Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Maan, Debora Cynthia; ten Veldhuis, Marie-Claire; van de Wiel, Bas Johannes Henricus

doi:https://doi.org/10.5194/egusphere-2022-104

Preprints

https://doi.org/10.5194/egusphere-2022-104

Preprints

27 Apr 2022

| 27 Apr 2022

Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Debora Cynthia Maan, Marie-Claire ten Veldhuis, and Bas Johannes Henricus van de Wiel

Abstract. Plant roots are highly adaptable, but their adaptability is not included in crop and land surface models. They rely on a simplified representation of root growth, independent of soil moisture availability. Data of subsurface processes and interactions, needed for model set-up and validation, are scarce. Here we investigated soil moisture driven root growth. To this end we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (45 x 45 x 10 cm). The development of the vertical soil moisture and root growth profiles are tracked with a high spatial and temporal resolution. The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments suggest that vertical root extension only takes place if the soil moisture exceeds a threshold value at the root tip. We show that even a parsimonious one-dimensional water balance model, driven by the measured water input and output fluxes, can be convincingly improved by implementing root growth driven by soil moisture availability.

Received: 04 Apr 2022 – Discussion started: 27 Apr 2022

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Journal article(s) based on this preprint

29 Jun 2023

Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Cynthia Maan, Marie-Claire ten Veldhuis, and Bas J. H. van de Wiel

Hydrol. Earth Syst. Sci., 27, 2341–2355, https://doi.org/10.5194/hess-27-2341-2023,https://doi.org/10.5194/hess-27-2341-2023, 2023

Short summary

Debora Cynthia Maan et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-104', Anonymous Referee #1, 21 May 2022
The authors investigated soil moisture-driven root growth using a rhizobox experiment. Their results confirm that vertical soil moisture distribution regulates the root growth profile, while at the same time, the soil moisture dynamics are impacted by the root growth. This study is mainly supported by the parsimonious one-dimensional water balance model they developed, with the functional relationship between the soil moisture and the root density growth rate being its key. The manuscript is well organized and well written. However, despite this reviewer's great interest in this study, he found there is still some room to extend this study for real case applications (if not at the global scale, then at the regional scale, or at least using a range of in-situ sites). This reviewer will motivate his comments as below:

This reviewer agrees with the authors that the current LSM should consider the dynamic root growth model, which should take into account the impact of soil moisture. The parsimonious model as the author developed sounds ok, and the current toy example is good for raising the awareness of LSM community on this point. Nevertheless, this is not new. For example, NoahMP¹ has a dynamic root growth module, which considers its dependence on soil moisture and soil temperature. The recently published STEMMUS-SCOPE² model also considers the dynamic root growth as a function of air temperature, soil temperature (via water stress factor) and net assimilation.

With the above, this reviewer is trying to search for the unique contribution from this study. Perhaps the functional relationship between the soil moisture and the root density growth rate could be one significant contribution. On the other hand, this reviewer is not clear or the authors did not show how the current approach can be applied elsewhere in the field. Particularly, the most important parameters (e.g., u2/u3) are determined via sensitivity analysis, which makes this reviewer wonder how to obtain these parameters in the field, and further at regional/global scale?

Please find attachment with some minor comments.

References:

1. Gayler, S., Wöhling, T., Grzeschik, M., Ingwersen, J., Wizemann, H.-D., Warrach-Sagi, K., Högy, P., Attinger, S., Streck, T., and Wulfmeyer, V. (2014), Incorporating dynamic root growth enhances the performance of Noah-MP at two contrasting winter wheat field sites, Water Resour. Res., 50, 1337– 1356, doi:10.1002/2013WR014634.

2. Wang, Y., Zeng, Y., Yu, L., Yang, P., Van der Tol, C., Yu, Q., Lü, X., Cai, H., and Su, Z.: Integrated modeling of canopy photosynthesis, fluorescence, and the transfer of energy, mass, and momentum in the soil–plant–atmosphere continuum (STEMMUS–SCOPE v1.0.0), Geosci. Model Dev., 14, 1379–1407, https://doi.org/10.5194/gmd-14-1379-2021, 2021
Citation: https://doi.org/10.5194/egusphere-2022-104-RC1
- AC1:
  'Reply on RC1', Cynthia Maan, 21 Jul 2022
  Dear reviewer,
  Thank you for your constructive comments. Please find our reply to your remarks below
  The reviewer commented that there is still some room to extend the study for real case applications. We agree with the reviewer that there is room for extension to real case applications. However, we think that the results presented here – the parsimonious model with very direct relationships between the two state variables (soil moisture and root density) and the experimental setup for validation (including `constant’ and highly controlled flow rates through the drip lines) could already be ready to share with the public.
  
  The reviewer also notes that our parsimonious model is not new, while other LS models (e.g. NoahMP and STEMMUS-SCOPE model) have a dynamic root growth module.
  
