Please find our comments in the supplement. 

Review of the manuscript “Comment on “Are soils overrated in hydrology?” by Gao et al. (2023)” by Ying Zhao et al. submitted to HESS

Ying Zhao and co-authors have formulated substantial disagreement with the opinion paper by Gao et al. (2023) along three main arguments: The first addresses the “soil” vs. “ecosystem” perspective as false dichotomy of a scaling issue lost in complexity. The second details on unanswered  challenges arising from excluding small-scale soil function in “root zone centred”, Darwinian hydrology as proposed by Gao et al. (2023). The third emphasises on limits in the advancement of physics-based perceptual model frameworks in (catchment) hydrology. 

The comment is well founded, well organised and a worthy contribution to the debate. The authors work out persuasive arguments to critical aspects in the opinion paper without self-admiring their own achievements but conciliating the scientific field at large. Along the lines of HESS, the original “Opinion article” has been discussed in HESSD. In the same manner, the “Comment” is treated. Since I have criticised the earlier manuscript (https://doi.org/10.5194/egusphere-2023-125-RC1) along similar lines, my central suggestions to the authors are only i) to expand and illustrate the arguments to make them more easily accessible to catchment hydrologists and ii) to focus their lines of arguments towards pinpointing the (in my view) essential issue of the unanswered scaling of small-scale soil processes in any “root-zone centred landscape model”.

A heated debate due to a too provocative title?

I have criticised the choice of the title and the very reductionist perception of the term “soil” earlier (https://doi.org/10.5194/egusphere-2023-125-RC1). Since much of the debate relates to essential limits about how simplistic pedotransfer functions relate some soil properties to their hydrological function and fail to account for landscape- and vegetation-related properties, I find the reasoning of Gao et al. (2023) nevertheless reasonable. However, I still do not see that the proposed conceptual shift from a chained to a more nested perspective on soil hydraulic properties (Gao et al., 2023, Fig. 4) substantially extends over the achievements of the critical zone concept and the hydropedology debate (Lin et al., 2006). 

In this sense, Ying Zhao and co-authors rightfully stress this shortcoming along their arguments. Especially the essential and remaining difficulty to derive physically consistent concepts for any root zone as key element is a soil-biophysical challenge yet to solve. However and since both author teams refer to emergent properties at larger scales, I would not see the positions that opposite. Coming from a catchment hydrology perspective, the observation that parameters from very simple soil classifications suffice over the large uncertainties in other domains (i.e. vegetation activity) has its merits, when Carsel and Parrish (1988) parameters for the van Genuchten soil hydraulic model can be used. Coming from soil-biophysics, we will certainly highlight the essential requirements to depict details of internal states if we seek to inform water dynamics in changing climate and land use. 

Thus I see the main argument of both author groups addressing the question about what we should expect from hydrological models (and their theories). Both derive to the point that terrestrial ecosystem are essential in hydrology and that this includes soils and the scaling of processes. While Gao et al. (2023) work out their points with some sort of feature importance, Zhao et al. (here in review) take a theoretic argumentation about scaling and perceptual models. With this, Zhao et al. make a fundamental point with stressing the central importance of (soil-bio)physical principles and their scaling in opposition to mere correlation-based approaches, which start to be outperformed by machine learning approaches.

Could the focus become more balanced?

In my view, Gao et al. (2023) basically describe their unease with hydro-pedotransfer functions (PTFs) and how they are used in most hydrological models. Weber et al. (2024 in press) just published a more thorough suggestion on PTFs. There we argue that evaluating PTFs with respect to their scale and their level of system information could be a way forward (section 6, Fig. 9). Complementary, Zhao et al. (here in review) provide very helpful and compelling conceptual arguments, showing that the soil-biophysical and hydropedological theory has developed. I would add the debate about water potentials (Novick et al., 2022) which are often overlooked in hydrology but are the central (physical) driver for processes (in opposition to mere differences in soil moisture). 

Different infiltration models (like Germann, 2020; Ogden et al., 2017), the fill-and-spill concept (McDonnell et al, 2021), particle approaches to dual domains (Jackisch and Zehe, 2018), characteristic lengths in soil evaporation (Lehmann et al., 2008), soil controls on stomatal response (Carminati and Javaux, 2020) etc. could be used to show how much soil properties are essential and how we can incorporate these also in simplified, catchment or land surface models. Moreover, physics-informed machine learning hydrologic models (Feng et al., 2023) should be further elaborated on. On the other side, I am uncertain if the REW is really the essential benchmark for a coherent hydrologic theory and if it does or even should “parallel” the REV concept.

Again, I think the comment is a worthy contribution. If possible, I suggest that the authors might really embrace the chance to substantiate their points beyond the very condensed sections – or even the format of a comment? I think that the apparently opposing standpoints towards the role of soils can become more constructively aligned, when the authors could i) work out the essential biophysical controls by soils, ii) highlight the theoretical advances and how they could be incorporated in simplified hydrological models and iii) elaborate the theoretical necessity of biophysical principles more clearly. Conceptual sketches might be helpful to convey the points.

References:

Carminati, A. and Javaux, M.: Soil Rather Than Xylem Vulnerability Controls Stomatal Response to Drought, Trends Plant Sci., 25, 868–880, https://doi.org/10.1016/j.tplants.2020.04.003, 2020.

Carsel, R. F. and Parrish, R. S.: Developing joint probability distributions of soil water retention characteristics, Water Resources Research, 24, 755–769, https://doi.org/10.1029/wr024i005p00755, 1988.

