USLE K-Factor Method Selection for a Tropical Catchment
, Daniel F. Carvalho 2, Jamil A. A. Anache 3,*, Dulce B. B. Rodrigues 4 and Paulo Tarso S. Oliveira 4
Round 1
Reviewer 1 Report
The purpose of this study is to evaluate the performance of different methods to estimate soil erodibility for soil loss and sediment yield modeling using sediment yield observations. We computed K-factor values from three widely used empirical equations and using a portable rainfall simulator.
I believe that the experimental data are precious.
1. please specify the values of parameter m (dependent on the slope) in equation 8.
2. It may be better if the compare and contrast observed and is shown in the diagrams (predicted, observed).
3. The literature review is relevant. I suggest to extend it with recent works available in literature dealing with the experimental investigation of soil erosion (last 5 years).
Author Response
RC1-01: The purpose of this study is to evaluate the performance of different methods to estimate soil erodibility for soil loss and sediment yield modeling using sediment yield observations. We computed K-factor values from three widely used empirical equations and using a portable rainfall simulator. I believe that the experimental data are precious.
Thank you for recognizing the importance of the topic described by our manuscript. We replied and solved, when possible, all the concerns and remarks found along the text to improve its comprehension and quality.
RC1-02: 1. please specify the values of parameter m (dependent on the slope) in equation 8.
Thank you for the reminder. We added the required information. (P6L178).
RC1-03: 2. It may be better if the compare and contrast observed and is shown in the diagrams (predicted, observed).
We have compared the observed vs predicted sediment yield in Figure 4B. (P8L233)
“Figure 4. Monthly averages and monitored rates for 2010/2011 of rainfall (lines) and EI30 index (bars) distribution along the study period (A); sediment yield in the Córrego Água Azul catchment predicted by SDR-USLE using direct and indirect methods of K-factor; values measured in the outlet (primary axes); and observed rainfall in the study area during the monitoring period (secondary axes) (B)”.
RC1-04: 3. The literature review is relevant. I suggest to extend it with recent works available in literature dealing with the experimental investigation of soil erosion (last 5 years).
We inserted additional relevant literature reviews as suggested by the Reviewer (See some examples below):
Borrelli, et al. [1],Panagos, et al. [2],Couto, et al. [3],Lin, et al. [4],Anache, et al. [5].
References
1. Borrelli, P.; Robinson, D.A.; Fleischer, L.R.; Lugato, E.; Ballabio, C.; Alewell, C.; Meusburger, K.; Modugno, S.; Schutt, B.; Ferro, V., et al. An assessment of the global impact of 21st century land use change on soil erosion. Nat Commun 2017, 8, 2013, doi:10.1038/s41467-017-02142-7.
2. Panagos, P.; Borrelli, P.; Meusburger, K.; Yu, B.; Klik, A.; Jae Lim, K.; Yang, J.E.; Ni, J.; Miao, C.; Chattopadhyay, N., et al. Global rainfall erosivity assessment based on high-temporal resolution rainfall records. Sci Rep 2017, 7, 4175, doi:10.1038/s41598-017-04282-8.
3. Couto, A.A.; Da Conceição, F.T.; Fernandes, A.M.; Spatti, E.P.; Lupinacci, C.M.; Moruzzi, R.B. Land use changes associated with the expansion of sugar cane crops and their influences on soil removal in a tropical watershed in São Paulo State (Brazil). Catena 2019, 172, 313-323, doi:10.1016/j.catena.2018.09.001.
4. Lin, B.-S.; Chen, C.-K.; Thomas, K.; Hsu, C.-K.; Ho, H.-C. Improvement of the K-Factor of USLE and Soil Erosion Estimation in Shihmen Reservoir Watershed. Sustainability 2019, 11, 355, doi:10.3390/su11020355.
5. Anache, J.A.A.; Flanagan, D.C.; Srivastava, A.; Wendland, E.C. Land use and climate change impacts on runoff and soil erosion at the hillslope scale in the Brazilian Cerrado. Sci Total Environ 2018, 622-623, 140-151, doi:10.1016/j.scitotenv.2017.11.257.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript attempts to estimate the sediment loss at the outlet of the watershed with the universal soil loss equation and a sediment delivery function.
The USLE was developed for the eastern USA based on soil losses from plots 72 m long. Applying it the USLE has been done in many circumstances and is therefore an accepted practice. However, it means that none of the parameters in the USLE can be assumed to be known a priori. Therefore, the slope factor LS, the erodibility K, the crop and management factor C, and even the rainfall energy E (tropical rainstorms are different than the storms in the USA) are uncertain and not only the erodibility factor K as is assumed by the authors.
The sediment delivery function is even more uncertain than the USLE and therefore these parameters cannot be assumed to be certain.
Therefore, to have a title of the manuscript “Different approaches to compute the USLE K-factor in a tropical watershed” is just not correct. Considering the uncertainty of the other parameters the authors just cannot assume that they determine with any certainty the K factor.
Thus, the analysis is flawed in this manuscript. However, what is interesting but not shown are the soil losses obtained with the rainfall simulator and at the outlet. By emphasizing these results and showing and discussing the soil loss of the rainfall simulators and the daily sediment loss and discharge at the outlet might make the paper acceptable for publication. The authors can then at the end of the article show the soil loss calculated with the USLE on a monthly basis and compare that with the monthly soil loss at the outlet. Base on that a sediment delivery function for the watershed can be developed.
