Size of Sand Grains Controls Pore Structure and Water Dynamics: Implications for Water Retention and Hydraulic Conductivity

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear author, please address the comments given in the attachment

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the valuable comments. 

Abstract:  The abstract was revised to simplify methodological terminology, clarify that the conductivity results refer to unsaturated hydraulic conductivity, and include practical implications related to irrigation management and water conservation in sandy soils.

Introduction section: We added for introduction section line 91 - 99: "While the influence of particle size distribution on soil physical properties is a classic topic, studies that isolate the exclusive effect of sand particle diameter in monodisperse systems remain scarce. Unlike some research focusing on samples with preserved structure or natural mixtures of sand, silt, and clay [2, 5, 12], this work analyzes water dynamics by quantifying changes in pore size distribution, water retention, and unsaturated hydraulic conductivity across a wide range of matric tensions (from 10 to 15,000 hPa). By controlling the variability inherent to fine and organic fractions, this study provides quantitative parameters, allowing for the isolation of how the reduction in particle diameter specifically affects water retention and hydraulic conductivity."

Methods section: In response to the comments, the section was revised to improve the clarity of the experimental design. Additional goodness-of-fit metrics and residual analyses were not included because the modelling approach was primarily intended for comparative interpretation among sand fractions.   Results and discussion section: we have included citations. 

We fully agree that real soil particle packing is irregular and highly heterogeneous, and therefore cannot be fully represented by a single idealized geometric model. In this study, the tetrahedral pore geometry was intentionally adopted as a conceptual and simplified model to describe particle arrangement and to provide a theoretical reference for estimating the relative magnitude of pore diameters generated by different sand grain-size classes.

We edited discussion to “The other sand fractions showed intermediate α values, with fitted models presenting similar parameter estimates, ranging from 10 to 11 hPa.” We also revised the model fitting procedure and confirmed that the adjustment consistently results in these extreme parameter values for the very fine sand fraction. Although these parameters differ markedly from the other fractions, they reflect the distinct shape of the retention curve observed for this treatment and are not due to fitting errors. Therefore, the reported values were maintained as they represent the best fit obtained for the experimental data.

We revised and rewrote the two paragraphs in Section 3.3 to reduce redundancy and avoid repeated literature comparisons, improving clarity and conciseness of the Results and Discussion.

We revised the manuscript to include a clearer quantitative interpretation of the Kr curves and added the missing conductivity units in the text and figure caption. We also clarified that Kr values are shown on a logarithmic scale. Statistical comparison among full Kr curves was not performed because the curves were model-based estimations rather than direct replicated measurements at each matric tension. Please see the changes included in the section 3.4- Unsaturated hydraulic conductivity (Kr) by Van Genuchten.

We have edited graphs and tables. 

Conclusion section: We added conclusion number 5 and 6. 

Reference section: We have verified.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Review comments on land-4280486

The study presents a controlled experimental study investigating the influence of sand grain size on pore size distribution, water retention, aeration, and unsaturated hydraulic conductivity in monodisperse systems. The use of monodisperse sand fractions enables the authors to isolate the effect of particle size, a rare achievement in field-based studies. The results are generally consistent and clearly show the influence of particle size on pore distribution and hydraulic behavior. However, despite these strengths, the manuscript has the following shortcomings that must be addressed before it can be considered for publication in Land.