Feng, D., Beck, H., Lawson, K., and Shen, C.: The suitability of differentiable, physics-informed machine learning hydrologic models for ungauged regions and climate change impact assessment, Hydrol. Earth Syst. Sci., 27, 2357–2373, https://doi.org/10.5194/hess-27-2357-2023, 2023.

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.

Germann, P.: Viscosity Controls Rapid Infiltration and Drainage, Not the Macropores, Water, 12, 337–15, https://doi.org/10.3390/w12020337, 2020.

Jackisch, C. and Zehe, E.: Ecohydrological particle model based on representative domains, Hydrol Earth Syst Sc, 22, 3639–3662, https://doi.org/10.5194/hess-22-3639-2018, 2018.

Lehmann, P., Assouline, S., and Or, D.: Characteristic lengths affecting evaporative drying of porous media, Physical review. E, Statistical, nonlinear, and soft matter physics, 77, 354, https://doi.org/10.1103/physreve.77.056309, 2008.

Lin, H., Bouma, J., Pachepsky, Y., Western, A., Thompson, J., Genuchten, R. van, Vogel, H.-J., and Lilly, A.: Hydropedology: Synergistic integration of pedology and hydrology, Water Resour Res, 42, 2509–13, https://doi.org/10.1029/2005wr004085, 2006.

McDonnell, J. J., Spence, C., Karran, D. J., Meerveld, H. J. (Ilja) van, and Harman, C. J.: Fill-and-Spill: A Process Description of Runoff Generation at the Scale of the Beholder, Water Resour Res, 57, https://doi.org/10.1029/2020wr027514, 2021.

Novick, K. A., Ficklin, D. L., Baldocchi, D., Davis, K. J., Ghezzehei, T. A., Konings, A. G., MacBean, N., Raoult, N., Scott, R. L., Shi, Y., Sulman, B. N., and Wood, J. D.: Confronting the water potential information gap, Nat Geosci, 15, 158–164, https://doi.org/10.1038/s41561-022-00909-2, 2022.

Ogden, F. L., Allen, M. B., Lai, W., Zhu, J., Seo, M., Douglas, C. C., and Talbot, C. A.: The soil moisture velocity equation, Journal of Advances in Modeling Earth Systems, 9, 1473–1487, https://doi.org/10.1002/2017ms000931, 2017.

Weber, T. K. D., Weihermüller, L., Nemes, A., Bechtold, M., Degré, A., Diamantopoulos, E., Fatichi, S., Filipović, V., Gupta, S., Hohenbrink, T. L., Hirmas, D. R., Jackisch, C., de Jong van Lier, Q., Koestel, J., Lehmann, P., Marthews, T. R., Minasny, B., Pagel, H., van der Ploeg, M., Svane, S. F., Szabó, B., Vereecken, H., Verhoef, A., Young, M., Zeng, Y., Zhang, Y., and Bonetti, S.: Hydro-pedotransfer functions: A roadmap for future development, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1860, 2023.

I have carefully assessed the submission by Zhao et al. (this issue), which is a commentary on the earlier paper by Gao et al. (2023), and the reply by Gao et al. to this commentary, as well as the review comments by Conrad Jakish (this issue).

My first order responsibility to assess the acceptability of Zhao et al. (this issue) to be published as a commentary in HESS. My answer to that question is an unqualified YES. The commentary is well written, makes its arguments cogently, and makes a useful contribution to the debates centered around soil hydrology. They are of course welcome to revise their manuscript to accommodate the comments from Gao et al. and from Conrad Jakish. My recommendation is to accept the paper with minor/moderate revisions.

To me these are “details” – as the exchange between Gao et al. and Zhao et al. that appear on line here – the two sets of authors disagree strongly on their fundamental arguments. No amount of tinkering on the details will bring these arguments to a close.

To my way of thinking, this is a mess. The two sets of commentaries and the debates around them are sending a confused message to new entrants to the field. Both sides are partially right and wholly wrong. Both sides do however agree that we need a common theoretical framework that transitions seamlessly from small (point) to hillslope to catchment scales. I remember debating these issues at the workshop in Corvallis that led to McDonnell et al. (WRR, 2007), and close to 20 years have passed since then, we as a community has gained in confidence and process insights, and yet we are nowhere near resolving the differences that break out in these papers.

Like I said I do believe we are closer than ever and the time is right for someone to organize another workshop like the one in Corvallis, bring key players together and once and for all finalize the outlines (not all the details) of a new common (scale free) theoretical framework so that research communities  working across these ranges of scales can align their work towards this framework. I am confident that we can reach out beyond the HYDRUS-PDF view of soils and the lumpled Manabe bucket model view of soils, with just a little effort that extracts the insights that the Richards equation can provide us, and the coevolutionary view of how climate and vegetation control the development of soils and their ability to store and release water.

So in conclusion, I recommend acceptance of Gao et al (this issue) in its present form, but put out a call for a more organized activity to help resolve these differences and achieve some sort of consensus so that these debates continue to happen endlessly without resolution.

I enjoyed reading the paper by Gao et al and the commentary by Zhao et al. I've also read the reviewers' comments and they cover most of the comments I had in mind, so I won't repeat them.

Personally, I believe that both views, soil-centred and ecosystem-centred, are relevant and necessary to address different problems in hydrology. Recently, with the advent of new high temporal and spatial resolution satellite observations, it is very likely that we will finally have the data to reconcile the two views. This means that we can test our modelling approaches with spatially and temporally distributed data as never before. This is in line with Zhao et al., but I do not want to rely on my personal opinion to evaluate the nice discussion raised by the Gao et al. paper.

In summary, I believe that the comment can be published as is, and I hope that the different groups of authors and communities will work together to reconcile the two views.