-Arctic environments.

Instead of only focusing on parameter transferability, it would be valuable to compare this approach with the hillslope model approximation proposed by the CLM group (described in Swenson et al., 2019 and latest results presented by David Lawrence at AGU 2024). How similar or different is the method presented here compared to the CLM approach? This discussion would strengthen the study.



Overall, this is an interesting and relevant study addressing the critical issue of scaling hydrological processes from local high-resolution models to Earth system models. The calibration and parameter transferability approach is promising but needs a stronger discussion of its limitations, particularly in regions with different seasonal dynamics. Adding detailed model setup descriptions, more figures illustrating profiles and seasonal variations, and a clearer comparison with alternative modeling approaches (e.g., CLM hillslope parameterization) will significantly strengthen the paper.

Swenson, S. C., Clark, M., Fan, Y., Lawrence, D. M., & Perket, J.(2019). Representing intra-hillslope lateral subsurface flow in the community land model. Journal of Advances in Modeling Earth Systems, 11, 4044–4065. https://doi.org/10.1029/2019MS001833

H13R-03 Improving terrestrial hydrologic process representation in Earth System Models: Accounting for slope, aspect, and lateral water transfer through representative hillslopes, AGU 2024

Schädel, C., Rogers, B.M., Lawrence, D.M. et al. Earth system models must include permafrost carbon processes. Nat. Clim. Chang. 14, 114–116 (2024). https://doi.org/10.1038/s41558-023-01909-9

Editorial & Content Suggestions

1. L18: Replace physics-rich with physics-based or high-fidelity physics.
2. L50: Use Schaedel et al., 2024 as a better reference.
3. Introduction:

• Conclude by emphasizing the scaling challenge: ATS operates at meter-scale resolution, whereas ELM works at ~150 km scale.
• Discuss how this study attempts to bridge this scale gap, either through parameterization improvement in global-scale models or by showing how local-scale models inform global simulations.
• The latter angle may be a safer framing.

Model Description Section

• Provide more details on the model setups for ATS and ELM:
• Initial and boundary conditions
• Number of soil layers
• Differences in thermal, hydrological, and surface/subsurface properties
• Include a figure comparing these configurations.

Ground Conditions

• The WT was initialized at 8.8 m. Show the initial profiles of:
• Liquid pressure vs. depth
• Liquid and ice saturation vs. depth
• Temperature vs. depth
• Add figures for seasonal profiles of ground temperature and liquid/ice saturation for both Sag River and Teller.

Meteorological Data

• Clarify the type of meteorological forcing used (e.g., daily vs. smoothed data).

Validation

• Were the models validated using measured temperature, runoff, streamflow, or discharge data? This should be clearly stated.

Figure-Specific Comments

• Figure 1:
• L174: refers to Fig. 1b
• L186: refers to Fig. 1a
• Mark the locations on the Teller map where streamflows were measured.
• ATS Model: Clarify whether it was a 2D or full 3D hillslope simulation. If 3D, specify whether runoff refers to streamflow over the entire hillslope.
• Figure 2b: Indicate whether discharges are distributed over the hillslope or limited to streamflow.
• Figure 3:
• Add titles for each subplot column (e.g., Base Case, Adjusted, Seasonally Adjusted).
• Include “Teller” and “Sag” in the caption.
• Figure 4:
• Avoid plotting ATS discharge as dots; use a continuous line.
• Consider plotting the difference (ATS – ELM) to highlight which seasons match better or worse.
• Use mL instead of 10^-5 units for easier interpretation.

Discussion Suggestions

• Explicitly use notation such as y^{ATS}_{runoff} and y^{ELM}_{runoff} for clarity.
• Discuss why RMSE, MAE, and NSE metrics were chosen and what different insights they provide.
• L269: Does the ground refreeze at the Teller site? If not, this may lead to increased water storage and higher subsurface discharge during summer.
• Consider mentioning that Teller’s topography, shrub cover, and snow-shrub interception promote warmer soils and influence hydrology.
• L270: Clarify whether the symbol used is a diamond or a triangle.
• Lines 290–295: Include a discussion comparing this study’s approach with the CLM group’s hillslope paper.