Summary

The manuscript evaluates seven runoff schemes in the Noah-MP land surface model for estimating global river discharge, comparing them to ERA5-Land data and streamflow observations. Results show varying accuracy, with TOPMODEL-based schemes underestimating runoff in some regions, while others like Schaake and BATS performed better. VIC overestimated runoff globally. The study indicated that despite good performance, biases in high-flow extremes highlight the need for further model improvements. The study emphasizes improving hydrological models for accurate water resource management and climate adaptation.

General Comments

The study addresses a quite interesting topic. The manuscript is well organized and neatly written with the appropriate scientific content. However, I have some suggestions and questions as follow:

Major Comments:

• The paper does not adequately address how the insights from these evaluations could be used to advance global hydrological modeling, particularly in the context of discharge simulation. While it provides a thorough assessment of the performance of seven runoff schemes within the Noah-MP Land Surface Model, its contribution to improving hydrological modeling remains unclear.
• Line 358-362: It is noted that the lags between peak runoff and peak discharge in large river basins, such as the Amazon, are attributed to the natural routing lag within the river network. Could these lags also be due to specific limitations within the CaMa-Flood global river routing model? Have you conducted any sensitivity analysis on the models to explore this?

Minor Comments:

• Abbreviations are used in the abstract that may be unclear to readers who are not very familiar with the study.
• Line 25: Rephrase this sentence to better highlight the importance of this study, e.g.: “These findings are critical for improving global hydrological models, which are essential for developing more reliable water resource management strategies and adapting to the growing challenges posed by climate change, such as shifts in water availability and extreme flood events."
• Line 39: Please use formal expression for the “On the flip side, ....”
• Line 202: Is it correct that the ERA5-Land variables were regridded from 0.1° to 0.2°? If so, perhaps using a term like "regridding," "spatial aggregation," or "extrapolation" would be more appropriate.
• Line 241: In this study, ERA5-Land runoff is used as a benchmark for evaluating the runoff simulated by Noah-MP. While ideally, direct runoff observations would be used for this purpose, such data was not available, as you mentioned. To further strengthen your evaluation, and ensuring that the simulations align with real-world observations, it would be helpful to cite studies that have assessed ERA5-Land runoff against direct runoff measurements to demonstrate the reliability of ERA5-Land as a reference dataset.
• Page 10, Figure 1: What was the reason behind selecting these specific river basins? Were they chosen based on their size as the largest river basins?
• Line 393-394: I suggest using a more complete version of your statement something like this “According to the water balance equation, within a defined area over a specific period, the total inflows (such as precipitation) must equal the total outflows (including runoff and evapotranspiration), plus any change in storage (such as changes in soil moisture, groundwater, or surface water reservoirs).”
• Page 18, Table 1: How do you interpret the potential reasons for why the performance metrics for equatorial, and warm temperate regions almost for all EXPs outperform those of other regions?

The authors compare the impact of different runoff schemes on the hydrological simulations in different basins at a global scale. The paper is well-written and organized overall which is easy to read. The methodology is described in sufficient detail and provides a clear description of the results. I reviewed the paper, and I would highlight the following concerns.

Major comments:

Throughout the paper, it emphasizes the improvements in hydrological models, specifically highlighting this necessity.  However, as the paper is written, it seems more focused on the evaluation of the different runoff generation schemes within the Noah-MP model. While this analysis is valuable, the paper could benefit from a more thorough exploration of how these findings can be applied to enhance global hydrological modeling, particularly in the context of discharge simulation, which I understand is the central aspect of the paper.

Given that the results align with expectations across different regions and are not particularly surprising (lines 399 – 406, 461 – 462, and 469 – 470), it raises the question of what the primary contribution of this study is. If the findings largely confirm well-established patterns, it would be helpful to clarify how this research advances current understanding or introduces novel insights into hydrological modeling. A clearer articulation of the contribution of this study would strengthen its impact and ensure that it is seen not just as a validation of existing knowledge, but as a meaningful step forward in hydrological research.

The paper offers a detailed analysis of the biases in different runoff generation schemes related to discharge, which is valuable. However, it would benefit from a deeper discussion of the underlying physical processes that contribute to these differences. By incorporating a more thorough exploration of the hydrological mechanisms driving these variations, the paper could provide a more comprehensive understanding of the findings and their implications.

Minor comments:

• it would be beneficial to replace informal phrases like: "On the flip side" (line 39) with more formal language.

The data and methods are well described. However, there are some areas where this section could be further clarified and improved:

In the section 2. Materials and Methods:

• Section 2.1.1 would benefit from incorporating a table listing the different experiments, the runoff scheme used and their corresponding equations, making the text easier to read.
• In section 2.3, model evaluation, some paragraphs describe the actual models/data used (lines 243-248, and 272-276), I would suggest reorganizing these paragraphs in the corresponding sections.

In the Results and Discussion section:

• The text describing Figure 2 sometimes uses the “mm/year” units and sometimes %, I would suggest using the same units that appear in the figure (%) and indicate in brackets (mm/year), since using different units makes the text confusing.
• Figure 3 could be improved to show only the basins mentioned in the text, this would make it easier to follow the findings described in the text directly in the figure. The rest of the basins could be included as a complementary figure.
• Lines 357 – 362, While the central idea is clear, the text is somewhat difficult to follow due to its current structure and phrasing. I recommend restructuring this paragraph to improve coherence and enhance the link with the next paragraph.
• Line 405: David et al., 2019. That reference was previously cited in the text and should be properly referred to again in this section. Instead of using a link, it would be more appropriate to use the established citation format.
• I suggest reordering Section 3.4 to enhance clarity. Starting with the description of the global performance metrics (and not at the end as it is presented now) and then moving to the findings in detail for each of the regions (cold, warm, etc.), would improve section structure.
• The authors highlight that further improvements are necessary, such as refinement across diverse climatic regions, and calibration at finer resolutions (lines 336-338, 490 – 494). However, they could provide suggestions or hypotheses for improving global hydrological models and discuss potential refinements in more detail.

Conclusions

• While the conclusions offer valuable insights into the regional outcomes, as they are written, it seems that they do not adequately reflect the global perspective outlined in the paper:  "Our study transcends the boundaries of individual schemes and specific regions, highlighting the need for a holistic assessment…". It would be beneficial to rephrase the conclusions to present a more unified global argument, aligning with the objective of the research