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the Creative Commons Attribution 4.0 License.
| 23 Jan 2026
Water storage and redistribution effect evaporation, retention, and infiltration of forest floor sites
Abstract. The forest floor (FF) possesses a significant water retention capacity, facilitating the transfer of water between the atmosphere and the soil. However, knowledge on the water retention characteristics and water redistribution effects of the FF remain limited. Due to the dominance of laboratory data regarding the storage capacity of a forest’s litter layer, we used a combined FF weighted grid-lysimeter and soil moisture network to directly and in-situ measure the dynamics of water storage of the FF and fluxes from and into the FF. The objective was to quantify storage capacities, retention durations, and resulting water redistribution patterns, as well as evaporation from the FF. We present the results of our network at three sites with different altitudes located in the Black Forest, southwest Germany. The three sites have an annual mean temperature gradient from 6.3 °C to 10.3 °C, leading to humus forms that vary from typical F-Mull to typical Moder. Throughout the monitored period in 2024–2025, the storage capacity of the FF ranged between 1.4 and 4.2 g/g FF and was not only influenced by the type of litter but also by the rainfall characteristics themselves. With our field setup we could show that longer, low intensity rainfall events fill the FF storage more efficiently than shorter heavy rainfall events (−24 %). Our gridded lysimeter design revealed small-scale spatio-temporal infiltration patterns, caused by a redistribution of rainfall along the passage through the FF. The findings of the lysimeter network provide a comprehensive understanding of the influence of the FF mass on the water cycle within forest ecosystems.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-284', Anonymous Referee #1, 23 Feb 2026
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RC2: 'Comment on egusphere-2026-284', Seyed Mohammad Moein Sadeghi, 23 Apr 2026
General comments:
This manuscript presents an interesting and valuable field-based investigation of forest floor (FF) water storage, retention, evaporation, and redistribution, using a novel grid-lysimeter approach across three sites in the Black Forest. The dataset is rich (1,570 events), and the in situ measurement of FF processes is a clear strength, addressing a well-known gap in the literature that has long been dominated by laboratory-based studies.
Importantly, the study also brings much-needed attention to forest floor hydrology—an aspect of the water balance that is often overlooked or, in some cases, not explicitly considered in ecohydrological analyses. From my perspective as someone who works extensively in forest systems, I strongly disagree with this tendency. The forest floor is not a passive layer; it is a dynamic interface that regulates water storage, delays runoff, and mediates energy exchange. In that sense, I particularly appreciate the authors’ effort to quantify these processes under field conditions and to elevate the role of the forest floor within the broader hydrological framework.
The manuscript would benefit from clearer terminology regarding the measured compartment. At present, “forest floor,” “litter layer,” and “organic layer” are used somewhat interchangeably in the Introduction and Methods, whereas the lysimeter setup appears to include not only the organic horizons but also a portion of the upper mineral soil. Because this distinction can influence the interpretation of storage, drainage, and evaporation dynamics, I encourage the authors to clarify exactly what component is being measured throughout the manuscript and to use the terminology consistently. In particular, please clarify whether the reported Cmin and Cmax values should be interpreted as properties of the litter layer, the full forest floor, or the combined organic–mineral surface layer represented in the lysimeter design.
The manuscript compares hydrological responses across the three study sites (e.g., Figure 7); however, these sites differ not only in altitude but also in precipitation regimes, temperature, and environmental conditions, which are key drivers of the observed fluxes. While this comparison is informative, it would be helpful to clarify whether the results are intended to be descriptive (process-based) or statistical comparisons of site characteristics. This distinction is particularly important because differences among sites may reflect not only intrinsic forest floor properties, but also variations in forcing conditions (e.g., rainfall amount, intensity, and duration). This point becomes especially relevant in Figure 7, where statistical differences in Cmin and Cmax among sites are presented using t-tests. Given the differing precipitation regimes, it would be useful to clarify how these comparisons should be interpreted, and whether the observed differences are driven primarily by site properties or by differences in precipitation forcing. A brief clarification or discussion of this aspect (e.g., whether comparisons are normalized, event-based, or influenced by precipitation variability) would strengthen the interpretation of the results.
