the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Aug 2026
Plant Effective Conductance Fails to Predict Soil–Plant Hydraulics in Sandy Soils
Abstract. Atmospheric and soil dryness impose increasing limitations in the hydraulic pathway of the soil–plant–atmosphere continuum (SPAC), leading to a decrease in the hydraulic conductivity of many elements, such as soil, soil–root interface, xylem, and leaves. Many SPAC models simplify the flow through this series of hydraulic conductances to a single effective conductance that varies as a function of the xylem water potential. This effective conductance is expected to implicitly account for soil characteristics, including soil texture-specific hydraulic limitations. However, it is unclear whether such an approach can properly reproduce the non-linear effects of soil texture-specific conductivities. This paper aims to explore under what conditions reducing the SPAC to a function of internal plant water status accurately captures the relation between transpiration rate and leaf and soil water potential, and when larger model complexity is required. For this purpose, a plant-only model, where the conductances along the SPAC solely depend on plant water potential, was compared to a more detailed model incorporating the hydraulics of varying soils. We demonstrate that the former simplification works well in fine-grained soils, such as clay and silt loam, but fails in more coarse-textured soils like sandy loam, where the pronounced non-linearity of its hydraulic functions requires explicit description. Additionally, a sensitivity analysis of key parameters within the full SPAC model reveals that the soil hydraulic properties have a significantly stronger impact on plant transpiration in sandy soils, as opposed to finer soils, where internal plant traits are more influential on plant–water relations.
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Status: open (until 30 Oct 2026)
- CC1: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 18 Sep 2026 reply
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CC2: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 18 Sep 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-5107/egusphere-2026-5107-CC2-supplement.pdf
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CC3: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 21 Sep 2026
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4) The source code archived for this manuscript is currently not runnable due to a file path mismatch. For example, in the file hydraulicmodel_comparison.py, the code attempts to load data using the following line: with open('2_Data/soil_Data.json') as f:
However, the actual folder structure provided does not match this path. The file soil_Data.json is located inside the 3_Data folder rather than 2_Data. This path discrepancy causes the model to crash immediately upon execution. The authors must update the archived repository submitted to the journal to ensure it contains a fully functional, runnable version of the code that successfully reproduces the results presented in the manuscript.
5) The hardcoded limits for the bulk soil water potential array (h_b) artificially restrict the model and do not account for the specific soil types (Clay, Silt Loam, Sandy Loam) evaluated in the manuscript. In hydraulicmodel_comparison.py, the array is defined as: h_b = -1 * np.arange(10, 15010 + 100, dx, dtype=int)
This implementation restricts the model evaluation strictly to a cutoff of -15,010 cm. While -15,010 cm is conventionally used as a biological threshold for the permanent wilting point, it does not represent the physical endpoint of the soil water retention curve during extended simulations, drying phases, or evaporation blocks. While a Sandy Loam might dynamically stay close to the -15,000 cm range when dry, fine-textured soils like Silt Loam routinely extend to -30,000 cm, and Clay soils easily sustain matrix potentials moving far past -100,000 cm due to tight adsorption forces in micropores. By forcing an identical, rigid -15,000 cm boundary across all soil types, the simulation treats these diverse soil parameters as a static mathematical box rather than a dynamic environmental system. The authors must modify the code to dynamically scale h_b according to the specific, realistic hydraulic bounds of each soil type being tested to ensure the code matches the work presented in the manuscript.
6) There is a significant mismatch between the implementation of the leaf water pressure head calculation in the source code and Equation (7) presented in the manuscript.
Within the Calc_hleaf(E, h_b, params_plant) function, the final line of code executes as follows: h_leaf = -1 * np.abs(E / c_x + h_x.astype(np.complex128)**(1 - tau_x))**(1 / (1 – tau_x))
This implementation introduces two distinct discrepancies relative to Equation (7):
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Incorrect Reference Potential: The final Python statement passes h_x (the calculated potential at the root-xylem junction/stem bottom) into the leaf calculation. However, Equation (7) in the text explicitly uses h_root (the bulk soil/root boundary potential).
