This paper assesses the impact of accounting for irrigation when calculating root zone storage based on a wealth of large hydroclimatic datasets applied to numerous catchments across the globe. The authors evidence a general reduction of root zone storage estimates, more marked in absolute value in locations with more than 10% of irrigated area (17-22mm reduction) or in a humid tropical climate, while the relative reduction is largest in temperate locations (16-22% reduction).

In revising the estimates plant-accessible storage estimates in irrigated crops, this paper tackles an issue of key importance for estimating current and future ecohydrological feedback in the Earth System. It is therefore an important step, and I found the manuscript pleasant to read, concise and clear for the most part. In my view it may actually a bit too concise, lacking contextualization through a more in-depth discussion. After this and some clarifications, I think it will be suitable for publication in HESS.



General comments

The discussion is quite short, and a significant part of it is a synthesis of the results, I think it could dig deeper in the implications and robustness of the method and results. These could be (but not limited to):

- the comparison with the impact of using snow accumulation is very interesting, all the more that the present study also considers snow storage. It would be quite interesting to see the relative effects of snow and irrigation in catchments where both are significant, with a “no-snow” case (e.g. by forcing P_sn to zero).

- one of the key assumption of the methodology is a sustainable water use, but this is not the case in many locations, as mentioned L264-266. Beyond this sentence, a more detailed discussion of potential impact on the methodology (e.g. irrigation exceeding sustainable use by XX% implies XX% changes in S_r) would be quite interesting to put results into perspective

- another assumption in the methodology is that there is single succession of excess/deficit periods within a year. How robust is that, also in relation to cited efforts to quantify root zone storage? How do other patterns such as double cropping system (where irrigation may happen in two periods), and hydroclimatic patterns such has bimodal monsoon (which e.g. affects significant parts of India) alter this framework?

- between the ongoing irrigation expansion and improved irrigation efficiency, what would be the net effect on irrigation volumes and thus root zone storage estimates ? Perhaps a more detailed perspective relating to McDermid et al. (2023) and other review literature on irrigation would be interesting

Specific comments

L23-24: Vegetation also mediates soil evaporation and perhaps more importantly evaporated interception ; these fluxes are generally smaller, but amount land evaporation to transpiration is misleading.

L41-42: Kleidon and Heimann (1998) and Kuppel et al. (2017) also used a similar approach, albeit using potential evaporation

L160-164 / Fig. 3: since the RMSE(f_IAF, f_IWU) is computed for all catchments (for each beta value), why not showing the spread of RMSE (e.g. interquartile range) instead of a single line? Can you justify with beta should be constant for all catchments ? i.e., why not computing 4511 catchment-specific beta values that minimize the RMSE, as I doubt it is strictly 0.9 everywhere? Could the authors clarify this point, as the potential impact of this variability (or computing choice) upon S_r estimates in the IAF case could be quite interesting to discuss.

L169-179: I had to read this section several times to understand how S_r was finally derived, and I am not sure I did. It is announced in the first sentence but referring to a Table 1 which is actually more a reminder list of notations than explaining S_r. In the last sentence it is said that it is the mean of three values, while S_r is separately computed for NI, IWU, and IAF right? What is meant by “Sd,M-values with occurrences closest to T = 2 years”? Perhaps a supplementary figure with an example of S_r calculation across return periods for a (given set of) catchment(s) would help.

L174-176: Here or in the Discussion, a tentative/summary (and if possible physically-based) explanation for why a 2-year return periods fits best would be welcome.

L213-214: From this text it seems Fig. 8a only shows catchements with I_a > 0.05, but this is not mention in the Figure or its caption (contrary to Fig. S3b), could the authors clarify? I actually wonder if this selection if I_a > 0.05 does not also apply to parts of Table 2?

L224-232: For this discussion, consider adding a third panel to Fig. 6 with the relative difference between IWU and IAF, to look for patterns?

L268-270: Not necessarily true if irrigated area fractions increased since then.

L270-271: How did the authors conclude on the lack of impact? What does that refer to? At present, it is not very convincing and may perhaps be extended as part of the discussion (see General Comments).

Technical comments

L185: Fig. S2-S3 are referred to before Fig. S1 (which is a referred though Fig. 4 a bit later). Perhaps consider swapping Fig. S1 for Fig. S3, etc. ?

References

Kleidon, A., Heimann, M. (1998). A method of determining rooting depth from a terrestrial biosphere model and its impacts on the global water and carbon cycle. Glob. Change Biol. 4, 275–286. http://dx.doi.org/10.1046/j.1365-2486.1998.

Kuppel, S., Fan, Y., & Jobbágy, E. G. (2017). Seasonal hydrologic buffer on continents: Patterns, drivers and ecological benefits. Advances in Water Resources, 102, 178-187.

McDermid, S., Nocco, M., Lawston-Parker, P. et al. Irrigation in the Earth system. Nat Rev Earth Environ 4, 435–453 (2023). https://doi.org/10.1038/s43017-023-00438-5

The manuscript “Influence of irrigation on root zone storage capacity estimation” assesses the impact of global irrigation practices on root zone water storage capacity. The findings are quite interesting suggesting a general reduction in storage capacity particularly for agriculturally areas.

The paper is generally well written and fairly easy to understand given the theoretical nature and complexity of the topic. I find it suitable for publication in HESS after addressing some concerns.

My main struggle when reading the manuscript was the lack of potential consequence of their estimations. For example, what are the consequences for landscape scale land-use and land management? I.e. you determined a decrease in root water storage capacity with irrigation, but would it not be more meaningful to try to explore “best” irrigation practices for a hydrologically resilient agriculture?

I would prefer some calculations, but at least this issue should be thoroughly discussed.

Discussion: in general the discussion is fairly short and not exactly spiked with literature comparison and contextualization. This could be improved. Aside from the suggestion above, one discussion point could be the process-based mechanisms underlying reduction in root water storage capacity. In the introduction the authors relate this mainly to anatomical changes in the rooting system, i.e. shallow and less dense root system under irrigation.

However, plants react to changes in water input regime in more ways than anatomical adjustments. E.g. how does changes in hydraulics or generally differences in hydraulics between species affect Sr?

Is it, i.e., possible that adjustments or species specific differences in plant maximum water potentials (ψ) affect Sr and how?

Specific comments:

LL25: actually phenological development especially in croplands is pretty important and can easily outrule other influences.

LL25ff: this definition of Sr is dominated by physical objectives and does not consider plant regulation at all, same goes for the description of T regulation. This lacks an understanding of physiological and ecological processes that regulate T and I find this troublesome.

LL44: do you truly mean evaporation or evapotranspiration?

Fig 2 and methods section: Why do you specifically need two years? Also: You start the hydrological year with the day of highest water availability. But how do you deal with consecutive years varying in precipitation regime? Or do you just define this for the starting point?

Fig. 4 and 6: the way the figure is plotted in the preprint this is very hard to read given the size and color palette.