Hongkai Gao and co-workers present an interesting contribution to the debate about key elements in the concepts of hydrological modelling. As an opinion paper, the authors argue that the affinity of hydrological model concepts to soil properties are more a relict than a substantial information basis. They propose to shift focus to the rootzone (as manifestation of the ecosystem) as alternative conceptual foundation.

I congratulate the authors for their work and I agree that our community has to keep challenging the conceptual assumptions and traditions. The role of soils in hydrology and land surface modelling is a particularly interesting debate. Recently Novick et al. (2022) have pointed to a “water potential information gap” in similar notion but opposite proposals. Discussing the role of pedotransfer functions (Looy et al. 2017, Vereecken et al. 2022) and soil hydraulic functions (Peters et al. 2023) together with structural adequacy of models (Gupta et al. 2012), perceptual model consistency (Wagener et al. 2021) and the data flow in model building (Gharari et al. 2021) and model analyses (Loritz et al. 2018) is in my view very important and promising. Hence I see the topic of this manuscript as worth an opinion paper.

However, I am not really convinced that the current arrangement of the arguments in the manuscript is really substantiating this timely debate. My main concern is that the authors use the word “soil” for different concepts and at different scales without much differentiation. The critical zone concept (Lin et al. 2006) was already much further than this. Also the debate about landscape organisation and hydrologic functioning (Jackisch et al. 2021) including a critical assessment of conceptual assumptions about processes and scaling is more advanced on the topic.

The authors do address such aspects in their manuscript and point to intertwined factors and sub-systems. However, the arguments are not really brought to consistently support the very fundamental claim of the manuscript. Without meaning to offend the authors, I would see many of the claims rather being rooted in conceptual limitations in the view of soil functions by the authors than in the lack of information or importance of soils in hydrological processes and models. I will substantiate this in the more detailed assessment.

In general, I do not really see, how the replacement of a soil-centred with a rootzone-centred concept deviated from the critical zone concepts (Lin et al. 2006). I also do not see, why the authors omit the main driving concept for fluxes (depletion of gradients) and thus the whole debate about potentials (Novick et al. 2022). I would have liked to see to which degree their arguments are essentially an expression of the conceptualisation of hydrological models i) as distributed and linked storages, ii) at a broader scale (in the sense of the scale triplet) and iii) with soils expressed by texture classes. 

Moreover, I find many claims very strong and confrontative (e.g. L21f, L111) and not well-balanced. To really spark the debate (and not a battle) I would have liked a more balanced and substantiated formulation.

Detailed comments:

L43: I would argue that this is the debate throughout in pedology. At least not only recently.

L49: Why do you limit the perspective to abiotic boundary conditions when you actually argure for an ecosystem perspective? Biodiversity, niches, disturbances, stressors, carbon pools are not all determined by climate and geology. Moreover, at least most temperate soils do not develop directly from bedrock material but on deposited material from rather old geomorphological processes (which include path-dependendtdevelopment options). Pointing to this, soil degradation and soil loss too is an important and largely irreversible process with severe implications on regional hydrological and biogeochemical cycles.

L59: I think I have an idea what you intend to express, but since this simplistic/reductionist pedotransfer approach has a couple of implications which could be challenged. I suggest to clarify this sentence a little more and link to the debates in soil physics and the pedotransfer community.

L63 (opposite): I am not sure if I can follow. The argument before was that simple pedotransfer and soil hydraulic property models are an issue and that they become coupled to rooting depth.

Consequently, rooting depth has to be defined somehow to eventually assess plant-available soil water storage. Why does it matter if in this step plant-available soil water storage becomes the dependent variable of the other involved variables, if it is used as independent variable in the proceeding calculation steps? Isn’t this a question about the perceptual model underlying any form of conceptualisation and numerical expression?

L63 (root zone storage): Yes, but maybe at different time scales? Plants and ecosystem may adapt and co-evolve (within a range of their survival). So why should the debate be solved by exchanging the depending/independent variables? Could this actually be a scaling issue?