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Inconsistent Operator Signs: The terms within the modulus in the code are added together (E / c_x + h_x...), whereas Equation (7) in the manuscript dictates that these terms should be subtracted.
Because Equation (7) bypasses h_x entirely, it neglects the crucial potential drop that occurs across the root domain before entering the stem xylem. The code's approach of basing the leaf potential on h_x is physically logical; therefore, it is highly likely that Equation (7) in the manuscript is mathematically incorrect and poorly formulated. The authors must carefully revise the manuscript text to ensure that Equation (7) is mathematically accurate, consistent with the physical principles of the soil-plant hydraulic model, and perfectly aligned with the actual code execution.
Citation: https://doi.org/10.5194/egusphere-2026-5107-CC3 -
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CC4: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 21 Sep 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-5107/egusphere-2026-5107-CC4-supplement.pdf
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CC5: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 23 Sep 2026
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7) The mathematical formulation of the underlying physics is detailed extensively in Section 2.1 of the current manuscript. However, it is noted that Figure 1 already incorporates almost all of the detailed mathematical formulations discussed within this section. Consequently, presenting the exact same equations in both the body text and the visual schematic introduces an unnecessary layer of repetition.
The rationale for keeping such an identical, dual formulation across both the text narrative and the figure layout is not clear. To improve readability, streamline the manuscript, and eliminate this redundancy, the authors should decouple these elements. It is highly recommended to either scale back Section 2.1 to focus strictly on the conceptual physical mechanisms and governing assumptions or strip the heavy mathematical symbols from Figure 1 so it serves purely as a conceptual flowchart.
8) The "Author Contributions" section indicates that the primary author (EW) performed the conceptualization, formal analysis, methodology, software development, visualization, and preparation of the original draft, while the co-authors (FJPW, SDB, AC) primarily provided supervision, funding acquisition, and editorial review.
If the primary author is a postgraduate or graduate student and this manuscript constitutes a core requirement for their degree completion, the inclusion of supervisors as formal co-authors raises a critical question regarding academic integrity and institutional evaluation procedures. A postgraduate degree is explicitly awarded based on a candidate's capacity to demonstrate rigorous, independent research. Because research supervisors frequently serve as the internal evaluators or committee members responsible for assessing the student's independent competency, claiming co-authorship on the primary degree output introduces a conflict of interest, potentially clouding the objectivity of the academic evaluation process.
Furthermore, standard international publishing ethics dictate that providing routine supervisory feedback, proofreading, or editing does not inherently justify peer-reviewed authorship credit. Extending authorship on these grounds sets a problematic precedent that could logically extend to journal reviewers and editors who provide structural edits.
To ensure full transparency, prevent supervisory favoritism, and preserve the integrity of the degree-granting process, the handling editor should require a formal, signed statement from the institutional supervisors. This declaration must explicitly verify that this specific manuscript is entirely independent of the first author’s primary degree requirements and graduation claims. The handling editor should carefully scrutinize this declaration to ensure that the work represents a fair, uncompromised evaluation of the student's research output.
9) In the current implementation of the forward simulation framework (HydraulicModel_comparison.py), the plant hydraulic calibration parameters are constrained by the following optimization boundaries defined within the run_de() function:
Bounds = [R_root: (0.03 * 10^7, 0.15 * 10^7), h0_x: (-27000, -15000), tau: (3, 9)]
However, a structural inconsistency arises when evaluating these bounds against the driving gradient defined in the same script:
h_b = -1*np.arange(10,15010+100,dx, dtype=int)
If the global optimization engine samples an h0_x value deep within the prescribed bounds (e.g., -25,000 cm), it parameterizes the plant to withstand extreme hydraulic tensions that the simulated bulk soil water potential matrix (h_b) never experiences. Consequently, the governing non-linear xylem cavitation equations are never physically triggered or evaluated during the forward simulation timeline. Because h0_x remains outside the active operating envelope of the soil drying sequence, the objective function loses sensitivity to this parameter. Therefore, to establish a mathematically unconstrained yet physically responsive calibration space, the search bounds for h0_x must directly intersect the window where the plant experiences severe, physiologically relevant drought stress.
