Review Comments on HESS-2024-340

General Comments

1. Research question on modeling impacts of tile drainage on hydrology and crop yield using ecosys model is new and worthy contribution to the literature.
2. Predicting crop yield using O2 modeling is novel
3. The manuscript could be significantly strengthened with additional details on the methods and a greater emphasis on novel results fo the work. 

Suggest that the authors consider the following points, regarding their methods:

1. General observations:

• Use past tense when referring to methods and results
•  
• Model setup:

• Hydrological processes lack clarity and better description of terms  and more consistent useage of terms is needed: In Fig S1, terms used are surface leaching, runoff, infiltration, discharge, subsurface leaching. What is meant by surface/subsurface leaching?
• No subsurface recharge is shown in Fig S1, but it is discussed in results. What is this? 
• “Subsurface discharge” used in results. Please define.
• “Subsurface recharge” used in results. Please define.
• What is the scale of the model and what are the boundaries of the setup?
• Calibration/Validation:

• more detail is needed here. Why use undrained for calibration and drained for validation? State reasons for doing so.
• List all parameters used in calibration, along with beginning and final values
• Why is precipitation different in tile vs no-tile teatments (Fig 6)?
• Increased precipitation experiment:

• Why stop at 30% increased precipitation? Why not go further?
• How was additional precipitation distributed? The seasonal/daily/hourly distribution of rainfall is important. Please comment on this.

Results:

1. Predicted no impact or increase of surface runoff in the tiled scenario (Fig S10 and line 385)—what is the explanation for this unusual result (normally, surface runoff decreases with tile drainage)?
2. What is the mechanism for tile drainage increasing ET in the summer months?
3. Fig 6 - keep tile drainage volume separate.
4. Fig 6 - is subsurface recharge (“water going into the field”) flow from adjacent fields? Please define this parameter?

Conclusions

1. Conclusions drawn by the authors do not really challenge or go beyond what we already know about the impacts of tile drainage on soils and crop growth--but I think there is a potential to do so. This seems like a missed opportunity with the novel approach taken by this study. Recommend reevaluating the discussion and conclusions to focus on unique aspects of this study.

The manuscript presents a modeling assessment of the impact of tile drainage on hydrology, biogeochemistry, and crop growth at a representative US Midwest agroecosystem. Using the model ecosys, the work reveals the interconnected processes that lead to increased subsurface discharge, subsurface nitrogen leaching, soil oxygen content, net mineralization, crop growth, and resilience to precipitation variability. Overall, the manuscript presents comprehensive and solid analysis that contributes to advancing the current understanding of drainage-managed agroecosystems and supporting agricultural management. Below are specific comments aiming to help enhance the clarity and interpretation of the findings.

The work leverages the detailed mechanistic description of root-zone processes in ecosys, which offers the unique opportunity to diagnose interactive hydrological, biogeochemical, and ecophysiological processes. As pointed out multiple times in the manuscript, the root distribution, particularly rooting depth, is a critical factor that drives the benefits of tile drainage for yield, including enhanced nutrient and oxygen supply and access to deeper moisture in summer. Are there data or relevant work supporting deeper roots in tile drainage settings? It would also help to include explicit quantification or discussion on how the uncertainty of model-simulated rooting depth might affect the robustness of the findings, e.g., on yield and plant water stress.

Model validation & Fig. 4: Based on the description of available site data, the crop yield record includes the typical wet and dry years presented in Fig. 10. Can the validation be shown in a time series so that it illustrates whether field data supports the advantages of tile drainage in extreme (wet and dry) years (Fig. S13), apart from the long-term averages presented in Table 3? The comparison could also help gauge the confidence in the benefits of yield resilience to precipitation variability derived from simulations (Fig.S18).

Section 2.2.1 Field Data: It would help to describe the distance between the two sites, tile and no tile, and if there are additional differences in site characteristics such as slope, soil texture etc. The distance between the two sites might be relevant as the precipitation amount for the two sites is not trivial (Fig. 6). Why is this the case?

Fig. 6: Related to the above, the differences in hydrological budget between the tile and no tile sites seem comparable or smaller than the difference in precipitation. How do the confounding factors, including precipitation and possibly other site characteristics, affect the interpretation of differences in hydrological budgets?

The results and discussion on tile-induced ET change could be clearer. It is unclear how the moisture profile and plant water stress affect ET in wet and dry years, which can be improved by including subpanels showing ET time series in Fig. 10. The discussion in L554-565 makes it difficult to draw the message clearly, and separate between findings here and in the previous studies.

Fig. 10: It could help to compare the profiles shown in panels f-h but on the driest days in August to understand drought-induced stresses.  

Fig. 11 & L460-464: Should the low variability of yield to precipitation increase be interpreted by high resilience or low sensitivity to precipitation rather than high resilience?  Also, while the Midwest is projected with increasing average precipitation, the variability is also increasing. How does tile drainage impact yield in droughts, e.g., when the rainfall amount ratio falls below 0.9? Quantification or discussion on this aspect could help clarify the interpretation of the impact of tile drainage. Also, should “1.3 times greater than” in the legend of Fig. 11 be “1.3 times of”?

Line 595-599 & L617: Related to the above, deeper roots allow access to deeper water but might also expose the roots to low soil water potential in thicker dry soils in extreme droughts. Would it induce a potential risk of tile drainage?  

L 628: Regarding controlled drainage, does the finding here offer implications on optimal schedules of controlled drainage for yield depending on the interplay of seasonality of coupled processes? A brief discussion on the potential or future work might help.

The growing season was considered from April to October (L345). Did the model account for different planting schedules across the years?

Equations 1-5: The definition of  D_{d,O_2} in Eq. 1 is missing. Mathematical syntaxes in Equations 2-5 are inconsistent with those in the text. Examples include Q_{a,O_2}, C_{a,O_2}, D_{a,O_2}, and D’_{a,O_2}.

Fig. S13 needs a more complete caption, e.g., stating the difference between green & yellow lines and the black & red lines.

Fig. 12 and 13 summarize the key mechanisms nicely. In Fig. 13, should there be downward arrows in the boxes of surface runoff and surface N loss?

L554: The statement needs rephrasing. As of now, “tile drainage did not significantly change crop growth” seems contradictory to the findings and the summary in Fig. 13.

L659: There are two repetitive phrases of “subsurface discharge”