In this paper, O'Donnell evaluated hydroclimatic biases in the CESM1 within the Mississippi River basin. The evaluation data include USGS gauge data of river discharge, ERA5 reanalysis, GPCC precipitation observations, and LIvneh ET. They also compared the CESM1 simulated runoff with the simulations from several CMIP6 models, including the newer version of CESM - CESM2. They demonstrated that CESM1 has substantial biases in simulating runoff and river discharge and attributed the model discrepancy to the deficiency in the RTM river model. They showed that CESM2 with the more advanced MOSART river model performs better in the river basin. While the results are clearly presented, I find that the motivations of this study are not clear and there are likely serious errors in the CEMS1 configuration or simulation or both. As such, the study has limited values and I cannot recommend its publication in this journal.

There are two major gaps/issues in the paper. First, the authors have not explained clearly why we need to know the biases of the old CESM1 given that the newer version CESM2 has been used by CMIP6. Does CESM1 have unique features that are not available in CESM2? Is there still a large user base who is using these features for important studies? What are the obstacles that hinder the users to adapt to the new version? Without good reasons, I would question why not to evaluate CESM2 instead.

Second, the model simulations look suspicious. Table 3 indicates that the modeled surface runoff, subsurface runoff, total runoff and snowmelt are two orders of magnitude smaller than the observations or benchmark data. Given this unbelievably poor performance, I would honestly think the model is useless. It is reasonable to question whether the authors have configured the model or extracted the outputs correctly because CESM1 has been well tested before. Furthermore, there are also several other variables with odd values: 1) the reported precipitation values (Figure 1b and Line 89) are less than 200 mm/year which if true would mean that the Mississippi River basin would be a desert; and 2) the reported runoff values in Figures 3 and the reported snowmelt values in Figure 4 if converted to mm/yr are unrealistically large (1e-6 m/s > 3e4 mm/yr). Also, it is very strange that the model and data are not shown at the same scales in the model-data comparisons (Figures 3, 4, and 7).

My other comments are shown below.

L104: Why did you choose the USGS 07289000 which is only available since 2008 for the Lower MS? Why not choose 07295100 Mississippi River at Tarbert Landing, Mississippi which has much longer data records for investigation?

Table 2: Could you explain why these years can be regarded as the separation of pre-modification vs. post-modification? For example, for the Missouri River, many of the dams were constructed in the 1930s. As a result, I do not think you can see much difference by comparing the model simulation before and after 1967.

Section 2: Given the importance of runoff generation and river routing in this study, wouldn't it be necessary to describe CLM and RTM briefly? Particularly, how is water management represented in RTM?

L135: QOVER is only a part of surface runoff and does not include surface runoff from standing water (QH2OSFC). 

L144: Could you describe briefly these 13 ensemble members? Under what configurations these members were simulated?

L151: What software do you use to calculate lagged correlation and spectral angle?

L265: It is probably not true. To my understanding, RTM does not represent two-way land-river coupling. As such, subsurface runoff affects river routing but not vice versa.

Section 3: Please separate results and discussion. The current structure prevents a cohesive storytelling.

Section 3.3: Why isn't this metric introduced in the methods?

The authors investigated the biases in monthly atmospheric and land surface variables in CESM1 data in the Mississippi River Basin (MRB). They found that there exist large seasonal biases in CESM1 precipitation, runoff, and discharge simulations. By comparing with other CMIP6 model data, they showed that the CESM2 model had better precipitation seasonality than CESM1 model. While this study provides useful information about the CESM1 model quality in the MRB, a few major issues need to be addressed before this paper can be accepted.

Major comments:

1. The primary concern is the value of evaluating only the CESM1 model when CESM2 large ensemble project data are available for this region. I suggest reorganizing this study to evaluate both CESM1 and CESM2 data and highlight the improvements in the CESM2 model.
2. The division of the MRB into western and eastern parts may not be sufficiently justified. The major sub-basins in the western part, i.e., Upper Mississippi, Missouri, and Arkansas River Basins, have very different atmospheric and hydrological properties. I recommend conducting comparison individually for each major river basins.
3. Figures 3, 4, 7: These figures have two y-axes with different ranges on each side. Does this indicate that the climate model data have significantly different magnitudes compared to reanalysis/observations? I suggest using consistent y-axis ranges for these figures to avoid misleading interpretations, but I understand that the authors may want to compare seasonality patterns rather than magnitudes.
4. Line 110: The gauge record was divided into pre- and post-modification periods. Please provide references to support the chosen separation year. Also, I suggest a more thorough discussion of the influences of human modification on river discharge in the MRB. The main stem of the river was heavily controlled by dams and flood protection structures. The influence of these structures on river flows should be mentioned and highlighted.

Minor comments:

1. Line 150: Please provide more details about the lag correlation and spectral angle methods. What formulas were used to calculate these two metrics?
2. Table 3: If possible, consider using figures instead of tables to present these results.
3. Figure 2: Adding the names of gauges used for each sub-basin would be helpful.
4. Figure 7: Please specify the uncertainty range in the figure caption. Is it a 95% uncertainty interval? Also, clarify the source of uncertainty. Is it coming from inter-annual variability or ensemble variability?