L65: This might depend on what exactly we see as detailed. As you open your argumentation with coevolution, maybe a broad idea about the general type of soil (not texture class) and biome (including its ecohydrological properties) could be sufficiently detailed? If so, remote sensing claims various solutions to gather such data…

L67: Again, i would see this as a scale issue: Ecosystem and climate are both terms referring to large scales (in the sense of the scale triplet). Hydrology is not referring to a specific scale.

L72f: This assumes that the ecosystem is somewhat in equilibrium with determinable drivers of its development. However, path-dependent trajectories, dynamic deviations from equilibrium more or less buffered by the ecosystem and any application for global changes (climate, land use, cohabitation…) but severe challenges to this view.

L78: Why do you refer exactly to these citations? I would think that e.g. the work of Gardner including his famous lab experiments have been far more important for propagating this perception.

L87ff: Yes, and this might be one of the actual issues to address here. Linear Darcy filter flow has been coupled to highly non-linear retention properties with the Richards equation and as a first-order diffusive flow model, it does an ok job for diffusive flow in somewhat well-defined porous media. However, especially infiltration (as initial soil water redistribution into the soil during rain events) is often not dominated by diffusive flow but by advection (Newtion Shear Flow equation (Germann, 2020), soil moisture velocity equation (Ogden et al. 2017), particle model (Jackisch and Zehe 2018), non-equilibrium flow (Vogel et al. 2023)). To my understanding, this deficit is rooting back to the very limited means to measure antecedent state-dependent infiltration and to use such data in hydrologic models. But why this is an argument for soils not being central in the question for one of their fundamental services to mediate the local soil water cycle is not clear to me. Especially because infiltration is state dependent, precipitation may not be retained after drought conditions, requiring vast amounts of light rains, slow snow melts or similar to replenish the water stocks, while storm events will simply lead to preferential flow and possibly erosion…

L93: Well, it is not dominant when it comes to storm events, yes. But these experiments use rather steep gradients with a lot of water. The debate about when and to what degree soil water flow is preferential is ongoing. If this was the full story, soils could hardly sustain the ecosystems.

L98: Partly yes. But preferential flow can also simplify our models. Anyways, I suggest to ease the dispute opened by this statement with a slightly more balanced view on achievements towards unifying forms of non-uniform infiltration.

L108: Yes. But this again can be seen as a scaling issue. At the hillslope- and plot-scale, these parameters/concepts have been very central. Only at the catchment-scale they could be easily subsumed as general soil property parameters not requiring for a dual domain definition. And this is true for the hindcast of our observations…

L111: Again a strong claim. I can agree that the pref flow debate has always struggled to connect to Darcy-scale soil physics. But fog? No progress? Is this claim really needed for your argument?

L113 (a priori assumption): Well, they are ABOUT the description of soil water flow. If they are key for describing hydrological processes is part of the actual model conceptualisation, its numerics and the respective regimes under study. Again, I would argue that this is no other “a priori assumption” as most other parts of the perceptual model. And since its actual effect in the model can be and is challenged (Glaser et al. 2018), I would rather see it as a positive example for advancing hydrologic models.

L118: So far soil variability has not been motivated. This is especially difficult, because the effect of soil variability is again a matter of scale (including the respective range of processes). After reading subsection 3.1, I can think of quite a number of papers, providing good evidence for the opposite: When you have the average soil right, you can easily reproduce observed hydrologic patterns (e.g. Loritz et al. 2017).

L123f: Ok. Known and well established. Maybe citing some of the many studies would be nice.

L131f: This argument is not really sound. Studies fully agree that plants and ecosystems strongly moderate the net ET flux of a stand. But without soil as the part of the ecosystem which can actually store water for weeks and beyond, this percentage cannot be reached. We exactly see this in data based on Budyko-like assessments that more draining locations (sandy, karstic) have very little ETact simply because precipitation is largely drained.  