Overall, this is a strong and well-executed study with a rich dataset and clear relevance to forest ecohydrology. The manuscript would benefit from some targeted clarifications and modest improvements in conceptual framing and methodological explanation, but these are minor and can be addressed without substantial reanalysis. I therefore recommend major revision.
Specific comments (line-by-line / section-based)
** Title:
1. The title reflects the general scope of the study; however, I found it somewhat difficult to follow on first reading. The phrasing is not fully clear (e.g., “effect” vs. “affect,” and “forest floor sites”), and the sentence structure makes it harder to quickly grasp the main message. Beyond identifying issues, I believe it is helpful to suggest possible improvements. I therefore encourage the authors to revise the title for clarity and readability, using a more direct structure that clearly highlights the key processes and the role of the forest floor. For example, a formulation such as:
“Forest floor water storage and redistribution affect evaporation, retention, and infiltration in mixed temperate forests”
may improve clarity and better reflect the ecological context of the study. That said, this is only a suggestion—the authors may choose an alternative phrasing. From a forest management perspective, explicitly indicating mixed stands (or mixed temperate forests) could be valuable, as hydrological processes in mixed systems are often more complex and less studied than in pure stands.** Abstarct:
2. First, I suggest introducing the ecological context more explicitly at the beginning of the abstract. In particular, mentioning that the study was conducted in mixed Picea abies and Fagus sylvatica stands would provide important context and improve clarity from the outset. Second, the description of the study sites as “three sites with different altitudes located in the Black Forest” is somewhat misleading. Based on Table 1, the sites differ not only in altitude, but also in temperature and precipitation regimes, which are critical drivers of hydrological processes. I recommend revising this statement to better reflect the full range of environmental gradients represented in the study. Third, there are a few minor language issues (e.g., “effect” should be “affect”), and some sentences could be streamlined for better readability. Finally, I do not fully agree with the last sentence of the abstract. It is currently too general and does not reflect a specific contribution of this study. In its present form, it reads more like a broad statement that could be made independently of the results. I recommend revising this sentence to more clearly emphasize the novel findings or specific insights derived from your dataset and experimental approach.
** Intro:
3. Several aspects would benefit from clarification and refinement to strengthen the conceptual framing and improve the overall impact. The manuscript would benefit from clearer and more consistent use of terminology at the beginning of the Introduction. Terms such as “forest floor,” “litter layer,” and “organic layer” are used closely and at times interchangeably, although they can represent different components of the soil profile. I recommend that the authors explicitly define these terms early in the Introduction, and clearly state which component is the focus of the study. This is particularly important because the hydrological behavior of the litter layer can differ from that of the entire forest floor, and later in the Methods, the measurement system appears to include not only organic material but also part of the upper mineral soil.
4. L 30: While the Introduction summarizes relevant literature effectively, the research gap is not yet clearly articulated. The manuscript notes that field-based data are limited, but it would strengthen the paper to explicitly state: what specific limitations exist in previous studies (e.g., lack of in situ, event-scale, or spatially distributed measurements), and how the present study addresses these limitations.
5. L 43-44: The statement that “broadleaf litter has a larger interception capacity than needle litter due to its higher surface area-to-weight ratio” is supported by laboratory-based studies at the leaf scale. However, I encourage the authors to be more cautious in presenting this as a general conclusion. At the forest floor scale, interception and storage are not only controlled by leaf morphology, but also by litter accumulation, decomposition rates, and structural properties of the litter layer. For example, needle litter often decomposes more slowly and can form thicker and more porous layers, which may result in greater overall water storage at the ecosystem scale, even if the per-mass storage capacity is lower. I suggest clarifying the scale of interpretation (e.g., per unit mass vs. per unit area or whole-layer storage) or slightly rephrasing this statement to avoid overgeneralization.