  The authors agree that the proposed model is not the first one considering dynamic root growth. However, only few models take soilmoisture driven root growth into account. STEMMUS scope considers the effect of the total root zone soil moisture availability, but does not consider vertical variations in the soil moisture profile. The authors are aware (now) of only the coupled NoahMP/VOM-ROOT that considers the impact of the vertical soil moisture profile on the root growth profile. Last model makes another assumption about the bulk root growth.
  Furthermore, the existing dynamic root growth models are unambiguous about first principles. E.g. should the bulk root growth rate be related to the biomass growth rate, as Adiko et al. suggest, to the difference between the water demand and the actual water uptake (Schymanski et al./ NoahMP/VOM-ROOT; growth of the root bulk in case of water shortage), or could it be related to the soil water status directly/only (as proposed here). It generally is a good idea to keep the model as simple as possible, while including the most dominant (proven) relationships. Therefore we propose a very simplified model, which nevertheless includes a dominant functional relationship between root growth and soil moisture.
  Unique about this study is the following:
  - In the last version of the model we propose to link the (local) root growth rate very directly to the local soil moisture, with no other (e.g. above ground) dependencies. This is a new proposal/hypothesis.
  - In our experiment we maintain the water flow relatively constant. In other studies irrigation is concentrated in short periods of irrigation or rain, which makes it more difficult to distinguish between signals of the forcing and of the response of the plant. By applying constant `irrigation forcing’ it is easier to recognize the adjustment processes to constant irrigation forcing.
  Also by simulating a single plant and perform direct measurements on both the soil moisture and root density growth rates simultaneously, i.e. it can give more insight in the direct interactions between the two variables. This information is missing when root densities/weights are measured after the soil moisture measurements. Also we measure both variables (roots densities and soil moisture) directly instead of indirect proxies (like evaporation or temperature) that depend on a translation by more complicated models with more assumptions.
  The reviewer is not clear or the authors did not show how the current approach can be applied elsewhere in the field. Particularly, the most important parameters (e.g., u2/u3) are determined via sensitivity analysis, which makes this reviewer wonder how to obtain these parameters in the field, and further at regional/global scale?
  
  We propose two different model-versions for root growth that can be applied in different settings/situations.
  The first one is only the relationship between (vertical varying) soil moisture and root density growth. This part of the model is easy to couple with a LSM, because the vertical extension velocity u1 is a parameter that is already included in LS models with a dynamic root growth module (however not yet including soil moisture driven root growth, like SWAP), and is therefore already estimated/determined for different plants and crops. The most easy way to implement our model in the most basic form (only the impact of vertical varying soil moisture) is to incorporate only equation 1 (possibly in combination with adding a threshold value at the root tip as we also propose), and leave the bulk (depth integrated) root growth of the LSMs intact.
  The water extraction rate per centimeter of roots in saturated conditions u2 was only introduced to make a simple translation between the vertical root profiles and the (measureable) soil moisture. Most existing LSMs contain a much better described and well validated module to calculate water extraction from given root profile and soil moisture profiles. So this part of the model is not meant for incorporation or practical application.
  In the second version we propose to link the root growth rate directly to the normalized soil moisture. This is a new proposal/hypothesis. We however fully agree that further measurements and experiments are needed to test this hypothesis, and to determine the root density growth rate in optimal conditions (saturation) u3 for different plant types. Note however, that we make use of only one and the same value to simulate the root growth in all time spans and at each level in Figure 8 . In practical applications, to implement this model in LSMs, it will be necessary to maximize the total root growth to (a part of) the total biomass growth.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC1
RC2:
'Comment on egusphere-2022-104', Anonymous Referee #2, 28 May 2022

Dear Editor,

This work investigates the driven factor of root growth. It provide a simply but practical method to model root growth. Here the manuscript needs some improvements to enhance its clarity and readability. The specific comments are as follows.

Specific comments:

L 21-24: some models described the maximum rooting depth by a linearly increasing function with accumulated temperature.

L 47: As I know, Some crop (e.g. Spacsys and STICS) and land surface models (e.g. CLM 5.0) have implemented dynamic root growth.

L 85: equation (1): the theta_n in bottom right should be followed by 'dz'

L 142-144: It should be the reason why exponential root profile is widely used in crop models and land surface models.

Fig. 7 and Fig. 8: “rooting dencity” should be “root density”.

Citation: https://doi.org/10.5194/egusphere-2022-104-RC2
- AC2:
  'Reply on RC2', Cynthia Maan, 21 Jul 2022
  Dear reviewer,
  Thank you for your constructive comments.
  The reviewer comments that some models described the maximum rooting depth by a linearly increasing function with accumulated temperature.
  
  The authors agree that temperature can be a dominant driving factor. We propose to add the following line after “In most crop … i.e., both independently of soil moisture.” :
  ”Some exceptional models treat root growth more dynamically by relating root growth to soil related parameters as accumulated temperature or root zone soil moisture. Models that take the vertical profiles of soil moisture into account, however, are scarce.”
  2. As I know, some crop (e.g. Spacsys and STICS) and land surface models (e.g. CLM 5.0) have implemented dynamic root growth
  These models have indeed a dynamic root growth component, but do not include a dependency on the vertical variation in soil moisture.
  3. The relative insensitive soil moisture should be the reason why exponential root profile is widely used in crop models and land surface models
  Our data indeed suggests that roots do not only `follow moisture’, but the moisture also `finds roots’ if roots are locally not present (by diffusion), reducing the resulting error. However, the experimental setup in an enclosed box favores this process unrealistically. At a larger scale the ‘root follows moisture routine’ is expected to have a more pronounced effect on the soil moisture.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC2
RC3:
'Review of egusphere-2022-104', Stan Schymanski, 31 May 2022

Dear editor,

The manuscript by Maan et al. describes an experiment where a maize plant was grown in a transparent rhizobox and root growth was monitored along with soil moisture in the vertical soil profile. The authors compare different model simulations with the observations, one with a prescribed exponential root distribution, and several simulations based on dynamic root distributions with different levels of observation data assimilated into the simulations. The authors find that root growth rates follow the soil moisture distribution, and that this could be simulated by assuming that root growth rate is a function of soil moisture and only happens at a normalized water content greater than 0.075.

