the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Feb 2023
Mammalian bioturbation amplifies rates of both, hillslope sediment erosion and accumulation, in coastal Chile
Abstract. Soil bioturbation activity affects soil texture, bulk density, soil water content and redistribution of nutrients. All of these parameters influences sediment redistribution, which shapes the earth surface. Hence it is important to include bioturbation into erosion models. However, up to present, the inclusion of bioturbation into erosion models was limited. This is because to realistically include bioturbation into the modelling, the interplay between bioturbation, sediment redistribution and environmental parameters is not understood.
Here, we included bioturbation into a soil erosion model and interpreted the impacts of bioturbation on sediment redistribution. To do this, we measured the needed soil properties and location of burrows created by bioturbating animals in four research sites located along the Chilean climate gradient. Then, we parametrized a semi-empirical erosion model by applying machine learning algorithms to upscale soil properties and burrow distribution. We ran the model for a time period of 6 years under two conditions: With and without bioturbation. We validated the model using several sediment fences in the field. We estimated the modelled sediment redistribution and surface runoff in all climate zones. Lastly, we identified environmental parameters determining the positive or negative impact of bioturbation on sediment redistribution.
We found that the model with integrated bioturbation performed much better (R2 = 0.71, RMSE = 0.63) than the model without integrated bioturbation (R2 = 0.17, RMSE = 1.18), meaning that model runs which considered bioturbation predicted the sediment redistribution more realistically. Furthermore, bioturbation increased sediment redistribution in all but the humid climate zone, especially in the Mediterranean zone. The quantity of sediment redistributed due to bioturbation was reliant on an interplay between elevation, slope, surface roughness and sink connectivity. Overall, bioturbation enhances sediment erosion in areas where more erosion is expected, and enhances sediment accumulation in areas which are more prone to accumulate sediment. In other words, considering bioturbation when studying earth surface evolution means an amplification of existing tendencies in sediment redistribution, and leads to a faster hillslope relief equalisation.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-84', Emmanuel Gabet, 02 Mar 2023
The authors clearly did a lot of work both in terms of field measurements and modeling. The results and the framing of the manuscript of the manuscript, however, left me feeling a bit underwhelmed. The main conclusions seem to be that (1) bioturbation increases rates of sediment redistribution, (2) that the rate of sediment transport by bioturbation increases with slope, and (3) models that incorporate bioturbation will yield better results than models that don't when applied to landscapes where bioturbation is an important process. First, conclusions (1) and (2) are not novel - both of these have been know for a long time and have already been demonstrated in previous studies. Second, conclusion (3) seems trivial - of course models that incorporate the main sediment transport processes will be more accurate than those that don't. I would encourage the authors, therefore, to identify what is truly new and unique about their study.
Also, the framing of the project felt a bit misleading. In the introduction, the authors argue that few models have accounted for bioturbation and that, when they have, the effects of bioturbation are 'hard-coded' into them. First of all, this is not accurate, and I provide examples of models where the effects of bioturbation are emergent properties. Second, the model presented by the authors has several instances where processes related to bioturbation are hard-coded into the model and do not arise naturally from more fundamental processes like competition, access to resources, compaction, etc. Therefore, I would recommend that the authors become more familiar with other models that have incorporated bioturbation and, also, not oversell their own model.
Not enough information was provided regarding the model. I understand that the model is described elsewhere, but individual manuscripts must stand on their own. For example, the authors report that, at some sites, including bioturbation in the model increases runoff rates but decreases in others; however, no explanation was provided describing why bioturbation would affect runoff rates and, without a description of how the model works, it's impossible for a reader to evaluate or even understand this result. I think that the most interesting result of this study is how well the modeled sediment flux matches the measured sediment flux, but without a good description of the model, that result was difficult to evaluate. Also, given the importance of that comparison, I would recommend putting Figure A3 into the main text.
Finally, I would recommend cutting down on the number of figures. There were figures presented as results that were just a simple function of how the model was parameterized (eg., Fig. A5) or figures that didn't seem to be relevant (eg, Fig. A6).
