MS#: hess-2023-91

Title: Root water uptake patterns are controlled by tree species interactions and soil water variability

It has been a pleasure reading through this contribution. This research underlines the lack of research on how throughfall patterns influence root water uptake patterns. The authors propose to close this knowledge gap by examining the role of throughfall patterns, soil water variability, soil properties, and biotic factors on root water uptake patterns using a statistical model.

I will organize my comments following the structure, per section, of the manuscript.

INTRODUCTION

The introduction provides a detailed background on below-canopy precipitation, specifically focusing on throughfall. It covers previous research about throughfall, its spatial distribution, and its impact on soil moisture patterns.

1. Line 62: Consider rephrasing the phrase "Previously proposed explanations" to "Previous studies have suggested". This would be more direct.
2. Line 86: The term "water scarcity" is introduced without any context or explanation. A brief explanation or definition would enhance understanding. In fact, this is the only part of the manuscript where "water scarcity" is mentioned.

MATERIALS AND METHODS

The Materials and Methods section is well-written. Some areas that could be improved to make it clearer and easier to understand follow:

1. Line 174: Provide units for variables, such as λ (latent heat of vaporization).
2. Line 196-197: Elaborate why only 56 sensor locations provided high-quality data, and why only 34 of these provided data for root water uptake estimation. What qualifies as ‘high-quality data’?
3. Linear Mixed Effects Model: Explain the terminology used (like 'random effects', 'fixed effects') for readers unfamiliar with these statistical methods.
4. Linear Mixed Effects Model: Add a few sentences as justification or rationale for including each of the factor (fixed and random) and their interactions (for fixed) in the model. The reader may be able to identify the rationale by referring to the texts in Introduction. But it pays to be redundant in the Methods so that it is clear to the reader which factors were included, and more importantly ‘why’.

DISCUSSION

This discussion section is generally well-written, but there are some areas for improvement to enhance clarity and readability:

1. Structure and Organization: The text is divided into several subtopics, which makes it easier to follow. However, it could benefit from a clearer outline or roadmap at the start of the discussion section that provides an overview of what will be discussed.
2. Data Presentation: Some results are mentioned without an explicit description of how they were obtained. For example, in lines 477 to 479, the authors state they found that bulk density of the monitored soil layer did not affect local water uptake, but there is no explanation of how this conclusion was reached. Providing a more detailed explanation would enhance the credibility of the findings.
3. Clearer takeaways: The section could benefit from more direct conclusions or ‘takeaway’ after discussing each main point. For instance, after discussing the influence of tree species richness on root water uptake patterns (lines 504 to 533), a one-sentence conclusion summarizing the main takeaway could be beneficial.
4. Hypotheses and Expectations: It might be useful to explicitly state the original hypotheses or expectations before explaining how the results confirm or contradict them. This can provide the reader with a clearer understanding of the study's purpose and significance. Perhaps, these hypotheses can be explicitly stated towards the end of Introduction. The authors can then ‘revisit’ these hypotheses in Discussion following their presentation of results.
5. Broader context of the literature. This study appeared broadly consistent with the finding of Knighton, Singh, Evaristo (2019, DOI: 10.1029/2019GL085937), which showed that monoculture catchments dense with trees reliant on shallow soil water exhibited reduced transpiration losses compared to deep‐rooted and mixed‐species forests. It is an important confirmation to make considering that this study is based purely on a statistical framework whilst that of Knighton et al. (2019) was based on the Budyko framework.
6. Broader context of the literature. Demir et al. may find use in placing their finding within the larger context of the topic (root water uptake studies) that used other techniques, particularly stable H and O isotopes in water. That is, the hours-long timescales used in this study for estimating transpiration losses are orders of magnitude shorter than what stable isotopes would show. See, for example, the study by Evaristo et al. (2019, DOI: 10.1029/2018WR023265), which showed that transpiration water was between 17 and 62 days. How do the timescales of Demir et al. (and their findings) compare and contrast to the timescales (and findings) of studies using tracers? A few sentences that place Demir et al. within the larger context of the topic would be useful for future researchers to recognize.

Reviewer#: Review of hess-2023-91 " Root water uptake patterns are controlled by tree species interactions and soil water variability " by Gökben Demir et al.

The manuscript provides valuable insights for quantifying the role of throughfall patterns on root water uptake patterns and the drive factor of root water uptake patterns. It is of great significance to understand the effects of throughfall patterns on subsurface water dynamics and the interaction between plant-soil-water systems in forest ecosystems. The study seems interesting Overall, I would recommend this manuscript for publication in HESS after some redivision. The comments and suggestions as follow:

In the introduction section,

1. The research methods of root water uptake patterns should be introduced so that we can better understand the subsequent analysis.
2. Further statements on the effect of throughfall patterns on the variation of soil moisture should be added. For example, some previous studies showed that the weak and short-term influence of throughfall patterns on the soil moisture patterns. However, when the proportion of throughfall is greater, whether this relationship will change.
3. The influence of abiotic factors on water use patterns should also be mentioned.

In the Materials and methods section,

1. What you mean” We measured gross precipitation and throughfall on rainless days”? It is confusing how you can measure total rainfall when there is no rain.
2. In 2.4 section, what indicators are used to reflect root water uptake?
3. How to integrate the water uptake pattern of a single site to the plot scale?

In the Discussion section:

1. L454-455: What exactly does root activity mean? How does this study prove that root activity does not alter canopy-attributed heterogeneity in drainage pathways?
2. How do you prove that bulk density of the monitored soil layer did not affect local water uptake?
3. This study focused on the effect of soil bulk density on root water uptake, but other soil properties, such as texture and organic matter, affect soil structure and aggregate characteristics, which will indirectly affect water transport in soil and thus affect the water use pattern of plants.
4. Trees with different ages have different physiological structures, such as root system and leaf characteristics, which will affect the water use pattern of plants. The effects of different tree ages on the water use pattern of plants should be discussed.