The results presented are interesting and potentially insightful, but unfortunately, the paper is not well organized and lacks details and clarity. Therefore I found the results difficult to interpret and the conclusions difficult to verify. The paper consists of essentially three separate methods and results sections, one for each model version. It is not clear which parameters were calibrated in each model run and how, e.g. whether in 2.4 only u3 was calibrated, or also u2, and on what data precisely. It is also not clear in which soil depth how much irrigation was applied when, and whether the vertical soil moisture differences were induced by differential irrigation or root water uptake. The main conclusion is that root growth is strongly determined by soil moisture, with high soil moisture promoting root growth. However, since irrigation was "adjusted in steps to follow the plants growth and demand for water" (L68), and the soil moisture increases over time in the deeper soil (Fig. 2C), I am a bit confused in how far the vertical soil moisture distribution was controlled and if it really triggered differential root growth. More information about the experimental strategy would be helpful.

I would propose to consolidate the methods and results in two separate sections as per standard convention, and include sufficient details about the experimental and modelling methods to enable reproduction of the experiment and anlysis. The authors promise to publish the data for producing the plots and results in the future, and provided links for the referees, but unfortunately, the links did not work for me, so I was not able to assess if the promised material will be indeed adequate to enable reproducibility of results.

I added detailed comments in an annotated pdf-file, as they are too many to list here. Some of the equations provided are incorrect, but I cannot tell if they were correct in the analysis. For linguistic glitches, I just highlighted bits of text in yellow. I hope that my review will help to revise the manuscript and add more clarity about the methods and results.

Citation: https://doi.org/10.5194/egusphere-2022-104-RC3
- AC3:
  'Reply on RC3', Cynthia Maan, 22 Jul 2022
  Dear reviewer, dear dr. Schymanski
  Thank you for your remarks and suggestions, your constructive comments are very helpful. Please find our reply below as well as in the attachment.
  The reviewer found the results difficult to interpret and the conclusions difficult to verify. `The paper consists of essentially three separate methods and results sections, one for each model version. It is not clear which parameters were calibrated in each model run and how, e.g. whether in 2.4 only u3 was calibrated or also u2, and on what data precisely. “
  
  In each section, one extra parameter is calibrated, while the other is taken from the previous step:
  In section 2.2 (‘root follows moisture’ model) the vertical extension velocity u1 is calibrated on the measured root growth profile/distribution. Note however that the results are not that sensitive to this parameter (figure 4)
  
  In section 2.3 (soil moisture and water uptake model) The water extraction rate per centimeter of roots in saturated conditions u2 is calibrated based on the soil moisture data, while u1 is taken from section 2.2 (based on the root growth data)
  
  In section 2.4 (the prognostic model) the root density growth in saturated (assumed to be ideal) conditions u3 is calibrated on the root density data. u1 (calibrated against the same root growth data) and u2 are taken directly from the previous sections and thus not calibrated for the `updated’ version of the model. We fully agree that it would be necessary to repeat these experiments for different datasets, in order to get more confidence about the parameter values.
  
  This will be clarified in the MS.
  2. It is also not clear in which soil depth how much irrigation was applied when, and whether the vertical soil moisture differences were induced by differential irrigation or root water uptake.
  The authors will make the levels of irrigation visible in figure 2. The irrigation level was increased in steps (but normally remained constant in the order of days). We will indicate the days at which the irrigation level was adjusted in figure 2. At each moment only one level is irrigated; so the flow rate at the applied level equals the total flow rate which is indicated in figure 2A. We noticed that the dotted lines in the first panels in figure 2 C were missing by omission, this will be corrected in the manuscript. During the first 14days water was supplied at 0cm, on day 15 the level was adjusted to 10 cm depth (the roots already appeared around the 10 cm line around day 8, but until the 17th there was no significant growth in the 10cm-20cm interval. We will make the sequence of steps, and the implication for causality, better visible in the MS.
  3. The reviewer is a bit confused in how far the vertical soil moisture distribution was controlled and if it really triggered differential root growth. More information about the experiment strategy would be helpful.
  The vertical soil moisture differences were induced by a combination of differential irrigation and differential water uptake. The irrigation supply, controlled at (relatively constant) low rates during the experiment, is the truly independent system driver (however not constant). The irrigation depth was generally adjusted only after the rooting depth seemed to stabilize (so the soil moisture was the first factor that changed, followed by active root growth). The spatial correlation between the water content and the root density growth rates (figure 2) suggests a causal relationship. By adding more information about the sequence of events, we will be able to clarify this.
  4.The reviewer proposes to consolidate the methods and results in two separate sections as per standard convention, and include sufficient details about the experimental and modeling methods to enable reproduction of the experiment and analysis.
  The authors agree with the reviewer that sufficient details should be given to enable reproduction. The review process helped the authors to identify the missing information in the paper, which we will add to the MS. The authors are able to provide sufficient details to enable reproduction of the experiment and analysis. However, the authors think that sufficient clarity can be given without changing the structure of the paper.
  5. The authors promised to publish the data for producing the plots and results in the future, and provided links for the referees, but unfortunately, the links did not work for the reviewer,, so he was not able to assess if the promised material will be indeed adequate to enable reproducibility of results.
  The authors are very sorry that the data has not been visible before. We will have an active DOI soon, but the data is now `already' accessible via the following link: https://figshare.com/s/f206405c95ebfbbf9bf7
  6. The reviewer notes that some of the equations provided are incorrect, but that he cannot tell if they were correct in the analysis.
  The authors revised the equations in the code, and were delighted to find that the indicated errors were omissions in the text (not in the code). However, the remarks led us to discover another omission in the code; the `effective saturation van Genuchten’ (equation 2) was used instead of `soil wetness’ in equation 6 and 7. This has been corrected and the results were analyzed. Fortunately, the resulting differences in the graphs are small and do not affect the main results and conclusions of the paper. All the figures and calibration values that are affected (figure 5,6,8) will be adjusted in the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-104', Anonymous Referee #1, 21 May 2022
The authors investigated soil moisture-driven root growth using a rhizobox experiment. Their results confirm that vertical soil moisture distribution regulates the root growth profile, while at the same time, the soil moisture dynamics are impacted by the root growth. This study is mainly supported by the parsimonious one-dimensional water balance model they developed, with the functional relationship between the soil moisture and the root density growth rate being its key. The manuscript is well organized and well written. However, despite this reviewer's great interest in this study, he found there is still some room to extend this study for real case applications (if not at the global scale, then at the regional scale, or at least using a range of in-situ sites). This reviewer will motivate his comments as below:

This reviewer agrees with the authors that the current LSM should consider the dynamic root growth model, which should take into account the impact of soil moisture. The parsimonious model as the author developed sounds ok, and the current toy example is good for raising the awareness of LSM community on this point. Nevertheless, this is not new. For example, NoahMP¹ has a dynamic root growth module, which considers its dependence on soil moisture and soil temperature. The recently published STEMMUS-SCOPE² model also considers the dynamic root growth as a function of air temperature, soil temperature (via water stress factor) and net assimilation.

With the above, this reviewer is trying to search for the unique contribution from this study. Perhaps the functional relationship between the soil moisture and the root density growth rate could be one significant contribution. On the other hand, this reviewer is not clear or the authors did not show how the current approach can be applied elsewhere in the field. Particularly, the most important parameters (e.g., u2/u3) are determined via sensitivity analysis, which makes this reviewer wonder how to obtain these parameters in the field, and further at regional/global scale?

Please find attachment with some minor comments.

References:

1. Gayler, S., Wöhling, T., Grzeschik, M., Ingwersen, J., Wizemann, H.-D., Warrach-Sagi, K., Högy, P., Attinger, S., Streck, T., and Wulfmeyer, V. (2014), Incorporating dynamic root growth enhances the performance of Noah-MP at two contrasting winter wheat field sites, Water Resour. Res., 50, 1337– 1356, doi:10.1002/2013WR014634.

2. Wang, Y., Zeng, Y., Yu, L., Yang, P., Van der Tol, C., Yu, Q., Lü, X., Cai, H., and Su, Z.: Integrated modeling of canopy photosynthesis, fluorescence, and the transfer of energy, mass, and momentum in the soil–plant–atmosphere continuum (STEMMUS–SCOPE v1.0.0), Geosci. Model Dev., 14, 1379–1407, https://doi.org/10.5194/gmd-14-1379-2021, 2021
Citation: https://doi.org/10.5194/egusphere-2022-104-RC1
- AC1:
  'Reply on RC1', Cynthia Maan, 21 Jul 2022
  Dear reviewer,
  Thank you for your constructive comments. Please find our reply to your remarks below
  The reviewer commented that there is still some room to extend the study for real case applications. We agree with the reviewer that there is room for extension to real case applications. However, we think that the results presented here – the parsimonious model with very direct relationships between the two state variables (soil moisture and root density) and the experimental setup for validation (including `constant’ and highly controlled flow rates through the drip lines) could already be ready to share with the public.
  
  The reviewer also notes that our parsimonious model is not new, while other LS models (e.g. NoahMP and STEMMUS-SCOPE model) have a dynamic root growth module.
  
  The authors agree that the proposed model is not the first one considering dynamic root growth. However, only few models take soilmoisture driven root growth into account. STEMMUS scope considers the effect of the total root zone soil moisture availability, but does not consider vertical variations in the soil moisture profile. The authors are aware (now) of only the coupled NoahMP/VOM-ROOT that considers the impact of the vertical soil moisture profile on the root growth profile. Last model makes another assumption about the bulk root growth.
  Furthermore, the existing dynamic root growth models are unambiguous about first principles. E.g. should the bulk root growth rate be related to the biomass growth rate, as Adiko et al. suggest, to the difference between the water demand and the actual water uptake (Schymanski et al./ NoahMP/VOM-ROOT; growth of the root bulk in case of water shortage), or could it be related to the soil water status directly/only (as proposed here). It generally is a good idea to keep the model as simple as possible, while including the most dominant (proven) relationships. Therefore we propose a very simplified model, which nevertheless includes a dominant functional relationship between root growth and soil moisture.
  Unique about this study is the following:
  - In the last version of the model we propose to link the (local) root growth rate very directly to the local soil moisture, with no other (e.g. above ground) dependencies. This is a new proposal/hypothesis.
  - In our experiment we maintain the water flow relatively constant. In other studies irrigation is concentrated in short periods of irrigation or rain, which makes it more difficult to distinguish between signals of the forcing and of the response of the plant. By applying constant `irrigation forcing’ it is easier to recognize the adjustment processes to constant irrigation forcing.
  Also by simulating a single plant and perform direct measurements on both the soil moisture and root density growth rates simultaneously, i.e. it can give more insight in the direct interactions between the two variables. This information is missing when root densities/weights are measured after the soil moisture measurements. Also we measure both variables (roots densities and soil moisture) directly instead of indirect proxies (like evaporation or temperature) that depend on a translation by more complicated models with more assumptions.
  The reviewer is not clear or the authors did not show how the current approach can be applied elsewhere in the field. Particularly, the most important parameters (e.g., u2/u3) are determined via sensitivity analysis, which makes this reviewer wonder how to obtain these parameters in the field, and further at regional/global scale?
  
