Manuscript number: egusphere-2025-3294

Authors: Zixi Li, Fuqiang Tian

Comments to authors

Overall comments

This study derived and validated raindrop kinetic energy estimation model for tree canopy based on microphysical processes. The model was developed based on Li and Tian 2025 and Li et al. 2025. The series of papers that take physical properties of leaf, microstructure of the canopy and microphysical processes into consideration are novel and have potential for development. Overall, this present manuscript is well organized and well written.

The weak point is that variables and constants are difficult to follow because there are numerous of them. That makes the manuscript hard to read. Relevant to that, for some variables and constants, there are no explanations and descriptions. I advise making the list of variables and constants at the end of the text like Li and Tian 2025 did, in addition to describing and defining them in the text firmly. However, it is not feasible to explain all variables and constants perfectly in the manuscript due to large number of them. Instead of omitting exact and detailed explanations on variables and constants, authors should refer readers to Li and Tian 2025 and Li et al. 2025 by explicit description, although I think most readers will likely do so without such description.

There are many points to be modified regarding format and lack of references. Please refer to https://www.hydrology-and-earth-system-sciences.net/submission.html

Specific comments

L18

“external variations”

I think the following description is better.

“variations in rainfall characteristics”

L19

“an unchanged”

I think the following description is better.

“a certain”

L35 to 36

“the number of raindrops …in kinetic energy”

I suggest modification as follows.

-> large raindrops are formed and the number of raindrops beneath the canopy is reduced, which results in a broader distribution range and an increase in kinetic energy

L38

measurement, mainly -> measurement,

L47

DSD -> raindrop size distribution

L50 to 53

This paragraph looks like the index of the paper. Readers would like to know the objectives of this study at the end of Sect 1 reflecting the background of the study stated earlier in the section. Authors need to add one or more specific objective(s) of this study here.

Between L90 and 91

It is advisable to explicitly encourage readers to refer Li and Tian 2025 and Li et al. 2025. Additionally, compiling a list of variables and constants and presenting it at the end of the main text would be useful for readers.

L96

Please delete “is the total kinetic energy calculation”.

Eq. (2)

E(D) -> E(D_i)

Please note that subscripts and superscripts are in the standard font, not italics.

L99

the unit area unit rainfall depth kinetic energy -> kinetic energy per unit area and unit rainfall depth

L99

What does “(KE)” mean? Does it mean E_Kp is a function of kinetic energy? If so, I think the expression is redundant and “(KE)” should be deleted.

L100

Please replace "t is the drop spectrometer observation time (60s)” with “P is the amount of rainfall”, because t does not appear in Eq. (3).

L102

Please delete “, A is the drop spectrometer observation area (54cm²),” as A has already been defined on line 100.

L104

the canopy density -> fractional vegetation cover (FVC)

Eq. (6)

There is no explanation for σ.

In the upper equation for s_max:

The denominator in the root symbol includes sinα, but Eq. (1) in Li et al. 2025, which I think original equation, does not.

In the lower equation:

Inside the root symbol in Eq. (2) in Li et al. 2025, (sinα)/2 is multiplied, but not in this present manuscript.

What is the cause of these differences?

L114

Please insert “that reflects wind load effect and” after “a coefficient”.

Eq. (7)

Please add explanation for "s”.

L119

Pleas insert “Li and Tian, 2025” after “function”.

Eq. (10)

There is no description on G. It appears on L184 to 185 but should be presented here.

L139

the proportion of splash drops -> the proportion of canopy and stem splash drops

Please consider that in Table 1 the terms “Leaf splashing” and “Stem splashing” are used.

L140

canopy drip -> canopy and stem drip

L164

the splash intensity (mm/h) -> the portion of canopy and stem splash drops

L164 to 165

the canopy dripping intensity (mm/h) -> the portion of canopy and stem dripping

L169 to 175

Please add the stand characteristics like average tree height, average diameter at breast height and stem density.

L187 to 188

“The model was… in Table 3.”

This sentence describes on validation not the method and should be moved after “these nine rainfall events” on line 198 as in, “these nine rainfall events along with accumulated rainfall, mean wind speed and mean rainfall intensity.”

Table 2

Symbol “R” should be replaced with “I” as already shown on line 158.

Symbol “LAD” should be replaced with “f(α)” or “α” because Li and Tian 2025 used them as leaf inclination distribution function and leaf inclination angle, respectively.

L209

Please insert “through the coefficient k” after “wind load effects”.

L215

Please insert “in total and dripping energy” after “performance”.

Figure 4

The vertical axis in Figure 4, f(ds), means raindrop size distribution but the maximum values in the figure range from 0.8 in Fig. 4a to 2.5 in Fig. 4b. Does it mean the number of raindrops in the measurement volume in OTT Parsivel2 laser spectrometer?

Figure 5: vertical axis labels

For Fig. 5a and 5d, please replace E_k with E_K.

For Fig. 5b and 5e, please replace E_k with E_s.

For Fig. 5c and 5f, please replace E_k with E_d.

Figure 6a, b

Please replace E_kt with ΣE_K considering Eq. (4).