6. L 59-62: The hypotheses are clearly stated and relevant. However: Hypotheses (1) and (2) are somewhat overlapping (both related to forest floor thickness and storage), and could be streamlined. Hypothesis (4) (“barrier for evaporation”) would benefit from a slightly stronger mechanistic framing (e.g., reference to capillary barrier effects or vapor transport limitations).
7. L 64: I recommend removing it to improve the structure of the manuscript.
** Methods:
8. Table 1: Table 1 provides useful background information; however, it would benefit from additional site descriptors to improve clarity and reproducibility. I recommend including the geographic coordinates (latitude and longitude) for each site, ideally representing the center point of the study area. In addition, including topographic information, such as the mean slope (or slope range), would be valuable, as slope is an important control on hydrological processes (e.g., runoff, infiltration, and lateral redistribution).
9. The manuscript refers to the study sites as mixed stands, but the stand composition is not sufficiently described. Please clarify the proportion of each species (e.g., percentage of Fagus sylvatica vs. Picea abies) at each site. This could be reported based on tree frequency, basal area, or another standard forest metric. This information is important because different mixtures (e.g., beech-dominated vs. spruce-dominated stands) can result in distinct canopy structures and, consequently, different hydrological responses.
10. In addition to stand composition, I recommend including information on canopy structure, such as canopy cover and/or leaf area index (LAI), if available. These variables are particularly important because they strongly influence understory rainfall partitioning (e.g., throughfall and stemflow) and are also indirectly related to forest floor development and litter accumulation, which are key controls on interception and storage processes.
11. L 70: The manuscript refers to humus forms such as F-Mull and Moder, which are well-established classifications in European forest ecology. However, these terms may not be familiar to all international readers, particularly those working in North America or other regions where different classification systems are more commonly used. I recommend briefly clarifying the key characteristics of these humus forms in the main text (e.g., differences in litter decomposition, thickness, and organic matter structure), so that readers can better interpret their hydrological implications. In addition, the authors may consider providing a more detailed description or classification framework in the Supplementary Material, for readers who are less familiar with this system.
12. L 81: The manuscript states that the lysimeters contain the forest floor organic layer together with the upper mineral soil (top ~15 cm). While this is clearly described, it introduces some ambiguity in the interpretation of results. Please clarify whether the system is intended to represent: the litter layer, the entire forest floor, or a combined organic–mineral surface layer.
13. L 75-90: Please clarify how was disturbance minimized during installation?
14. Table 2 is useful but could be moved to the Supplementary Material, as it serves mainly as supporting information. This would improve the flow of the manuscript.
15. L 106: There is a typo erorr: May is correct.
16. L 115: The assumption that negative storage change (ΔS < 0) in the absence of drainage represents evaporation is reasonable, but I recommend briefly acknowledging potential sources of uncertainty, such as: load cell measurement noise, OR temperature effects on sensor readings OR possible small, unrecorded drainage fluxes
17. L 139: The criteria used to define precipitation events (threshold and 6-hour separation) are clearly described; however, it would be helpful to clarify whether these thresholds were selected based on prior studies or tested within this dataset. In addition, under closed canopy conditions, the forest floor can remain wet for several days after rainfall, suggesting that a 6-hour dry period may not fully represent hydrologically independent events, particularly with respect to storage and pre-event moisture.