  We propose two different model-versions for root growth that can be applied in different settings/situations.
  The first one is only the relationship between (vertical varying) soil moisture and root density growth. This part of the model is easy to couple with a LSM, because the vertical extension velocity u1 is a parameter that is already included in LS models with a dynamic root growth module (however not yet including soil moisture driven root growth, like SWAP), and is therefore already estimated/determined for different plants and crops. The most easy way to implement our model in the most basic form (only the impact of vertical varying soil moisture) is to incorporate only equation 1 (possibly in combination with adding a threshold value at the root tip as we also propose), and leave the bulk (depth integrated) root growth of the LSMs intact.
  The water extraction rate per centimeter of roots in saturated conditions u2 was only introduced to make a simple translation between the vertical root profiles and the (measureable) soil moisture. Most existing LSMs contain a much better described and well validated module to calculate water extraction from given root profile and soil moisture profiles. So this part of the model is not meant for incorporation or practical application.
  In the second version we propose to link the root growth rate directly to the normalized soil moisture. This is a new proposal/hypothesis. We however fully agree that further measurements and experiments are needed to test this hypothesis, and to determine the root density growth rate in optimal conditions (saturation) u3 for different plant types. Note however, that we make use of only one and the same value to simulate the root growth in all time spans and at each level in Figure 8 . In practical applications, to implement this model in LSMs, it will be necessary to maximize the total root growth to (a part of) the total biomass growth.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC1
RC2:
'Comment on egusphere-2022-104', Anonymous Referee #2, 28 May 2022

Dear Editor,

This work investigates the driven factor of root growth. It provide a simply but practical method to model root growth. Here the manuscript needs some improvements to enhance its clarity and readability. The specific comments are as follows.

Specific comments:

L 21-24: some models described the maximum rooting depth by a linearly increasing function with accumulated temperature.

L 47: As I know, Some crop (e.g. Spacsys and STICS) and land surface models (e.g. CLM 5.0) have implemented dynamic root growth.

L 85: equation (1): the theta_n in bottom right should be followed by 'dz'

L 142-144: It should be the reason why exponential root profile is widely used in crop models and land surface models.

Fig. 7 and Fig. 8: “rooting dencity” should be “root density”.

Citation: https://doi.org/10.5194/egusphere-2022-104-RC2
- AC2:
  'Reply on RC2', Cynthia Maan, 21 Jul 2022
  Dear reviewer,
  Thank you for your constructive comments.
  The reviewer comments that some models described the maximum rooting depth by a linearly increasing function with accumulated temperature.
  
  The authors agree that temperature can be a dominant driving factor. We propose to add the following line after “In most crop … i.e., both independently of soil moisture.” :
  ”Some exceptional models treat root growth more dynamically by relating root growth to soil related parameters as accumulated temperature or root zone soil moisture. Models that take the vertical profiles of soil moisture into account, however, are scarce.”
  2. As I know, some crop (e.g. Spacsys and STICS) and land surface models (e.g. CLM 5.0) have implemented dynamic root growth
  These models have indeed a dynamic root growth component, but do not include a dependency on the vertical variation in soil moisture.
  3. The relative insensitive soil moisture should be the reason why exponential root profile is widely used in crop models and land surface models
  Our data indeed suggests that roots do not only `follow moisture’, but the moisture also `finds roots’ if roots are locally not present (by diffusion), reducing the resulting error. However, the experimental setup in an enclosed box favores this process unrealistically. At a larger scale the ‘root follows moisture routine’ is expected to have a more pronounced effect on the soil moisture.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC2
RC3:
'Review of egusphere-2022-104', Stan Schymanski, 31 May 2022

Dear editor,

The manuscript by Maan et al. describes an experiment where a maize plant was grown in a transparent rhizobox and root growth was monitored along with soil moisture in the vertical soil profile. The authors compare different model simulations with the observations, one with a prescribed exponential root distribution, and several simulations based on dynamic root distributions with different levels of observation data assimilated into the simulations. The authors find that root growth rates follow the soil moisture distribution, and that this could be simulated by assuming that root growth rate is a function of soil moisture and only happens at a normalized water content greater than 0.075.

The results presented are interesting and potentially insightful, but unfortunately, the paper is not well organized and lacks details and clarity. Therefore I found the results difficult to interpret and the conclusions difficult to verify. The paper consists of essentially three separate methods and results sections, one for each model version. It is not clear which parameters were calibrated in each model run and how, e.g. whether in 2.4 only u3 was calibrated, or also u2, and on what data precisely. It is also not clear in which soil depth how much irrigation was applied when, and whether the vertical soil moisture differences were induced by differential irrigation or root water uptake. The main conclusion is that root growth is strongly determined by soil moisture, with high soil moisture promoting root growth. However, since irrigation was "adjusted in steps to follow the plants growth and demand for water" (L68), and the soil moisture increases over time in the deeper soil (Fig. 2C), I am a bit confused in how far the vertical soil moisture distribution was controlled and if it really triggered differential root growth. More information about the experimental strategy would be helpful.

I would propose to consolidate the methods and results in two separate sections as per standard convention, and include sufficient details about the experimental and modelling methods to enable reproduction of the experiment and anlysis. The authors promise to publish the data for producing the plots and results in the future, and provided links for the referees, but unfortunately, the links did not work for me, so I was not able to assess if the promised material will be indeed adequate to enable reproducibility of results.

I added detailed comments in an annotated pdf-file, as they are too many to list here. Some of the equations provided are incorrect, but I cannot tell if they were correct in the analysis. For linguistic glitches, I just highlighted bits of text in yellow. I hope that my review will help to revise the manuscript and add more clarity about the methods and results.