1. The manuscript states in the Introduction, Line 75, that such studies are “still in their infancy”, but this claim is not convincingly supported with a critical review of recent literature. Please clearly define what is new; is it the magnitude of effects? The quantitative relationships? The behavior under high matric tensions? Also, provide a critical comparison with recent studies (last 5–10 years) and avoid presenting well-known trends as novel findings
2. The study is entirely based on monodisperse quartz sand, which is useful for isolating physical processes but highly artificial compared to real soils. The manuscript acknowledges that natural systems are polydisperse, but this limitation is not sufficiently integrated into the discussion. The conclusions are presented in a way that implies direct applicability to sandy soils, which may overextend the applicability of the results. Please clearly separate the experimental findings (ideal system) from the real-world implications (complex soils), and discuss how silt, clay, and organic matter would modify the results. Also, add a dedicated subsection on limitations and applicability
3. The manuscript frequently describes trends but does not always provide deep mechanistic explanations. For example, in pore formation, the tetrahedral packing model is used, but it assumes ideal spherical particles, ignores particle shape irregularities, and does not account for packing variability. Similarly, in hydraulic conductivity, the discussion correctly states that flow is proportional to the fourth power of pore radius but does not sufficiently explain why very fine sand maintains higher conductivity at high tension, nor does it discuss connectivity vs. pore-size trade-offs. In adsorption vs capillarity, the manuscript mentions both mechanisms but does not quantify their relative contributions or explain transitions across tension ranges. Please strengthen process-based explanations and distinguish between geometric effects and physicochemical effects.
4. One of the most striking results is that very fine sand shows higher unsaturated hydraulic conductivity at high matric tensions. While this is plausible, the explanation is not fully convincing. The manuscript attributes this to better pore distribution, capillarity and adsorption. However, finer materials typically reduce conductivity due to smaller pores, and the balance between pore continuity vs pore size is not discussed. Please provide a deeper explanation of why fine sand maintains flow under dry conditions and compare with classical soil hydraulic theory. Also, discuss whether this is a modeling artifact or a physically robust phenomenon.
5. The manuscript does not sufficiently translate the findings into practical implications. Given the scope of Land, the authors should better connect the findings to land management, irrigation scheduling, sandy soil amendment, drought-prone agricultural systems, and soil-water conservation strategies.
6. The manuscript is conceptually rich but lacks synthesis diagrams and conceptual models. Please add a diagram of particle size → pore distribution → water behavior and a conceptual model linking sand size, pore classes and hydraulic response.

Author Response

We thank the reviewer for the positive evaluation of the manuscript. 

1. The manuscript states in the Introduction, Line 75, that such studies are “still in their infancy”, but this claim is not convincingly supported with a critical review of recent literature. Please clearly define what is new; is it the magnitude of effects? The quantitative relationships? The behavior under high matric tensions? Also, provide a critical comparison with recent studies (last 5–10 years) and avoid presenting well-known trends as novel findings

Included: While the influence of particle size distribution on soil physical properties is a classic topic, studies that isolate the exclusive effect of sand particle diameter in monodisperse systems remain scarce. Unlike some research focusing on samples with preserved structure or natural mixtures of sand, silt, and clay, this work analyzes water dynamics by quantifying changes in pore size distribution, water retention, and unsaturated hydraulic conductivity across a wide range of matric tensions (from 10 to 15,000 hPa). By controlling the variability inherent to fine and organic fractions, this study provides quantitative parameters, allowing for the isolation of how the reduction in particle diameter specifically affects water retention and hydraulic conductivity.

2. The study is entirely based on monodisperse quartz sand, which is useful for isolating physical processes but highly artificial compared to real soils. The manuscript acknowledges that natural systems are polydisperse, but this limitation is not sufficiently integrated into the discussion. The conclusions are presented in a way that implies direct applicability to sandy soils, which may overextend the applicability of the results. Please clearly separate the experimental findings (ideal system) from the real-world implications (complex soils), and discuss how silt, clay, and organic matter would modify the results. Also, add a dedicated subsection on limitations and applicability

Conclusion: Reviewed

Limitations and implications for complex soils

While our monodisperse system isolated the fundamental physical effects of sand grain size on water retention and hydraulic conductivity, it is important to acknowledge that soils are polydisperse (clay, silt, and sand) and structurally complex. Our findings provide the physical foundation for the sand matrix, but the inclusion of finer particles (silt and clay) and organic matter in natural soils modify these results by altering pore geometry and adding water retention mechanisms (capillarity and adsorption). The presence of these finer components would likely increase total water retention and change the hydraulic conductivity compared to our monodisperse quartz sand, as they contribute to pore clogging and enhance surface charge interactions. Therefore, while our results clarify the contribution of sand fractions, they should be applied to natural soil simulations as a baseline, considering the interactive effects of soil structure and clay and silt fractions."