18. Figure 2: I suggest adding a short sentence in the text explicitly linking this conceptual framework to how these phases are quantified in the dataset (e.g., using storage change, drainage, and timing thresholds).19. The manuscript includes several statistical comparisons (e.g., t-tests); however, a dedicated Statistical Analysis subsection is currently missing. I recommend adding a short subsection at the end of the Methods to clearly describe: the statistical tests used, assumptions (e.g., normality, independence), significance levels, and the software or packages applied.** Results:20. I recommend briefly explaining why this event was selected (e.g., representative, average, or specific hydrological characteristics). Without this clarification, it is unclear how representative this example is relative to the full dataset (1,570 events).21. Table 3 (and so on): If abbreviations are used in figures or tables (e.g., PTF, CE, CM), please ensure that they are clearly defined in each figure or table caption (or as a footnote). Figures and tables should be able to stand alone, and readers should not need to refer back to the main text to interpret abbreviations.22. In Figure 3, the blue bars are labeled as “Precipitation,” whereas in the text and Table 3 the corresponding variable is referred to as canopy throughfall (PTF). This creates some ambiguity, as precipitation and throughfall represent different hydrological inputs. Given that the measurements are conducted below the canopy, it appears that the figure is showing throughfall rather than gross precipitation.23. Figure 5 (and whole parts of manuscript): In Section 3.2 (and Figure 5), the term “event size” is used to describe rainfall characteristics. I recommend revising this terminology to “rainfall amount” or “event magnitude”, which are more standard and precise in hydrological studies. The term “size” may be ambiguous and could be misinterpreted (e.g., as referring to drop size or spatial extent), whereas “rainfall amount” more clearly reflects the total precipitation depth (mm) associated with each event.24. L 203 (and table 1): The manuscript reports retention values in g/g, but in the text it is also written as “g/g mm,” which is unclear and potentially inconsistent.25. Table 4: Please use one consistent term (“water retention” or "retention") throughout.26. L 206: Please indicate which figure or table supports the statement regarding increased retention with higher precipitation totals.27. L 210: The statement “Given the absence of significant differences in retention amounts and durations among the three sites” requires clarification. The three study sites differ in key climatic drivers, particularly precipitation amount, intensity, and event characteristics (e.g., duration and intra-event variability). These factors directly influence retention dynamics. Therefore, direct statistical comparison of retention metrics across sites may not be fully appropriate without accounting for these differences in forcing conditions. For example, even if the number of events is similar across sites, the events themselves may differ substantially in total rainfall, intensity (including short-term intensities), and duration, which can strongly affect retention behavior. I recommend clarifying how these differences were addressed in the analysis (e.g., normalization, event-based comparison, or inclusion of precipitation characteristics as covariates), or revising the statement to reflect that comparisons are descriptive rather than strictly comparable across sites.28. In Figure 6, the x-axis labels (“yes” and “no”) are not sufficiently self-explanatory. Please clearly define the meaning of these categories in the figure caption (e.g., whether they refer to the occurrence or absence of initial rainfall retention). Given that many readers may come from different disciplines or may be students, the figure should be fully interpretable without requiring prior familiarity with the terminology or referring back to the main text.29. Fig 6: The x-axis label in Figure 6 is currently phrased as a question (“Does retention of initial rainfall occur?”), which is unconventional for figure labeling. I recommend revising it to a more concise and descriptive format, such as: “Initial rainfall retention” with categories “Yes” and “No”, or “Retention occurrence” (Yes/No). In addition, please ensure that the x-axis category labels are consistently formatted, with the first letter capitalized (i.e., “Yes” and “No” instead of “yes” and “no”).30. L 222-223: In the text, Cₘₐₓ is reported as: “1.5 g/g at Waldkirch to 4.4 mm at Kandel” This mixes g/g and mm, which is inconsistent. Please ensure that units are consistent when reporting ranges (either all in g/g or all in mm).31. L 229: Figure 7 caption: Figure 7 refers to: “different precipitation durations” However, panel (a) shows site comparison, not duration classes.32. Table 5: Please ensure that intensity units are consistently expressed (mm/h) across figures and tables, as “mm” alone is ambiguous.33. L 255: Figure10 OR 9?34. Section 3.5 presents evaporation patterns based on specific short time periods (Spring 