Citation: https://doi.org/10.5194/egusphere-2022-104-RC3
- AC3:
  'Reply on RC3', Cynthia Maan, 22 Jul 2022
  Dear reviewer, dear dr. Schymanski
  Thank you for your remarks and suggestions, your constructive comments are very helpful. Please find our reply below as well as in the attachment.
  The reviewer found the results difficult to interpret and the conclusions difficult to verify. `The paper consists of essentially three separate methods and results sections, one for each model version. It is not clear which parameters were calibrated in each model run and how, e.g. whether in 2.4 only u3 was calibrated or also u2, and on what data precisely. “
  
  In each section, one extra parameter is calibrated, while the other is taken from the previous step:
  In section 2.2 (‘root follows moisture’ model) the vertical extension velocity u1 is calibrated on the measured root growth profile/distribution. Note however that the results are not that sensitive to this parameter (figure 4)
  
  In section 2.3 (soil moisture and water uptake model) The water extraction rate per centimeter of roots in saturated conditions u2 is calibrated based on the soil moisture data, while u1 is taken from section 2.2 (based on the root growth data)
  
  In section 2.4 (the prognostic model) the root density growth in saturated (assumed to be ideal) conditions u3 is calibrated on the root density data. u1 (calibrated against the same root growth data) and u2 are taken directly from the previous sections and thus not calibrated for the `updated’ version of the model. We fully agree that it would be necessary to repeat these experiments for different datasets, in order to get more confidence about the parameter values.
  
  This will be clarified in the MS.
  2. It is also not clear in which soil depth how much irrigation was applied when, and whether the vertical soil moisture differences were induced by differential irrigation or root water uptake.
  The authors will make the levels of irrigation visible in figure 2. The irrigation level was increased in steps (but normally remained constant in the order of days). We will indicate the days at which the irrigation level was adjusted in figure 2. At each moment only one level is irrigated; so the flow rate at the applied level equals the total flow rate which is indicated in figure 2A. We noticed that the dotted lines in the first panels in figure 2 C were missing by omission, this will be corrected in the manuscript. During the first 14days water was supplied at 0cm, on day 15 the level was adjusted to 10 cm depth (the roots already appeared around the 10 cm line around day 8, but until the 17th there was no significant growth in the 10cm-20cm interval. We will make the sequence of steps, and the implication for causality, better visible in the MS.
  3. The reviewer is a bit confused in how far the vertical soil moisture distribution was controlled and if it really triggered differential root growth. More information about the experiment strategy would be helpful.
  The vertical soil moisture differences were induced by a combination of differential irrigation and differential water uptake. The irrigation supply, controlled at (relatively constant) low rates during the experiment, is the truly independent system driver (however not constant). The irrigation depth was generally adjusted only after the rooting depth seemed to stabilize (so the soil moisture was the first factor that changed, followed by active root growth). The spatial correlation between the water content and the root density growth rates (figure 2) suggests a causal relationship. By adding more information about the sequence of events, we will be able to clarify this.
  4.The reviewer proposes to consolidate the methods and results in two separate sections as per standard convention, and include sufficient details about the experimental and modeling methods to enable reproduction of the experiment and analysis.
  The authors agree with the reviewer that sufficient details should be given to enable reproduction. The review process helped the authors to identify the missing information in the paper, which we will add to the MS. The authors are able to provide sufficient details to enable reproduction of the experiment and analysis. However, the authors think that sufficient clarity can be given without changing the structure of the paper.
  5. The authors promised to publish the data for producing the plots and results in the future, and provided links for the referees, but unfortunately, the links did not work for the reviewer,, so he was not able to assess if the promised material will be indeed adequate to enable reproducibility of results.
  The authors are very sorry that the data has not been visible before. We will have an active DOI soon, but the data is now `already' accessible via the following link: https://figshare.com/s/f206405c95ebfbbf9bf7
  6. The reviewer notes that some of the equations provided are incorrect, but that he cannot tell if they were correct in the analysis.
  The authors revised the equations in the code, and were delighted to find that the indicated errors were omissions in the text (not in the code). However, the remarks led us to discover another omission in the code; the `effective saturation van Genuchten’ (equation 2) was used instead of `soil wetness’ in equation 6 and 7. This has been corrected and the results were analyzed. Fortunately, the resulting differences in the graphs are small and do not affect the main results and conclusions of the paper. All the figures and calibration values that are affected (figure 5,6,8) will be adjusted in the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2022-104-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (31 Jul 2022) by Bob Su

AR by Cynthia Maan on behalf of the Authors (01 Dec 2022) Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 Dec 2022) by Bob Su

RR by Anonymous Referee #1 (06 Jan 2023)

Suggestions for revision or reasons for rejection

Dear authors, thanks for your efforts to adjust your manuscript. However, some key questions were not answered. I re-put my comments as below:

1. this reviewer is not clear or the authors did not show how the current approach can be applied elsewhere in the field. Particularly, the most important parameters (e.g., u2/u3) are determined via sensitivity analysis, which makes this reviewer wonder how to obtain these parameters in the field, and further at regional/global scale?

The author responded: "The parameters first need to be determined by parameter studies on small scale (single soil-plant systems). Then the principles can be coupled to Land-atmosphere models for larger scale application."

The above response give me more questions. How are you going to do this "parameter study"? It is understandable this is now at small scale with one single soil-plant system, if you want to extend this to larger scale applications, my original questions are still remained open.

2. This reviewer commented on "the top soil was covered with plastic to prevent evaporation from the soil" (section 2.1), and the author responded that they are using actual ET as the boundary condition.

Considering the top soil surface is covered with plastic, can we still define the boundary condition with actual ET?

3. This reviewer commented "This reviewer is wondering why not using the soil sample in this experiment to determine the SWRC?" on line 118 or the original version.

The author responded "We agree that this would have been the more elegant option. We however assumed that the differences between the real SWRC and the standard values for loamy sand are small compared with other uncertainties (like in the tracking of the roots and the limited homogeneity of the soil moisture ). Hence, with this experiment (with considerable error flags) we aim to focus on very clear dominant aspects and relationships (like soil moisture driven root growth). We however did play with the model and did some parameter sensitivity tests for these paramters. The qualitative/main results are not sensitive for variation in a wide range around these values."

