General remarks:

The thematic of parameter stationarity in climate impact studies is an important topic when aiming at generating suitable future water resources projections. The authors explore here a framework to ingrate possible evolution of root zone storage capacity (S_R) in hydrological modelling experiments. The focus on alpine catchments in Austria and focus on impacts on streamflow response to changing soils. I like the topic and the way the authors present it. The findings represent a sample on the topic that might help future developments in this direction.

I list here below some thoughts and issues to be addressed.

Issues to be addressed (Line(s)):

30 – 109:         When accepting this review, I wondered if the authors would make a comment on Gao et al. (2023). I witnessed an interesting discussion on that between a hydrologist and a soil scientist. This was rather interesting, and I really wish the authors make their thoughts on it.

69-84              The introduction on optimality is very well formulated. At line 534 you compare your findings to the ones presented in Speich et al. (2018). In the paper you cite optimality approaches are used and discussed. You could consider adding this study also in the introduction.

103-105         Staying with Speich et al research, they published in 2020 a study with transient evolution of root zone storage capacities in an alpine catchment according to different scenarios (Fig 9 in Speich et al., 2020). This might be relevant for you. Also, in Speich et al. simulations with and without dynamic S_R are presented and discussed.

113-115          The authors focus on alpine catchments with shallow soils. Is this more than an opportunistic choice stemming from the Hanus et al. (2011) study? I agree that there are the places where possibly most of soil genesis might occur, but is S_R not more dynamic in low-land areas? More in general, I see the trade-off in terms of “duplication” between this section 2 and section 2 in Hanus et al. (2021) as unproblematic.

Table 1: Is there any soil information to be included here?

Figure 2 is a well-designed graphical abstract of the envisaged methodology. The indication of the colour schemes used is also very useful here. At the size presented in the submitted manuscript I don’t see the reason for keeping the text at such small fonts. A couple of more points everywhere would ease the reader.

148 ff              Considering the large number of assumptions that are declared in the 5 steps of the methodology, wouldn’t be useful to introduce some very basic benchmarks such has prescribed increase or decrease of S_R between the current and future time slice?

206-215          When you introduce the concept of supply and demand limited systems (also know as water and energy limited systems), wouldn’t be beneficial to elaborate on the blue and green water paradox in the Alps? Cfr. Mastrotheodoros et al (2020).

Figure 3          This Figure is presented in the Methodology but has also some results. As ω is fixed, we see no scatter in how the six basins respond to future climate. Have you done this exercise with past data (e.g. 1961-1990 vs 1991-2020) to see if such projections are realistic? (e.g. if ω is constant).

247-248          How did you partition for different vegetation types? Or did you just miss to refer to S2 also here?

251      When you speak of observed and modelled past, I think you mean “past obtained from simulations with observed data” and “past obtained with data from GCM/RCM”. If I am wrong, please explain me if I am correct, please state it clearly somewhere.

Figure 4          The violins show especially in RCP85 a clustering of the outcomes in to two families. Does this relate to specific GMCs or RMCs?

Figure 5          I like This plot, which speaks a lot for plausibility of your approach, as you get decreasing S_R (and chance to survive) for forest in high altitude basins, while grassland seems to have more chance to survive. Is this an independent achievement from your procedure, or does this follow one of several constraints you defined? (lines 269-271)

Figure 7          Did I miss a discussion on the cause of larger spread of “Modelled S_R_,clim” in the Pitztal? Glacier I guess?

The whole analysis of signatures is solid and follows very closely the Hanus et al. (2021) pattern. I see here potential for shortening the paper. Instead of replicating all these analyses, I would prefer you explore the sensitity of S_R with more simple benchmarks as I suggest for lines 148 ff.

539-552          I like this “Broader implication section”. As far as the catchment selection part is concerned, it is a pity that the study only concentrated on the Hanus et al. basins, all of them being humid and energy limited. So this counts also as limitation

Final considerations:

I like the organization and focus of this study. Having focus and analysed strongly connected with Hanus et al. (2021) is in first sight a good idea, but when looking at the results and analyses. I really considered a missed chance not including water limited catchments here. I fear that a follow-up study with addition of such basins would also be “jeopardized” by this “in between study”. My honest request is here major revision with addition of ~6 water limited basins. Another option would the to keep it in this form but adding some simple S_R scenarios to test the sensitivity against your sophisticated approach.

Best regards

Massimiliano Zappa, WSL

04.10.2024

References:

Speich, M. J. R., Zappa, M., Scherstjanoi, M., and Lischke, H.: FORests and HYdrology under Climate Change in Switzerland v1.0: a spatially distributed model combining hydrology and forest dynamics, Geosci. Model Dev., 13, 537–564, https://doi.org/10.5194/gmd-13-537-2020, 2020.

Speich, M. J. R., Lischke, H., and Zappa, M.: Testing an optimality-based model of rooting zone water storage capacity in temperate forests, Hydrol. Earth Syst. Sci., 22, 4097–4124, https://doi.org/10.5194/hess-22-4097-2018, 2018.

Gao, H., Fenicia, F., and Savenije, H. H. G.: HESS Opinions: Are soils overrated in hydrology?, Hydrol. Earth Syst. Sci., 27, 2607–2620, https://doi.org/10.5194/hess-27-2607-2023, 2023.

Hanus, S., Hrachowitz, M., Zekollari, H., Schoups, G., Vizcaino, M., and Kaitna, R.: Future changes in annual, seasonal and monthly runoff signatures in contrasting Alpine catchments in Austria, Hydrol. Earth Syst. Sci., 25, 3429–3453, https://doi.org/10.5194/hess-25-3429-2021, 2021.

Mastrotheodoros, T., Pappas, C., Molnar, P. et al. More green and less blue water in the Alps during warmer summers.at. Clim. Chang. 10, 155–161 (2020). https://doi.org/10.1038/s41558-019-0676-5