L137f: Yes, difficult but steeply advancing. Please see Peters et al. (2023) and Hohenbrink et al. (submitted to ESSD) for examples. The most critical part might be the reduction of such data to van Genuchten/Mualem SHP model parameters and the weakly informative relation to the broad texture classes, BD and Corg. But the issue of pedotransfer models is a discussion on its own, and which is currently gaining momentum.

L144: The issue here might be that soil mapping is not particularly done for hydrological purposes. On the one hand, pedological classes are not always directly convertible to hydrological properties. On the other hand, soil stratification and the respective hydrological properties are rarely conveyed into land surface models with sufficient degree of vertical resolution. Moreover, the uncertainty about the hydrological properties of the mapped soil classes is largely unknown and very different from region to region. Given all of these points, I am not quite sure if “interpolation and upscaling” is the core issue here. Maybe it is more a disconnection between soil mappers and hydrologic modellers? 

L148: I fully agree that unnatural lab conditions are a fundamental difficulty. However, many measurements are conducted using “undisturbed” samples for soil hydraulic property analyses. It is unnatural because the samples are extracted from their capillary context, exposed to free evaporation at the surface and a no flow boundary at the bottom (for the standard HYPROP protocol). However pedotransfer functions are then correlating lab measurements (soil hydraulic properties) to lab measurements (texture) and the scaling and transfer involved in its application to field conditions remain hidden.

L152: I fully agree, but again this is an issue with quite a bit of literature from hydropedology to cite here.

L153ff: I do not get this point. The discussion about parameter regionalisation has a long standing in hydrology. E.g. mHM (Samaniego et al. 2017) exactly works because it modifies the initial lab scale parameters to match its distributed effects on fluxes in the landscape. Showing one odd model result can have so many reasons that I find it very difficult to support your argument through it.

L157: Which is a nice example for model extrapolation and the shift in parameter sensitivity under climate change (Melsen and Guse 2021).

L177: Again, a difficult claim. They test if texture classes and soil depth is informative. However Novick et al. (2022) point nicely to soil water potential being most informative and often omitted in LSMs. The model you are referring to are not particularly strong in soil physics as they conceptualise soils as stores instead of any framework of potentials as drivers. So your assessment might actually pinpoint that soil hydrology based on a storage concept is not very informative? As stated in the general section, I find it very difficult that you do not discern between weak conceptualisations of soils and the actual physical properties and dynamics linked to soils.

L188: These intertwined factors mostly manifest at “soil scales”, which are not necessarily very small.

L194f: Again, I would argue that the concept of infiltration capacity as rigid site property maybe the root of the issue here? Infiltration capacity to my understanding does not necessarily entail a constant or any specific model (e.g. Horton which is subsuming site properties and antecedent condition into an exponential decay function for infiltration rate or Green and Ampt which indeed is rarely proven in natural soils). Since infiltration is the passage of water into the soil domain, I would argue that soil structures (draining macropores and storing finer pores) facilitate it and that antecedent conditions plus the rainfall supply dynamics govern the individual initial (non-uniform) soil water redistribution (see comment to L87ff). The ecosystem modifies the boundary conditions, state dynamics and structure formation in the long run (Lange et al. 2015 and other publications from the Jena experiment).

L203ff: I agree and I admire the authors for their very nice contributions to these examples. However, this comparison is not fair since the intended applications of more complex models are often more than rainfall-runoff modelling. Especially when models are used to analyse effects of changes in land use , climate regime, management etc. the stationarity assumption collapses and we require parameters and submodels with physical meaning. Once we have a good understanding about how the modified hydrologic system can be conceptualised, the simple models are much more efficient and maybe even less error-prone again. But the transition (in system characteristics or scale) remains very challenging for these kind of models. 

