This manuscript is well written and presents a solid summary of work to incorporate reservoirs as simple idealized model objects. I have a few comments that may be worth addressing, but they are all suggestions and should not hold up publication of this work.

I've attached a copy of the pdf with inline comments.

Summary:

This study implements a reservoir scheme into ParFlow, an integrated hydrological model that simulates many other hydrological components but lacks reservoirs and dams. Specifically, the study uses idealized and real-world situations to demonstrate the proposed scheme functions as expected, ensure mass conservation, and is computationally reasonable. The manuscript presents formulations on the new scheme and details how it works in idealized and real-world settings. The East-Taylor watershed is used as a test bed toward the end of the manuscript. Overall, I was excited about the topic and agreed to review, also because the work comes from two leading groups in hydrological modeling. After reading the manuscript, I didn’t find my expectations to be met, primarily because the manuscript entirely lacks validation. Please see my detailed comments below.

Major Comments:

• A major concern I have on this manuscript is the lack of validation. I waited until the end to see if some validation (real validation with observed flow or reservoir storage variation) is presented, but I found none. Validation is fundamental to any modeling study, which applies to reservoir modeling as well. A number of previous studies I note below have presented validation across the US and worldwide with available data on reservoir storage and release. If this manuscript were to be published in HESS, the authors should demonstrate model applicability across a range of basins in the US (and even beyond) and present a detailed validation of whether the model reproduces observed flows in the downstream of dams and storage variations. Substantial data exist for many reservoirs, at least those in the US (please see numerous studies I note below). Without such explicit validation, I’m not sure what the value of the present study is.

• Page 2: Literature review is very limited in scope and doesn’t include many of the early studies; reservoir modeling has been there for over two decades and numerous studies have attempted to incorporate reservoir in different kinds of models; I suggest that the authors acknowledge the previous students and expand the introduction; for example: (Dang et al., 2020; Haddeland et al., 2006; Hanasaki et al., 2006; Hanazaki et al., 2022; Shin et al., 2019; Vanderkelen et al., 2022; Wada et al., 2014) and many more. Even Hanasaki et al. (2006), one of the very early studies, has not been mentioned.

• Introduction: It is generally well-written but it overly emphasizes implementation of dams in ParFlow. I suggest that the authors place more emphasis on: why this is needed (given existing models) and what is the novel contribution of the study?

• Page 5, Line 162 and elsewhere: how is dead, active, and flood storage considered in the model? Please describe in the manuscript.

• Equation 4: Are reservoirs considered to be of rectangular shapes?

• Line 257: Where are the “release curves” taken from? It is critical to elaborate this point.

• Page 10, Line 267: How are the volumes of 7 MCM and 5 MCM determined? Please provide the basis for these numbers.

• Line 275: there are certain model parameters noted here, which also seem arbitrary; the authors should present a sensitivity analysis to demonstrate that these parameters are reasonable/robust.

• L331, “maximum possible storage”: Is this calculated? Maximum storage is available in reservoir database such as the GRaND data.

• Figures: Figures including 4 and 6 do not provide important information; these could be placed in a supplemental document.

• Figure 5, “maximum possible storage”: I was expecting some validation with observed data but seems like this is just a comparison of simulated vs. “calculated” max storage. Please present actual validation.

• Line 355-359: seems like this is AI-written and not checked by the authors; for example, it reads: “insert citation”.

• L359: xx et al.?

• Figure 7: please present validation with observations; rich observations are available from USGS, US Bureau of Reclamation, and US Army Corps of Engineers for US reservoirs.

• Figure 9: I wasn’t able to understand the point of this figure; looks like a supplemental figure.

• L492: How is it novel? Please clearly describe this in the introduction and here.

• L498: how is it “user friendly”, especially compared to existing reservoir models.

• Overall, this manuscript can be a valuable contribution only if sufficient validation is presented to demonstrate that the reservoir module can reproduce observed release and storage.

Minor issues:

• Line 23: there is a time stamp, perhaps put by AI
• L243: please fix citations
• Line 265: n.d.?

References

Dang, T.D., Vu, D.T., Chowdhury, A.K., Galelli, S., 2020. A software package for the representation and optimization of water reservoir operations in the VIC hydrologic model. Environmental Modelling & Software, 126: 104673.

Haddeland, I., Skaugen, T., Lettenmaier, D.P., 2006. Anthropogenic impacts on continental surface water fluxes. Geophysical Research Letters, 33(8): L08406. DOI:10.1029/2006GL026047

Hanasaki, N., Kanae, S., Oki, T., 2006. A reservoir operation scheme for global river routing models. Journal of Hydrology, 327(1–2): 22-41. DOI:10.1016/j.jhydrol.2005.11.011

Hanazaki, R., Yamazaki, D., Yoshimura, K., 2022. Development of a reservoir flood control scheme for global flood models. J. Adv. Model. Earth Syst., 14(3): e2021MS002944.

Shin, S., Pokhrel, Y., Miguez‐Macho, G., 2019. High Resolution Modeling of Reservoir Release and Storage Dynamics at the Continental Scale. Water Resources Research, 55: 787-810.

Vanderkelen, I. et al., 2022. Evaluating a reservoir parametrization in the vector-based global routing model mizuRoute (v2. 0.1) for Earth system model coupling. Geoscientific Model Development, 15(10): 4163-4192.

Wada, Y., Wisser, D., Bierkens, M.F.P., 2014. Global modeling of withdrawal, allocation and consumptive use of surface water and groundwater resources. Earth Syst. Dynam., 5(1): 15-40. DOI:10.5194/esd-5-15-2014