2025 and Summer 2024). It is not clear why these limited periods were selected instead of using the full observation period, particularly since other sections of the manuscript appear to rely on more comprehensive datasets. This raises two points that require clarification: the rationale for selecting these specific time windows, and whether the results shown are representative of the overall dataset In addition, the temporal scope of the analysis appears inconsistent across sections, which makes it difficult to compare results directly.35. Table 6: Column headings: “Days without rainfall” “First day following rainfall” Consider standardizing wording slightly (e.g., “Rain-free days” vs “Post-rainfall day”).36. L 264: “proportion of precipitation between the left and right sides” However, based on the study design, measurements are taken below the canopy, meaning this likely represents throughfall, not gross precipitation.37. L 273-290: The manuscript refers to: “left and right sides” but these are not explicitly defined. Please clearly define what is meant by “left” and “right” (e.g., orientation, relative to tree stem, or lysimeter setup), either in the text or figure caption.38. L 289-290: the caption does not fully explain: how significance is determined what statistical test is used.** Discussion:39. Section 4.1: The discussion provides a reasonable interpretation of the observed relationship between antecedent moisture conditions and retention capacity; however, several aspects would benefit from clearer mechanistic justification and stronger linkage to the results. First, the statement that a saturated forest floor lacks the capacity to retain additional water is broadly consistent with hydrological theory, but the manuscript does not explicitly distinguish between capillary storage, gravitational storage, and dynamic storage thresholds (e.g., Cₘᵢₙ vs. Cₘₐₓ). Clarifying which storage domain is being referred to would improve precision and avoid oversimplification. Second, the attribution of outliers in the “no retention” class to hydrophobicity is presented as an assumption (“we assume…”), but no direct measurements or supporting indicators (e.g., water drop penetration time, soil organic composition, or temperature history) are provided. Given that hydrophobicity is a process-dependent and temporally variable phenomenon, this explanation should either be: supported with additional evidence, or more clearly framed as a hypothesis rather than a conclusion. Third, the discussion would benefit from a clearer connection to the results presented in Section 3.3 and Figure 6. For example, the observed relationship between lower pre-event soil moisture and higher likelihood of retention is consistent with the argument presented here, but this link is not explicitly stated. Finally, the statement regarding 2024 being an unusually wet year is important, but its implications are not fully developed. In particular, the limited occurrence of dry conditions may: constrain the range of observed FF states, and reduce the ability to robustly assess processes such as hydrophobicity or extreme dry-state behavior. A brief acknowledgment of this limitation would strengthen the interpretation (I would like to see section 4.5 as a new section for research limitation, and future directions. So you can move this limitation to a new subsection).40. Sectikon 4.2: L 300: Please define what is meant by “heavier” forest floor (e.g., mass per unit area, bulk density, organic matter content, or thickness).
41. L 301: The conclusion: “storage is influenced by the proportion of organic fine material (OFM) rather than just FF thickness” is interesting but not clearly supported by the results shown. Please clarify: how OFM was quantified, and whether a direct analysis (e.g., correlation or regression) supports this statement Otherwise, this should be framed more cautiously.
41. L 304: The discussion notes: storage was often not empty at the onset of events. This is a key methodological point. However, the manuscript does not clearly show: how initial storage conditions were quantified, or how strongly they influenced Cₘᵢₙ and Cₘₐₓ.
42. L 320-321: The discussion compares with: Sato et al. (2004) Keim et al. (2006) However, the explanation: “differences are due to lab vs field conditions” is somewhat broad. I suggest refining this by explicitly distinguishing: rainfall intensity distribution event duration structure water input magnitude rather than grouping all differences under “lab vs field”.
43. L 320: The sentence: “The most parsimonious explanation…” is quite strong. Consider softening to: “A likely explanation…” or “One possible explanation…” unless this is quantitatively demonstrated.
44. Section 4.3: The text refers to Penman–Monteith variables (radiation, humidity, wind), but: no full Penman–Monteith implementation or comparison is presented.
45. L 336-337: The discussion states that: temperature is not the primary controlling factor, while radiation, humidity, and wind dominate However, this conclusion is not directly supported by a formal sensitivity analysis or multivariate decomposition in the manuscript.