L210f and Fig. 2: I do not find it a logical proof of your argument that some models can succeed without soil information. If soil information is only texture class and porosity maybe it is more telling that these properties are not very informative for hydropedological characteristics and that the variable for the most frequent antecedent conditions (aridity) has far more influence because it is more informative for hydrological functioning? Hohenbrink et al. (submitted to ESSD, soon at https://doi.org/10.5194/essd-2023-74) show very nicely how these standard properties and texture-based soil classes do not inform hydropedologic functioning.

L217ff: I find it difficult to discern your “ecosystem”/“rootzone” approach from the hydropedology concepts (Lin et al. 2006). 

L226ff: Within the lines of arguments, I think you are jumping through different scales here (with concepts and properties which are known NOT to be scale-invariant). The assumption that the ecosystem will be able to become the dominant driver is only true if the system has sufficient degrees of freedom to do so. Mediterranean basins have been deforested long ago, soil has been lost and there is no sign of spontaneous ecosystem replenishing under the current climate conditions. Badlands, crusts, long-term unstable debris are examples contradicting your claim. Hence a more differentiated analysis would be more insightful?

L232: I fully agree that water can bypass the rootzone but is not necessarily reaching groundwater. In many soil systems of the mid latitudes we find laterally conductive layers formed by more distant ice ages leading to relatively quick drainage or even interflow. Your FLEX approach has nicely shown this for the Ardennes…

L235ff: With having FLEX in mind I can understand your reasoning but I find your PERCEPTUAL model rather inflexible in the first place. The notion to simplify as much as possible is fully legit but deterministic concepts are in my understanding rather a thing of the past when we were limited in computational powers. And I find that this stiffness weakens your argumentation.

L245 (and the paragraphs before and after): I do not see why this is an argument against the importance of soils. Just because modellers use non-informative variables about soils and just because they have not found laws to scale the scale-dependent concepts/models does not mean that soils are not important. If these observations are biased, this does actually point to a misconception of the soil system rather than serving as an argument for omitting soils altogether. I would claim that this only shows that soil function cannot be described by texture classes (alone).

L251: I find it very difficult to agree to your arguments at this too general level of characterisation of somewhat arbitrarily selected model examples. I suggest to build the arguments based on the state of the art about structural adequacy and model conceptualisation (see general comments)

L282ff (and the whole subsection): You are proposing a new conceptualisation in which you omit various central properties governing water retention and drainage, which are not only governed by vegetation alone. With most of the terrestrial surface of our planet being actively managed by humans and a massively changing climate and biosphere, I find it not very helpful from a physical and system perspective. Moreover, your concept does not evade the scale issues. Quite to the contrary the active rootzone is not a static thing (at many scales). When we look at root water uptake alone, the sourcing depth of water within the root zone is dynamic over the year and very different from site to site (with the very same tree species and ages) (Jackisch et al. 2020). Giving reference to ERA5 data for this is maybe a little too large scale to substantiate your arguments with?

L295ff: From a (soil and hydrologic) physics perspective the main fundament might be that fluxes are driven by gradient depletion and that the degrees of freedom for these fluxes are state dependent (including subscale properties subsumed as hysteresis). The fill-and-spill concept (McDonnell et al. 2021) is a very powerful description of dynamic connectivity and threshold behaviour resulting from the strong non-linearities in soils. However, the depletion of gradients is largely omitted in such models. You might argue (L298f?) that storage-based models do not require an explicit treatment of gradients since it is all implicitly covered by the individual storage and transfer functions. However, this is not an argument against the importance of soils nor does it solve the standing issue to be capable to convey changing landscape properties into the required storage characteristics.

L308ff: Why do you jump from the debate about the concepts back to the debate about available data (which has so far not been really opened)?

L320ff: Since I read your manuscript as a strong claim for a simplified hydropedologic perceptual model, I find the argument with Occams razor very problematic. I would claim that we are in a situation with plenty of data to challenge our perceptual models and we have the tools to do this (e.g. Höge et al. 2020, Guthke 2017). Occams razor is a perceptual assumption, too.

Again, I sincerely thank the authors for raising this debate. I hope that my review can contribute to sharpening the arguments and to raise awareness about the many aspects that might have fallen a little too short in preparing this manuscript.