Figure 6b

In Table 3, accumulated rainfall on 26 August was 6.71 mm but in Fig. 6b it is some 30 mm or more. Please explain or clarify the cause of the difference.

Technical comments

L13

9 field datasets -> nine field datasets

That holds true throughout the text.

L15

J/m² h -> J m^-2 h^-1

That holds true throughout the text.

L50

This section 2 delves into -> In Sect 2., we delve into

The abbreviation holds true throughout the text.

Figure 1

The combination of red, brown and green are not good for readers with color vision deficiencies. Please modify.

L105

E_d and -> and E_d

L106

Please put a space between “1.3” and “mm”.

L107

Please put a space between “0.8” and “mm”.

References

In the running text, please replace “&” with “and”, e.g. L49 Frasson & Krajewski, 2013.

L303 de Moraes Frasson et al. 2013 is in the reference list but not in the text.

L312 Fernández-Raga et al. 2010 is not in alphabetical order.

L352, 355 There are two Nanko et al. 2008 that are not distinguishable.

The following papers that appear in the text are not found in References.

L27 Angulo-Martínez et al., 2016

L29 Jeong et al., 2024

L39 Mosley et al., 1983

L40 Yan et al., 2021

L49, 74 Frasson and Krajewski, 2013

L36 Katayama et al., 2024 -> Katayama et al., 2023

L49, 79 Murakami et al., 2021 -> Murakami, 2021

L67 Gash and Morton, Valente and Gash -> Gash and Morton (1978), Valente and Gash (1997)

L75 Rutter et al. (1997) -> Rutter et al. (1971)

L120 Mou J (1983) -> Mou (1983)

Review of egusphere-2025-3294

Overview:

The manuscript develops a process-based model of under-canopy throughfall kinetic energy (KE) by partitioning throughfall into splash, drip, and penetration components, assigning each a size and velocity distribution, and integrating to obtain KE. The attempt to link canopy microphysics to under-canopy KE is novel and worth pursuing. However, several core elements—parameter definitions and provenance, observational partitioning of the components, explicit assumptions about drop-size distributions (DSDs), and the consistency of terminology—are currently underspecified. These issues reduce clarity, limit reproducibility, and make it difficult to evaluate the results. A thorough revision focused on reproducible methods, consistent definitions, and transparent validation would substantially improve the manuscript.

Major comments

1. Framing and objectives
   
   The Introduction reviews field work on under-canopy KE, but soon after states that the method for studying KE under canopy mainly uses experimental measurement. Please streamline the framing to avoid internal inconsistencies and clearly articulate the specific research objectives at the end of the Introduction.
2. Positioning relative to the throughfall partitioning literature
   
   Because component partitioning is central to your approach, the literature review should explicitly situate the work with respect to key studies on throughfall partitioning, such as Levia et al. (2019, Hydrological Processes, doi:10.1002/hyp.13432), Nanko et al. (2022, Journal of Hydrology, doi:10.1016/j.jhydrol.2022.128144), and Nanko et al. (2025, JGR-Biogeosciences, doi:10.1029/2024JG008340). Clarifying how your definitions and assumptions align (or intentionally differ) from these studies will help readers evaluate the contribution.
3. Consistent terminology for throughfall types
   
   Standard forest hydrology usage distinguishes free/direct throughfall, splash throughfall, and canopy drip (e.g., Levia et al., 2017, WIREs Water, doi:10.1002/wat2.1225). You use “penetration,” which risks confusion unless mapped clearly to prior terms. Please define precisely whether “penetration” is synonymous with free/direct throughfall or whether it includes drops secondarily generated by in-canopy splash (as your schematic may imply). If your category differs physically from free/direct throughfall, explain the rationale and adopt consistent wording throughout figures, equations, and text.
4. Equation (5): definition and coefficient consistency
   
   The symbol γ is described as canopy density/FVC in different places, and Eq. (5) scales a KE term by γ. If γ denotes fractional vegetation cover, the direct/free-throughfall fraction is commonly represented by (1−γ). Rather than asserting an error, I recommend clarifying the physical meaning of γ and revisiting the coefficient so that the term associated with direct/free-throughfall is consistently represented (which may imply using (1−γ)). Please ensure that the terminology (FVC vs canopy density) is consistent across the paper.
5. Section 2.2 parameter definitions and provenance
   
   Several parameters critical to reproducibility lack values, units, or sourcing: leaf surface contact angle θ (and how measured or sourced), the pinning proportion p, the retention coefficient β, surface tension σ, the wind-load parameter k, and the contact-angle hysteresis term X. Please provide a single comprehensive parameter table listing, for every symbol in Eqs. (5)–(14): name, definition, units, typical value or range, whether measured/assumed/fitted, the measurement/estimation method, and literature sources.
6. Observational partitioning of splash, drip, and direct/free components
   
   Tabled statistics are reported separately for splash and drip KE, yet the Methods do not explain how the disdrometer observations were partitioned into these components. Please describe a transparent and reproducible diagnostic (e.g., thresholds in size–velocity space, spectral shape, timing relative to wind gusts or canopy shaking), provide sensitivity to threshold choices, and clarify how any ambiguity was handled. Without this, component-specific R² and RMSE are difficult to interpret.
7. Explicit DSD assumptions for each component
   