Citation: https://doi.org/10.5194/egusphere-2026-5107-CC5 -
CC6: 'Comment on egusphere-2026-5107', Sivarajah Mylevaganam, 23 Sep 2026
reply
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-5107/egusphere-2026-5107-CC6-supplement.pdf
Model code and software
Plant Effective Conductance Fails to Predict Soil–Plant Hydraulics in Sandy Soils Eileen L. Waeber https://doi.org/10.5281/zenodo.22093435
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- 1
It is a well-known fact that human survival undoubtedly relies on the survival of plants and trees for food production, respiration, natural filtration, and cooling functions. Therefore, research on plants and trees—specifically their critical survival factors—constantly grows in importance due to humanity's reliance on them.
In this research, the authors study water uptake by plants through a series of conductance. In other words, since the exact nature and mechanism of plant water uptake from soil is unknown, the authors evaluate two distinct models. The first is a plants-only model, which assumes that the hydraulics of varying soil types are not important. The second model considers both plant and soil hydraulics to create a fully defined framework, acknowledging that the actual underlying science remains partially unknown. Based on these models, the authors claim that the plants-only model works well in fine-grained soils such as clay and silt loam, but fails in coarser-textured soils like sandy loam. Based on the current version of the manuscript, the following points are brought forward for discussion:
1) There is a distinct lack of alignment regarding the designated soil classification across different sections of the paper. The title of the manuscript asserts that plant effective conductance fails to predict soil-plant hydraulics specifically in "sandy soils." Conversely, the abstract narrow-focuses this claim, specifying that the model fails in coarser-textured soils like "sandy loam" [Lines 10–11].
According to standard soil taxonomy, "sandy soil" is a generic broad class, whereas "sandy loam" represents a specific texture class with a distinct sand-silt-clay ratio. This mismatch introduces ambiguity regarding the true experimental boundaries and definitive outcomes of the study. The authors must harmonize the terminology across the title, abstract, and text to accurately reflect the exact soil textures tested.
2) Throughout the manuscript (e.g., figure 1, line 146, and line 153), the authors consistently distinguish between their two frameworks by calling them the "simple model" and the "complex model." Labeling frameworks as "simple" or "complex" introduces an unnecessary layer of subjectivity into a scientific text; complexity is relative and varies based on a reader's perspective. To establish a more structured and rigorous presentation, the authors should abandon these subjective descriptors. Instead, the frameworks should be renamed based on their underlying physics and integrated components—for instance, the "plants-only model" versus the "coupled soil-plant hydraulic model."
3) In the plants-only model [lines 154-155], the authors implement an assumption where ks approaches infinity. However, an explicit, formal definition of the parameter ks is entirely missing from the current text. Furthermore, the manuscript asserts that as ks tends to infinity, the root-surface soil-root interface water pressure head (hs,r) collapses to the bulk soil water pressure head (hs,b) [Lines 154-155].
It is presumed that this logic is derived from Equation (5). In Equation (5), if the parameter Cs is mathematically proportional to the saturated hydraulic conductivity (ksat), root length (Lroot), and supplementary architectural parameters, then an infinite ksat will cause Cs to approach infinity. Within the operational formula for hs,r, an infinite denominator drives the term E/Cs to zero, thereby simplifying the expression to hs,r = hs,b. If this represents the authors' underlying derivation, it establishes that ks is functionally utilized as the saturated hydraulic conductivity in Equation (5). The authors must explicitly clarify whether this is the intended mechanism. Furthermore, they must provide the explicit mathematical steps and parameter definitions that justify the assumption that hs,r = hs,b, as these formal derivations are currently absent.