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This opinion paper makes interesting and bold claims about the importance of soil properties for hydrology. I agree with many of the statements for natural soils and mature ecosystems. However, the majority of our earth is no longer a natural mature ecosystem. We have changed the surface cover drastically and very large areas are under agriculture or are so badly degraded and not in a mature “steady” state that the ecosystem perspective that is advocated in this paper is possibly no longer applicable. I think that this has to be mentioned in the text and that the reader needs to be reminded more frequently that these statements are made for mature natural ecosystems.

That soil is important is clear in situations where severely degraded ecosystems are restored. It is the restoration of the soil that leads to the very large changes in the flow pathways (from overland flow to subsurface flow) and thus streamflow responses. Indeed, it is the ecosystem that changes the soil properties that lead to the changes in the hydrological flow pathways and runoff responses, but this does not mean that the soil itself is not important at all. It means that the ecosystem has such a large effect on soil that the ecosystem would be a better predictor to be used in models (because ecosystem and soil properties become correlated as the ecosystem matures and the ecosystem is easier to observe), but it does not mean that soil is not important at all, especially not when one wants to understand processes. I think that some of the statements about soil not being important therefore require a bit more nuance. In particular, the model perspective (rather than process perspective) for some of the claims should be made clearer.

One of the confusing parts of the paper is that the authors state that the rooting zone is important but that soil is not important. This seems to suggest that they think that the rooting zone is not part of the soil. I think that what they mean is that soil texture is not important. To me it seems that most of the time when the authors say that soil is not important, they mean that soil texture is not important. For example when the authors refer to soil in the top down approach of catchment comparisons (Section 3.3), they actually refer to texture, not soil hydraulic properties. I urge the authors to more explicitly state that they focus on the soil texture. A better description of what parts of the soil they think are not important would be really helpful. It will also help if they give their definition of soil early in the paper.

The authors should point out much more clearly (and explicitly) that a major problem is that we use texture in pedotransfer functions to derive the soil characteristics that are related to water flow and storage, especially because these pedotransfer functions were developed for agricultural soils. The sand or silt content of a soil do not affect water flow or storage. We only attribute such an effect when we use pedotransfer functions to derive properties related to water flow and storage based on the texture. Because the pedotransfer functions were largely derived for agricultural soils, they do not take the effects of structure (and preferential flow) into account.

The writing of the manuscript could be a bit sharper. At several places, the authors make a good argument for why the ecosystem is important and then conclude that the soil is not important. I think that these sections need to be improved for two reasons. First, reasons are given for why the ecosystem is important but not for why the soil is not important. In particular, no references are given for this second part. In other words, the authors provide arguments for the first part (the ecosystem is important) but not for the second part (soil is not important). Thus either the second part (soil is not important) has to be taken out or arguments and references need to be included for the second part as well. Second, ecosystem and soil are interconnected. It is the ecosystem that changes the soil properties. So one can not directly argue that because the ecosystem is important, the soil is not important. It is still important but the ecosystem is perhaps the better predicting variable to be used in models because it is easier to observe and has a large effect on the soil properties that actually affect how water moves through the soil.

Other parts of the writing could also be improved. In several sentences words are missing and some other sentences are not clear and should be reformulated. The structure of the paper and individual sections was sometimes unclear to me. For example, section 4.1 consists of four paragraphs. Paragraph one highlights the importance of ET and states that hydrologists focus on discharge instead (but this point was already made on L128). The second paragraph then describes that ecosystems maximize storage and drainage. This section is interesting and fits the caption of this section. One would expect the next paragraph to get deeper into this but the third paragraph describes that the numbers for soil properties used in models don’t match the actual measurement values, and the fourth paragraph describes the rebalancing of soil properties that needs to be done in models. While the first two paragraphs sort of fit together and the last two paragraphs as well, the link between the first two and last two is not obvious. It also means that the second paragraph ends abruptly and this line of thinking could use some more elaboration. In addition, the part on the soil properties and the rebalancing starts abruptly without an introduction. The latter two paragraphs would probably better fit in a separate section on the problematic part of using pedotransfer functions based on texture (see comments above). This is just one example, there are other sections where the flow was unclear and I expect other readers to also wonder how the paragraphs are connected. I made some suggestions in the annotated pdf but there are more places where text could be reordered for a better flow. I don’t request that the authors use the suggested order but I do recommend that they carefully read through the paper to see if the order is logical for a reader.