   For splash, you indicate a range (e.g., 0.3–1.3 mm) and a central tendency, while for canopy drip you derive a representative size but do not state a distribution form. Because Eq. (14)-type integrals over size strongly depend on f(D), please specify explicit DSD functional forms for each component (e.g., gamma, lognormal, triangular, uniform), justify them with observations or literature, and ensure they are consistent with Fig. 4. State whether Fig. 4’s vertical axis is number-based or volume-based frequency.
8. Wind effects and leaf inclination
   
   You state that Eq. (6) accounts for wind load via parameter k, and leaf inclination α appears elsewhere. Please clarify the physical role and calibration of k, whether α varies with wind, and how these influences were implemented (event-wise or time-varying). If wind effects were ultimately small in your validation, it would help to show a brief sensitivity analysis.
9. Raindrop velocity relation
   
   Equation (8) uses a piecewise V–D relation with a breakpoint near 1.9 mm. Consider replacing or benchmarking against a widely used continuous relation (e.g., Atlas et al., 1973) to avoid artifacts at the breakpoint, and report the sensitivity of KE to the velocity model.
10. Fall height for drip and splash
    
    The fall height h appears to be fixed as the mid-height of the last canopy layer for both drip and splash. Because origins differ (leaf tips vs branch edges) and canopy structure is heterogeneous, please justify this assumption or, preferably, derive h from your LiDAR geometry (e.g., distributions of likely origin heights) and examine sensitivity.
11. Definition and wording for pt
    
    pt is defined as SAI/(LAI+SAI) but is referred to as “stem flow ratio” in places. In forest hydrology, “stemflow ratio” usually denotes the fraction of rainfall routed as stemflow, not an area fraction. Please adopt “stem area proportion” (or similar) consistently to avoid confusion.
12. Figure 4 — axis definition only
    
    Please clarify the axis definitions: specify whether the vertical axis represents number-based or volume-based drop-size frequency (and provide units), indicate any normalization (e.g., per mm, per bin, probability density), and state the bin widths used on both axes.
13. Treatment of branch/twig drips
    
    The model and figures emphasize leaf drip, but branch/twig drips can be important—especially in broadleaf canopies—and can alter DSDs. Please clarify whether branch-origin drips are included in your “canopy drip” category, whether they are distinguishable in the data, and how (or if) they are represented in the model. If neglected, state this explicitly and discuss implications.
14. Validation design, reporting granularity, and interpretability
    
    It is unclear whether R² and RMSE are computed from 5-min bins within each event, then averaged, or from another aggregation. Please report the exact time aggregation, the number of samples per event, and the comparison targets (modeled vs measured component KE). For each event, provide measured KE (with uncertainty), rainfall duration, and a bias metric (e.g., modeled/observed mean ratio). Without this context, the reported RMSE values (≈20 J m⁻² h⁻¹) are hard to judge.
15. Discussion depth
    
    The Discussion largely reiterates results. For example, please expand with (i) comparisons to prior under-canopy KE and partitioning studies (agreement/disagreement and causes), (ii) sensitivity to canopy traits (LAI, inclination, θ) and rainfall characteristics, and (iii) implications for erosion modeling and for transferring parameters to other sites/species.

Specific comments

• Lines ~95: For under-canopy drops, not all reach terminal velocity; “fall velocity” is more accurate than “terminal velocity.”

• Line ~104 and Table 1 note: Clarify whether γ denotes canopy density or FVC, and use one term consistently; revisit the coefficient in Eq. (5) accordingly.

• Eq. (6): Define σ (surface tension), k (wind-load parameter; units and calibration), and X (half the contact-angle hysteresis). Provide values and sources for θ and X for the study species.

• Line ~115: If wind effects are represented through k and/or through variations of α, please explain how α was measured or parameterized and whether it varies in time with wind.

• Line ~123: Clarify whether the fall height h is fixed at the mid-height of the last canopy layer for both drip and splash, and justify or test sensitivity.

• Line ~134: Avoid “stem flow ratio” for pt; use “stem area proportion (SAI/(LAI+SAI)).”

• Eq. (8): Consider adopting or benchmarking a continuous V–D relation (e.g., Atlas et al., 1973) to avoid artifacts near the current breakpoint.

• Fig. 2: State explicitly whether “penetration rain” includes splash-generated drops from upper layers. If yes, this differs from “free/direct throughfall” in prior literature; adjust terminology accordingly.

• Fig. 3 and Table 3: Report the number of time steps per event, the aggregation window used for statistics, event-level measured KE and duration, and a bias measure to contextualize RMSE.

• Section 3.1 and Fig. 4: Explicitly state that open-site and under-canopy KE were computed from Parsivel size–velocity spectra (referencing the equations), confirm that the open-site DSD shown is observation-based, and indicate whether the vertical axis is number-based or volume-based.

• Throughout: Define w, wl, and ws consistently, and supply units for every symbol on first use.