- AC1: 'Reply on RC1', Paulina Grigusova, 04 May 2023
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RC2: 'Comment on egusphere-2023-84', Anonymous Referee #2, 08 Mar 2023
This manuscript presents field and modeling data to explore the impact of bioturbation on erosion and sedimentation on hillslopes. I think this is a worthwhile study to publish. However, I did not understand some of the modeling methods, which made the results mostly uninterpretable for me. I think differences in nomenclature may have also made it difficult for me to follow.
My main challenges were that I didn't understand how the model worked, so it was difficult for me to interpret the results. Even if I understood the basics of the model, I did not understand how the sensitivity analysis was carried out, which I think, but I am not sure, led to the results presented in Table 3, which I also did not understand. Without understanding the model experiments, it was difficult for me to take away much about how the processes impacted erosion and deposition.
It seems like the authors did a very thorough experiment, so hopefully my questions and comments can illustrate where I got lost. With a bit more clarity, I think this paper would be a lot more impactful. For now I'm left with "bioturbation increases erosion and sediment accumulation", and especially in temperate climate zones. But if I actually understood Table 3, I would have a lot better chance of using what they found to understand how bioturbation might be impacting another landscape.
Detailed comments:
L 110-111: Don't process-based models need to be parametierized for each location? I'm confused by the statement "once calibrated, they can be applied to almost any site"L 140: What does high spatial and temporal resolution mean? Can you quantify? Do you need to model variability with depth? Later in the paragraph you mention 0.5 meter resolution and daily time step - are those minimums? Maximums? One person's high resolution can be another person's low resolution. In other words, the definition of high resolution can vary among readers.
In Figure 1, is the scale bar in the lower right the same for b - d?
L 246 What is "above-ground skeleton"
L 261 : How is dm different from Sd? If they are the same, I think you should probably use the same symbol.
Isn't equation 4 the ratio of non-organic matter, because Sc has the organic matter burned out?
L 292, 293 : I don't know what the equations for TPI and TRI are, or what they represent. It might be good to at least describe them.
L 364 : I'm a bit confused about the depth aspect of the model. Is this a 2-D or 3-D model? When you write that you include vertical movement of sediment particles, does that mean you are keeping track of the density of the subsurface? Or are you just piling loose material on the surface of the hillslope?
Section 3.5 I don't understand how parameters were chosen for each of the individual cites. You describe a sensitivity test in the first paragraph of Section 3.5. In the second paragraph on validation, there is nothing about what parameters were chosen, besides the different bioturbation experiments.
For the sensitivity tests, you "stepwise changed the input parameter values" (L 382). Table A2 shows the results of the sensitivity analysis. But isn't there co-variance among the parameters? And are all the relationships between parameter change and erosion change linear? It's impossible to tell that from this table. Also note in Table A2, I think some of the parameter values need units. Erosion also needs units.
You then refer to Fig. A2, ("To quantify the significance of the input variations, we conducted a t-test (Fig A2).") but this seems to me like a spread of variables for each site. I guess I didn't understand Fig. A2, and maybe that was critical for understanding the sensitivity analysis?Line 407, 408 - Couldn't a positive value also mean that bioturbation reduced erosion in comparison with no bioturbation? Is that possible?
Line 417 "We quantified the model performance ..." this implies to me that your sensitivity test is your calibration. Is that correct? And if you are only varying one parameter at a time, how do you find the optimal value for all parameters?
L 444 - How can you accumulate more sediment than you erode? Where does the sediment come from?
L 445 - 447: These numbers need some context for me. I don't know how significant a 15.6% rise in erosion rate is? What does this mean for the landscape? And aren't there error bounds in these numbers, because isn't there a whole suite of parameter values that can be tested with and without burrows? From Figure 5a, it looks to me like there is a lot of overlap in sediment redistribution rates between the with and without bioturbation. I'm not sure I udnerstood figure 5a though. Is each dot from a pixel on the landscape? How do we know how the dots align with each other? That is, what are the comparative values at a point with and without bioturbation?
Figure 6 - In general I think plots like this one are better with binned rather than gradational color schemes. In other words, distinct colors for erosion in contrast to accumulation. Same could be said for Fig. 7.
L 556 It might be helpful to explain a bit more about what the settings of these other studies. I don't know anything about those studies so I don't know if their methods were similar to yours.
L 563: I would be very cautious about comparing bioturbation results with other studies. I would guess that bioturbation impacts erosion almost everywhere, but the degree that it impacts erosion probably varies a lot based on local variables and parameters. So comparing between different settings should probably be done with more context.