Oter specific points:

• L56/139: I think that the problematic part of the use of pedotransfer functions based on texture to derive properties about pores should be described in more detail. Especially knowing that these pedotransfer functions were developed based on cores from agricultural fields and that texture does not really influence the hydraulic conducitivity (e.g., Jarvis et al., 2013; Gupta et al., 2021). See also comments above.
• Section 3.1: I don’t think that anyone claims that soil affects the long term water balance more than climate and vegetation. So, I think that it is fine to use this section to highlight that the ecosystem and climate are the main factors that determine the long term water balance but it makes less sense to use this as an argument that soils are not important.
• L129-132: Yes, land use change (if severe) alters runoff generation, exactly because of the large effect it has on soils. So, I don’t think that you can use this argument here to say that soils don’t matter. You can use it to make the argument that vegetation has a large effect on the soil properties that actually matter for water flow and storage. Also, it would be good to reference some field studies here (not only model studies).
• L158: But the comparison is basically between a model and a model with more data. I don’t think that one should call this observations.
• L162: But it also mimics the depth to the groundwater – maybe this has a different effect in the two models?
• L181: The problem is in part that we use texture here. Texture does not describe the soil pores that are important for storage or flow of water. The problem is that we use pedotransfer functions that are largely based on data from agricultural soils and are not appropriate for forested systems. See also the comments above. Furthermore, soil depth data is usually very rough and not very reliable. Maps of soil properties that actually describe water flow and storage are rarely available. Thus, one could also argue that the big problem is that we don’t have soil maps with sufficient information on the properties that actually matter and are related to water flow, and that instead we rely too much on texture and pedotransfer functions.
• L188: I agree that all these processes are intertwined or connected. Therefore, I think that the opinion paper should use more careful wording. It is OK to say that for hydrological modeling it is more useful to look at the ecosystem because the soil properties that matter for hydrology are highly correlated with land cover, and ecosystem properties are much easier to observe or measure. However, if we want to actually understand processes and the factors that affect these processes, it is important to look at the processes. In other words, then we have to look at the partitioning of rainfall into infiltration, overland flow, deeper drainage, etc. and soils are important. I think that this distinction between model application and process understanding should be made more clearly throughout the text.
• Section 4.2: I am sorry but I don’t understand what these ERA5 storage volumes contribute to the arguments of the opinion paper. The volume is one thing, the total flux from repeated filling and emptying is another. Certainly, I agree that the total storage is highest in the root zone but I consider the root zone to be part of the soil. So why is the root zone important but soil not? The paragraph on 277-284 goes some way into explaining this but it could have been added to section 4.1. It would be good if the authors give a definition of soil early in the paper. I have the feeling that often the authors mean soil texture instead of the soil itself.
• Several minor comments and suggestions are given in the annotated pdf.

References:

Jarvis, N., Koestel, J., Messing, I., Moeys, J., and Lindahl, A.: Influence of soil, land use and climatic factors on the hydraulic conductivity of soil, Hydrol. Earth Syst. Sci., 17, 5185–5195, https://doi.org/10.5194/hess-17-5185-2013, 2013.

Gupta, S., Hengl, T., Lehmann, P., Bonetti, S., and Or, D.: SoilKsatDB: global database of soil saturated hydraulic conductivity measurements for geoscience applications, Earth Syst. Sci. Data, 13, 1593–1612, https://doi.org/10.5194/essd-13-1593-2021, 2021.