This reviewer is not convinced by the above response. The lab experiment is there, this reviewer assumed the authors have access to the experiment column and can easily get the SWRC parameters. This step is important for proving the reproducibility of this work.

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RR by Anonymous Referee #2 (08 Jan 2023)

RR by Stan Schymanski (30 Jan 2023)

Suggestions for revision or reasons for rejection

Dear editor, dear authors,

Thank you for considering my previous comments and expanding on the missing explanations, which enabled me to understand the contents of the paper and results much better. I found a few more contradicting statements (see detailed comments below), which may suggest that I have still misunderstood something. However, based on my now more complete understanding, I cannot recommend publication of the paper in its current form in HESS, for the following reasons.

The main conclusions of the paper are:
C1) Soil moisture exerts a strong, dominant influence on root density growth and vertical root distribution. (L200, L212)
C2) Root profiles can be predicted realistically from information on soil moisture profiles only. (L201)
C3) The root system is "insensitive to above ground processes and overall biomass growth". (L204)
C4) Implementation of the proposed root growth model in regional Land-Atmosphere models would result in "more correct representation of soil-plant water fluxes, and a more realistic representation of root biomass". (L216-219)

These conclusions are based on a single experiment where water was applied to a soil profile at progressively deeper depths as the root system of a single maize plant developed over 50 days in a rhizobox. Since there was no control experiment with a different irrigation scheme, it is not possible to tell from the results presented here whether the root system development was indeed governed by soil moisture availability or whether the pattern found here is just a result of the seedling exploring an empty soil volume. The irrigation scheme was designed "in an attempt to follow the water demand of the plant" (L76) and to "control the vertical soil moisture gradients and stimulate vertical root growth" (L78), such that water was always applied towards the bottom of the expanding root system (compare Fig. 2C with Fig. 6A-D), resulting in a vertical soil moisture distribution that largely reflected the irrigation scheme (Fig. 2). Although C1) is plausible, due to the lack of a control experiment, I cannot confirm that the results support this conclusion. Consequently, C2) is not supported by the presented results, as it is not clear whether root growth responded to soil moisture at all in this experiment, or whether the implied correlation between root growth and soil moisture was due to selective irrigation in soil layers where root growth was expected to be highest (at the bottom of the advancing root system). The conclusion that the root system "is insensitive to above-ground processes" (C3) is entirely unsupported, as above-ground processes were neither controlled nor monitored. Similarly, there is little support for Conclusion C4) in the results presented here, as the dynamics of the root system development of a single seedling growing in an initially root-free soil is of limited relevance for the simulation of the root system of a plant community in a regional Land-Atmosphere model. There are other papers in the literature that point out the importance of representing root system dynamics in such models, some of which have been cited here, but it is really far fetched to conclude that the model presented here would improve land surface models based on the results presented in this study.

The different model versions presented here provide limited potential for new insights. The finding that the exponential root distribution led to very different simulations of the vertical water uptake profile than simulations based on measured or dynamically simulated root distributions is not surprising given that root water uptake was simulated as a linear function of root length density and soil moisture. In fact, the representation of root water uptake adopted here ignores the non-linearity of the water retention and hydraulic conductivity curves, so it is not clear what can be learned from these simulations.

The main insights I drew from the paper are:
1) Maximum root growth was observed predominantly at the bottom of the advancing root system (Fig. 2c). At the same time, this is where the watering was predominantly taking place. Therefore, it is not clear if this growth pattern was due to the watering or a natural root system development of this variety of maize, whereas the watering followed the root development. I assume that the decision to design the irrigation scheme in a way to "stimulate vertical root growth" was based on prior experiments where vertical root growth did not occur to the same extent. It might help to include these experiments in the paper as controls or different treatments, so that the effect of the watering scheme becomes more obvious.
2) Vertical root distribution deviated substantially from an exponential distribution, with an almost inverted exponential root density profile, having the highest root density at the bottom of the root system at the end of the experiment (Fig. 6D). Believing that root growth does respond positively to soil moisture, or at least that it is hampered when the soil dries out, the vertical root profiles found in this study are likely due to the specific irrigation scheme at the bottom of the root system while the top of the root system is left to dry out after 20 days. This does show that root systems can deviate from the exponential distribution under certain conditions, but it does not automatically mean that exponential root distributions are wrong representations in land surface models where water replenishment happens predominantly by infiltration at the soil surface.

My co-workers and I are in the process of preparing a manuscript ourselves where we documented the dynamic responses of maize root systems to water pulses, so I do believe that your Conclusion C1) is correct, but unfortunately, I cannot see clear support by the data presented here. Since the other conclusions are in my view even less supported by the results presented here, I cannot recommend publication of the paper in its current form. However, I hope that my detailed comments below help re-structure and expand the paper in a way that it can be published in a suitable journal in the future.

DETAILED COMMENTS:

Unfortunately, the manuscript structure is still confusing:
1 Introduction
2 Experiments
2.1 Experimental Set-up
2.2 Diagnostic model of root growth: root follows moisture
Time series and correlations
Model formulation
Model parameter evaluation and results
2.3 Soil moisture and water uptake model
Model formulation
Model parameter evaluation
Results and discussion
2.4 A prognostic model for coupled soil moisture and root growth
Model formulation
Model calibration
Results and discussion
3 Conclusions

As shown above, there are numbered and un-numbered sections, where the un-numbered section titles are repeated multiple times. This makes the reading confusing, so I would recommend numbering all sections, and ideally consolidate them into one methods section where all three model versions are described, followed by one results and discussion section.

L76: How was water demand in each layer measured? According to Fig. 2A, soil moisture was highest at the depths where water was added, which suggests that water was added beyond the amount of water taken up by roots.