Figure 8 : What does "NDVI" mean in the accumulation zone? Should NDVI have a high or low before it?
Table 3 is very confusing for me. For example, how is elevation a parameter? You could have two hillslopes with the exact same everything (slopes, vegetation, burrow density) but one is from 0 to 100 m asl and the other is 500 - 600 m asl. Why would that matter for sediment redistribution locally on the hillslope. Shouldn't they behave in the same way?
What is meant by slope? Is that local slope? Or average slope on the hillslope? Why does slope have a middle value in which there is accumulation? That's hard to envision process wise?
In general, this table seems like it might be a good summary figure, but I needed someone to walk me through it and explain it.Citation: https://doi.org/10.5194/egusphere-2023-84-RC2 - AC2: 'Reply on RC2', Paulina Grigusova, 04 May 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-84', Emmanuel Gabet, 02 Mar 2023
The authors clearly did a lot of work both in terms of field measurements and modeling. The results and the framing of the manuscript of the manuscript, however, left me feeling a bit underwhelmed. The main conclusions seem to be that (1) bioturbation increases rates of sediment redistribution, (2) that the rate of sediment transport by bioturbation increases with slope, and (3) models that incorporate bioturbation will yield better results than models that don't when applied to landscapes where bioturbation is an important process. First, conclusions (1) and (2) are not novel - both of these have been know for a long time and have already been demonstrated in previous studies. Second, conclusion (3) seems trivial - of course models that incorporate the main sediment transport processes will be more accurate than those that don't. I would encourage the authors, therefore, to identify what is truly new and unique about their study.
Also, the framing of the project felt a bit misleading. In the introduction, the authors argue that few models have accounted for bioturbation and that, when they have, the effects of bioturbation are 'hard-coded' into them. First of all, this is not accurate, and I provide examples of models where the effects of bioturbation are emergent properties. Second, the model presented by the authors has several instances where processes related to bioturbation are hard-coded into the model and do not arise naturally from more fundamental processes like competition, access to resources, compaction, etc. Therefore, I would recommend that the authors become more familiar with other models that have incorporated bioturbation and, also, not oversell their own model.
Not enough information was provided regarding the model. I understand that the model is described elsewhere, but individual manuscripts must stand on their own. For example, the authors report that, at some sites, including bioturbation in the model increases runoff rates but decreases in others; however, no explanation was provided describing why bioturbation would affect runoff rates and, without a description of how the model works, it's impossible for a reader to evaluate or even understand this result. I think that the most interesting result of this study is how well the modeled sediment flux matches the measured sediment flux, but without a good description of the model, that result was difficult to evaluate. Also, given the importance of that comparison, I would recommend putting Figure A3 into the main text.
Finally, I would recommend cutting down on the number of figures. There were figures presented as results that were just a simple function of how the model was parameterized (eg., Fig. A5) or figures that didn't seem to be relevant (eg, Fig. A6).
- AC1: 'Reply on RC1', Paulina Grigusova, 04 May 2023
-
RC2: 'Comment on egusphere-2023-84', Anonymous Referee #2, 08 Mar 2023
This manuscript presents field and modeling data to explore the impact of bioturbation on erosion and sedimentation on hillslopes. I think this is a worthwhile study to publish. However, I did not understand some of the modeling methods, which made the results mostly uninterpretable for me. I think differences in nomenclature may have also made it difficult for me to follow.
My main challenges were that I didn't understand how the model worked, so it was difficult for me to interpret the results. Even if I understood the basics of the model, I did not understand how the sensitivity analysis was carried out, which I think, but I am not sure, led to the results presented in Table 3, which I also did not understand. Without understanding the model experiments, it was difficult for me to take away much about how the processes impacted erosion and deposition.
It seems like the authors did a very thorough experiment, so hopefully my questions and comments can illustrate where I got lost. With a bit more clarity, I think this paper would be a lot more impactful. For now I'm left with "bioturbation increases erosion and sediment accumulation", and especially in temperate climate zones. But if I actually understood Table 3, I would have a lot better chance of using what they found to understand how bioturbation might be impacting another landscape.