3. The manuscript frequently describes trends but does not always provide deep mechanistic explanations. For example, in pore formation, the tetrahedral packing model is used, but it assumes ideal spherical particles, ignores particle shape irregularities, and does not account for packing variability.

Included - "Tetrahedral packing is the most predominant arrangement in granular media subjected to sedimentation and mechanical settling, as it minimizes void space and maximizes particle contact points, thereby achieving a state of greater mechanical stability."

Similarly, in hydraulic conductivity, the discussion correctly states that flow is proportional to the fourth power of pore radius but does not sufficiently explain why very fine sand maintains higher conductivity at high tension, nor does it discuss connectivity vs. pore-size trade-offs.

Included - "The higher hydraulic conductivity observed in finer-textured fractions under high matric tensions is sustained by enhanced capillary and adsorptive forces, which facilitate the maintenance of continuous water films at solid-liquid interfaces; this preserves hydraulic pathways in tension regimes where the dominance of macroscopic capillary forces in coarser fractions would otherwise lead to hydraulic disconnection."

In adsorption vs capillarity, the manuscript mentions both mechanisms but does not quantify their relative contributions or explain transitions across tension ranges. Please strengthen process-based explanations and distinguish between geometric effects and physicochemical effects.

Included - "Near saturation, water flow is governed by volumetric capillary forces in water-filled pores, yielding high hydraulic conductivity regardless of texture. As the soil dries and capillary continuity breaks down, transport becomes predominantly dependent on adsorbed water films at particle surfaces—a mechanism that is more efficient in finer fractions due to their higher specific surface area."

4. One of the most striking results is that very fine sand shows higher unsaturated hydraulic conductivity at high matric tensions. While this is plausible, the explanation is not fully convincing. The manuscript attributes this to better pore distribution, capillarity and adsorption. However, finer materials typically reduce conductivity due to smaller pores, and the balance between pore continuity vs pore size is not discussed. Please provide a deeper explanation of why fine sand maintains flow under dry conditions and compare with classical soil hydraulic theory. Also, discuss whether this is a modeling artifact or a physically robust phenomenon.

Included - "Contrary to classical expectations where finer textures exhibit lower K due to reduced pore sizes (according Poiseuille flow is proportional to the fourth power of the pore radius [3])Hagen-Poiseuille scaling), the higher unsaturated hydraulic conductivity in very fine sand at high matric tensions (>1000 hPa) reflects a phenomenon driven by enhanced pore connectivity and sustained capillary-adsorptive water films, which maintain continuous flow pathways despite smaller individual pore radii-consistent with theoretical models emphasizing network tortuosity over isolated pore geometry."

5. The manuscript does not sufficiently translate the findings into practical implications. Given the scope of Land, the authors should better connect the findings to land management, irrigation scheduling, sandy soil amendment, drought-prone agricultural systems, and soil-water conservation strategies.

Included - "These findings on sand grain size effects have direct implications for land management in drought-prone sandy soils. Understanding these differences enables technicians and farmers to more effectively plan appropriate management systems for specific soil types, while improving irrigation efficiency and water conservation to optimize water delivery during critical dry periods.”

6. The manuscript is conceptually rich but lacks synthesis diagrams and conceptual models. Please add a diagram of particle size → pore distribution → water behavior and a conceptual model linking sand size, pore classes and hydraulic response.