Eq. 1: As L is increasing over time (Eq. 4), local root growth would decline at constant theta_n over time. Is this realistic?

Eqs. 1-4: Please clarify in the equations that it is r(t), not r, and L(t), not L. So r(t) is the observed overall rate of increase in root length, whereas Eq. 1 provides the rate of increase in root length in a given soil layer. So in essence, Eq. 1 is:
dR(z, t)/dt = integral(d(R(z,t)/dt) dz) * theta_n/integral(theta_n dz)
The hypothesis formulated in Eq. 1 is hence that the vertical root growth distribution follows the vertical soil moisture distribution in a linear way. It would be good to make this clearer.

L119: This is not actually root growth, but extension rate of rooting depth, see Eq. 4.

L119: Based on what observations? Based on observed rate of extension of the rooting depth or calibrated to reproduce measured root distribution?

L124: Why is this an extra condition? According to Eq. 2, theta_n<0 if theta<0.075, and according to Eq. 1, root growth should be negative if theta_n<0, so this should be already satisfied.

L135: I would put the fraction in brackets to make very clear that the exponent applies to the whole fraction.

Eq. 9: This ignores the non-linearity of the water retention and hydraulic conductivity curve. A linear relation between normalised soil moisture and root water uptake rate seems very unrealistic.

L148: Driven by the measured root density data?

L161: Why "except for the first period"? I see 0.75 for the exponential profile, compared to 0.77 and 0.81 for the others.

L162-169: This suggests that the fact that the vertical soil moisture distribution was relatively similar between the exponential and modelled root profile simulations was due to a negative feedback loop between water depletion by root water uptake and reduced root water uptake by reduced soil moisture. However, this argument is not supported by the experimental evidence, as the experiment was designed in a way to avoid depletion of soil moisture by root water uptake. In fact, root water uptake was over-compensated, leading to a soil moisture profile with the highest soil moisture where the root density was highest (Fig. 6, except for 40-50 days). An alternative explanation is that the vertical soil moisture distribution is determined by the location of water input to the system. Fig. 2C illustrates that the soil moisture was generally highest where the water inputs took place. Since the top soil was left to dry out after 20 days (same figure), it is not surprising that root growth subsided thereafter in this part, eventually leading to the inverted vertical root distribution shown in Fig. 6D.

L175: I don't think normalization is the right term, as this would suggested that the maximal "local root growth tendency" would be 1.

L176: So now Eq. 1 is replaced by Eq. 10, i.e. root growth rate is a linear function of normalized local water content rather than the local fraction of total water content? I understand that r(t) from Eq. 1 had to be removed for prognostic simulations, but why was the division by integrated water content removed, too?

L179: "with identical overall root length": If the exponential root profile is set to have the identical overall root length as the dynamically simulated root profile, how come there is up to 54% difference in total modelled root length between the two in Table 3?

L193-: I do not understand this explanation, as according to L179, the exponential profiles were set to have the same overall root length as the dynamic root profiles, so I expect the profile to be always exponential and never become "off-shape".

L200-: This is not clear, as irrigation was applied locally where the largest root growth was expected, so it is not clear if root growth followed irrigation and soil moisture or if the dynamics would have been similar under homogeneous soil moisture.

L201: What do you mean by vertical rooting depth? Do you mean vertical root distributions?

L202: Not infiltration, but vertical water transfer model.

L206-: Not necessarily, as all the results show is that differences in vertical root distributions did not have much effect on the simulated soil moisture profiles, which could be due to the strong irrigation signal in the soil moisture profile, overwhelming the more distributed root water uptake profiles. This is underligned by the fact that soil moisture was highest where root abundance was highest in most cases, so there was no obvious effect of root water uptake on the vertical soil moisture distribution.

L212: Not necessarily, see above.

L215: Why would it prevent water demand?

L216: I would strongly advice against implementation of an empirical root growth model in LSMs, which was based on an experiment with maize seedlings.

L217: There is no evidence for such a benefit in LSMs in this paper.

Table 1: To avoid confusion, I would write in the top row: "measured RP", "modeled RP", and "exponential RP", and in the caption: "...root profiles (RP)."

Figure 2C: What do the units of root growth given in cm/cm mean? Should this be cm/cm2, or is it the root length added divided by the initial root length? The line colours in the legends and in the plots are not the same. Perhaps it would be enough to have only one legend, as it is always the same between the solid, dashed and dotted lines.

Figure 6: Why not use squares for soil moisture in the middle column to avoid confusion? What does "driver" and "results" mean?

Figure 8: The lines are wrong in the legend, as there are only solid and dashed lines in the plot.

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EF by Anna Mirena Feist-Polner (17 Jan 2023) Author's response

ED: Reconsider after major revisions (further review by editor and referees) (19 Feb 2023) by Bob Su

AR by Cynthia Maan on behalf of the Authors (03 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (16 Apr 2023) by Bob Su

AR by Cynthia Maan on behalf of the Authors (26 Apr 2023) Manuscript

Journal article(s) based on this preprint

29 Jun 2023

Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Cynthia Maan, Marie-Claire ten Veldhuis, and Bas J. H. van de Wiel

Hydrol. Earth Syst. Sci., 27, 2341–2355, https://doi.org/10.5194/hess-27-2341-2023,https://doi.org/10.5194/hess-27-2341-2023, 2023

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Debora Cynthia Maan et al.

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Their flexible growth provides the plants with a strong ability to adapt and develop resilience to droughts and climate change. But this adaptability is badly included in crop- and climate models. To model plant development in changing environments, we need to include the survival strategies of plants. Based on experimental data, we set up a simple model for soil moisture driven root growth. The model performance suggests that soil moisture is a key parameter determining root growth.


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