Detailed comments:
L 110-111: Don't process-based models need to be parametierized for each location? I'm confused by the statement "once calibrated, they can be applied to almost any site"L 140: What does high spatial and temporal resolution mean? Can you quantify? Do you need to model variability with depth? Later in the paragraph you mention 0.5 meter resolution and daily time step - are those minimums? Maximums? One person's high resolution can be another person's low resolution. In other words, the definition of high resolution can vary among readers.
In Figure 1, is the scale bar in the lower right the same for b - d?
L 246 What is "above-ground skeleton"
L 261 : How is dm different from Sd? If they are the same, I think you should probably use the same symbol.
Isn't equation 4 the ratio of non-organic matter, because Sc has the organic matter burned out?
L 292, 293 : I don't know what the equations for TPI and TRI are, or what they represent. It might be good to at least describe them.
L 364 : I'm a bit confused about the depth aspect of the model. Is this a 2-D or 3-D model? When you write that you include vertical movement of sediment particles, does that mean you are keeping track of the density of the subsurface? Or are you just piling loose material on the surface of the hillslope?
Section 3.5 I don't understand how parameters were chosen for each of the individual cites. You describe a sensitivity test in the first paragraph of Section 3.5. In the second paragraph on validation, there is nothing about what parameters were chosen, besides the different bioturbation experiments.
For the sensitivity tests, you "stepwise changed the input parameter values" (L 382). Table A2 shows the results of the sensitivity analysis. But isn't there co-variance among the parameters? And are all the relationships between parameter change and erosion change linear? It's impossible to tell that from this table. Also note in Table A2, I think some of the parameter values need units. Erosion also needs units.
You then refer to Fig. A2, ("To quantify the significance of the input variations, we conducted a t-test (Fig A2).") but this seems to me like a spread of variables for each site. I guess I didn't understand Fig. A2, and maybe that was critical for understanding the sensitivity analysis?Line 407, 408 - Couldn't a positive value also mean that bioturbation reduced erosion in comparison with no bioturbation? Is that possible?
Line 417 "We quantified the model performance ..." this implies to me that your sensitivity test is your calibration. Is that correct? And if you are only varying one parameter at a time, how do you find the optimal value for all parameters?
L 444 - How can you accumulate more sediment than you erode? Where does the sediment come from?
L 445 - 447: These numbers need some context for me. I don't know how significant a 15.6% rise in erosion rate is? What does this mean for the landscape? And aren't there error bounds in these numbers, because isn't there a whole suite of parameter values that can be tested with and without burrows? From Figure 5a, it looks to me like there is a lot of overlap in sediment redistribution rates between the with and without bioturbation. I'm not sure I udnerstood figure 5a though. Is each dot from a pixel on the landscape? How do we know how the dots align with each other? That is, what are the comparative values at a point with and without bioturbation?
Figure 6 - In general I think plots like this one are better with binned rather than gradational color schemes. In other words, distinct colors for erosion in contrast to accumulation. Same could be said for Fig. 7.
L 556 It might be helpful to explain a bit more about what the settings of these other studies. I don't know anything about those studies so I don't know if their methods were similar to yours.
L 563: I would be very cautious about comparing bioturbation results with other studies. I would guess that bioturbation impacts erosion almost everywhere, but the degree that it impacts erosion probably varies a lot based on local variables and parameters. So comparing between different settings should probably be done with more context.
Figure 8 : What does "NDVI" mean in the accumulation zone? Should NDVI have a high or low before it?
Table 3 is very confusing for me. For example, how is elevation a parameter? You could have two hillslopes with the exact same everything (slopes, vegetation, burrow density) but one is from 0 to 100 m asl and the other is 500 - 600 m asl. Why would that matter for sediment redistribution locally on the hillslope. Shouldn't they behave in the same way?
What is meant by slope? Is that local slope? Or average slope on the hillslope? Why does slope have a middle value in which there is accumulation? That's hard to envision process wise?
In general, this table seems like it might be a good summary figure, but I needed someone to walk me through it and explain it.Citation: https://doi.org/10.5194/egusphere-2023-84-RC2 - AC2: 'Reply on RC2', Paulina Grigusova, 04 May 2023
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Paulina Grigusova
Annegret Larsen
Roland Brandl
Camilo del Río
Nina Farwig
Diana Kraus
Leandro Paulino
Patricio Pliscoff
Jörg Bendix
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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