Included.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Line 186:  Results/discussion begins with Bulk Density?  I don’t recall reading about bulk density in the introduction or conclusions, and there is one word in the methods.  The beginning paragraph should be about the most important finding, or a factor that pulls the elements together.  I suggest moving the first section of the Results/discussion to the last section, which contributes to the explanation of the results. 

Figure 2:  awesome.  From the diagram, you could deduce that there is a ‘breakthrough point’ within a range of soil pore size.  This could be confirmed analytically, which would add to the strength of the paper. 

Line 301:  Reference (11)  This sentence would be more meaningful if you included the first author “Fidalski et al. (11), …..”

Figure 6:  The vertical axis (Kr log) does not appear to be a log scale, however, the Tension from 1 to 1 x 10^4 does appear to be log transformed.   Also, in English, the units are written as 1.0 – 2.0 mm (rather than commas).

 

Author Response

We thank the reviewer for the positive evaluation of the manuscript. 

Line 186:  Results/discussion begins with Bulk Density?  I don’t recall reading about bulk density in the introduction or conclusions, and there is one word in the methods.  The beginning paragraph should be about the most important finding, or a factor that pulls the elements together.  I suggest moving the first section of the Results/discussion to the last section, which contributes to the explanation of the results.

Corrected - Regarding the Introduction: We agree that Bulk Density should be introduced earlier. Following your suggestion, we have added a paragraph in the Introduction (Lines 75-77) explaining the relationship between sand particle size distribution and bulk density: Included -  "Furthermore, the sand particle size distribution directly influences soil bulk density, as particles of different sizes affect not only the total porosity and pore size distribution but also the arrangement of the soil matrix, which governs the overall bulk density."

While we appreciate the suggestion to move the BD section to the end, we have opted to maintain its original position at the beginning of the Results and Discussion. We believe that BD is a primary physical/structural property that fundamentally governs pore arrangement and, consequently, the hydraulic conductivity discussed later, presenting it first provides the necessary physical foundation for the reader to interpret the subsequent results.

 

Figure 2:  awesome.  From the diagram, you could deduce that there is a ‘breakthrough point’ within a range of soil pore size.  This could be confirmed analytically, which would add to the strength of the paper.

Response to Reviewer (Figure 2): We thank the reviewer for the positive feedback. We performed regression on the pore size data, identifying a breakthrough point at - the transition from drainage macropores to retention micropores. This analysis is discussed in lines [215-219] of Results/Discussion. Included - “Piecewise regression analysis of pore size distribution identified a breakthrough point at 10 kPa (Figure 2), marking the critical transition from drainage-dominated macropores to retention-dominated micropores (60 kPa), which explains the observed shifts in soil water dynamics across granulometric fractions.”

 

Line 301:  Reference (11)  This sentence would be more meaningful if you included the first author “Fidalski et al. (11), …..”   

Corrected: “This result aligns with field studies such as Fidalski et al., who, working with sandy soils under different cropping systems, found greater water availability where the fine sand fraction predominated.”

 

Figure 6:  The vertical axis (Kr log) does not appear to be a log scale, however, the Tension from 1 to 1 x 10^4 does appear to be log transformed.   Also, in English, the units are written as 1.0 – 2.0 mm (rather than commas).

Corrected: Regarding the Y-axis in Figure [6]: To clarify the presentation of our data, please note that the Y-axis represents log-transformed values (labels such as 0, -5, -10, etc.), but it is plotted on a linear spatial scale rather than a logarithmic coordinate system. This was done to facilitate the visual comparison of the magnitudes of change across different sand fractions while maintaining a consistent spatial distribution of the data points.

We added this observation in the Title: “The Y-axis displays log-transformed values plotted on a linear scale.”

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The majority of my comments well addressed except the modification of the article title. I agreed to be published in the journal after a few editorial works to be addressed in the production stage

Author Response

 

Thank you, we have changed the title and some values of Table 1 were revised according to the reviwer´s suggestions.

 

 

Author Response File: Author Response.pdf