46. L 339: The statement: higher evaporation in spring due to higher radiation under less dense canopy is plausible, but: canopy density is not quantified seasonally in this section spring vs summer also differs in VPD, soil moisture, and phenology.
47. Section 4.4: The text refers to “Figure 11” as supporting evidence for infiltration-related findings. However, there is no Figure 11 present in the manuscript.
48. The Discussion section would benefit from a clearer separation of content into distinct sub-sections. I recommend adding: Section 4.5 – Limitations and future directions: to explicitly address methodological constraints (e.g., temporal coverage, climatic variability, and limited observation of extreme dry conditions) and to outline clear avenues for future research. Section 4.6 – Management implications for forest ecosystems and water resources to translate the findings into applied insights for forest management, particularly regarding forest floor dynamics, water storage regulation, and hydrological functioning under different stand conditions.
** Conclusion:
50. The Conclusion currently repeats several detailed findings already discussed in the Results and Discussion (e.g., differences in storage capacity, precipitation controls, and evaporation patterns). I recommend restructuring it as a synthesis-oriented section, focusing only on the main take-home messages rather than restating results. Example improvement: Instead of: “We found significant differences in storage capacities among the three sites…” A more appropriate conclusion-style synthesis would be: “Storage capacity varied among sites, primarily driven by forest floor composition and moisture conditions.” Some expressions in the Conclusion are not sufficiently precise, such as “nearly all lysimeters” or “significant differences.” In a Conclusion section, such vague wording reduces scientific clarity. Please replace such statements with explicit quantitative summaries where possible or remove them if they are not essential.
I hope these comments are helpful in improving the manuscript.
Seyed Mohammad Moein Sadeghi
Citation: https://doi.org/10.5194/egusphere-2026-284-RC2
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This manuscript reports on experiments to quantify storage, drainage, and evaporation from the forest floor, using innovative instrumentation to obtain rare observations of these processes. The experiment and analysis are simple and the interpretations are clear, providing a remarkably clean story about the controls on water storage and release from this important store. I have only a few suggestions to improve the manuscript.
There is an inconsistency in how Cmax is defined. In Fig 8 and many other places in the results, Cmax is the same as or less than Cmin. I expected Cmax to be defined as in Fig 2 and L150, where it is defined as an absolute amount of water, not as excess above Cmin as it seems to be in most of the Results. I hope you will choose to define and report it as in Fig 2 and L150.
The methods and discussion unfortunately gloss over how much mineral soil is in these lysimeters. Are there data available on organic volume or mass vs mineral volume or mass? Though it might be difficult to compare across humus forms, this kind of analysis would help in extrapolating results to other settings.
In Fig 9A, why are there large hourly oscillations between high evaporation and zero? Can we be sure this is due to sunflecks and not to an error in the data logger or analysis? Even if there are no data errors, it may be worth explaining the limitations of Fig 9 when estimating evaporation at the ha or catchment scale; i.e., sunflecks affect these installations differently than they affect catchments.
I think Fig 10 is one of the most important figures in the manuscript. We almost never get to see spatially resolved estimates of infiltration, and I found the variation to be strikingly high. It’s interesting to see that the forest floor appears to be a stronger source of variability than redistribution by the canopy. Is it possible to add some discussion on how these magnitudes compare to spatial redistributions caused by other sources of heterogeneity?
I appreciate the discussion comparing intensity effects on storage capacity in the canopy vs forest floor, but I think there is a missing element, namely the contrasting mechanisms in how those two stores become wetted. Unlike in canopies, the role of drop velocity in wetting the forest floor is low, and wetness depends more on low-velocity flow into detrital pores. This difference (momentum vs van der Waals dominance) seems to me a more obvious explanation than the laboratory vs. field distinction emphasized in the manuscript. I do agree that the duration is
Figure 3 given that evaporation is always near zero and the details are also in Table 3, I suggest replacing that uninformative line with a time-varying estimate of C, as in Figure 2.
Table 5 last rows should be mm per … h?
Line 293: what does “saturated” mean here? This word must be chosen carefully.