In rewriting the manuscript please note that the EQs 1 and 2 have constant that depend on the unit used. So, the units need to be mentioned or the equation need to be written without the constants.
Hopefully with this review the manuscript can be improved is such a way that it can be accepted for publication
Author Response
RC2-01: The manuscript attempts to estimate the sediment loss at the outlet of the watershed with the universal soil loss equation and a sediment delivery function. The USLE was developed for the eastern USA based on soil losses from plots 72 m long. Applying it the USLE has been done in many circumstances and is therefore an accepted practice. However, it means that none of the parameters in the USLE can be assumed to be known a priori. Therefore, the slope factor LS, the erodibility K, the crop and management factor C, and even the rainfall energy E (tropical rainstorms are different than the storms in the USA) are uncertain and not only the erodibility factor K as is assumed by the authors. The sediment delivery function is even more uncertain than the USLE and therefore these parameters cannot be assumed to be certain.
We recognize the fact that USLE and SDR models were developed within the USA to make predictions considering the USA conditions [1]. However, those methods are widely used worldwide [2] and our role as scientists is to test these models in different conditions. In countries where field data is scarce (e.g. Brazil), any attempt to test USLE parametrization is a valid effort to evaluate how uncertain the method can be. We included SDR estimates because of the observed data, which is the sediment yield monitored in the catchment’s outlet. Thus, we selected an SDR method that uses distributed modelling and presented a satisfactory performance for distributed modeling. (See P6L198)
RC2-02: Therefore, to have a title of the manuscript “Different approaches to compute the USLE K-factor in a tropical watershed” is just not correct. Considering the uncertainty of the other parameters the authors just cannot assume that they determine with any certainty the K factor.
Following the Reviewer suggestion, we changed the manuscript title to “USLE K-factor method selection for a tropical catchment”.
RC2-03: Thus, the analysis is flawed in this manuscript. However, what is interesting but not shown are the soil losses obtained with the rainfall simulator and at the outlet. By emphasizing these results and showing and discussing the soil loss of the rainfall simulators and the daily sediment loss and discharge at the outlet might make the paper acceptable for publication. The authors can then at the end of the article show the soil loss calculated with the USLE on a monthly basis and compare that with the monthly soil loss at the outlet. Base on that a sediment delivery function for the watershed can be developed.
As described in the manuscript Introduction (P1L45), the soil erodibility factor (K-factor) represents the rate of soil loss per unit of erosion index in a standard plot 22.1 m long, on a 9% slope under a continuous bare cultivated fallow, tilled in slope direction [3]. As it is necessary to perform experimental fieldwork under natural or artificial rainfall for each soil class, the determination of the K-factor is expensive and time-consuming [4,5]. Therefore, in large countries such as Brazil, the determination of this factor become more difficult, taking a gap or inaccurate information about its values. We cannot compare the soil loss from the rainfall simulator with the “soil loss at the outlet” because the observations made in the outlet consists of the sediment yield and these data are not comparable. They can be comparable when the soil loss in the basin is multiplied by the sediment delivery ratio (SDR). The SDR represents the fraction of soil loss that is transported to the stream and became sediment yield that reaches the catchment’s outlet. Thus, to compute the soil loss for the whole catchment using the rainfall simulator data, we calculated the K-factor for each soil class, in order to perform the distributed application of the USLE equation as described in the Material and Methods section (P2L74), however, such data is not comparable to the catchment sediment yield. The idea to develop an SDR equation to the catchment seems to be a good idea using the USLE modeled erosion using the rainfall simulator data, however, the rainfall simulator plots (0.7 m2) are not computing rill erosion (it only measures interrill erosion) [6,7], and this is the main reason for the poor predictions for sediment yield estimations using the rainfall simulator K-factor (we added this argument on P11L309).
RC2-04: In rewriting the manuscript please note that the EQs 1 and 2 have constant that depend on the unit used. So, the units need to be mentioned or the equation need to be written without the constants.
We changed Equations 1 and 2 descriptions to improve its comprehension (P3). Thank you for the suggestion.
RC2-05: Hopefully with this review the manuscript can be improved is such a way that it can be accepted for publication, Regards, Tammo Steenhuis
We appreciated the discussion proposed by reviewer 2 and his insights were very helpful to improve the quality of our manuscript.
References
1. Kinnell, P.I.A. A comparison of the abilities of the USLE-M, RUSLE2 and WEPP to model event erosion from bare fallow areas. Sci Total Environ 2017, 596-597, 32-42, doi:10.1016/j.scitotenv.2017.04.046.
2. Borrelli, P.; Robinson, D.A.; Fleischer, L.R.; Lugato, E.; Ballabio, C.; Alewell, C.; Meusburger, K.; Modugno, S.; Schutt, B.; Ferro, V., et al. An assessment of the global impact of 21st century land use change on soil erosion. Nat Commun 2017, 8, 2013, doi:10.1038/s41467-017-02142-7.
3. Wischmeier, W.H.; Smith, D.D. Predicting Rainfall Erosion Losses. A guide to conservation planning. USDA Agriculture Handbook 1978, 537.
4. Nearing, M.A.; Romkens, M.J.M.; Norton, L.D.; Stott, D.E.; Rhoton, F.E.; Laflen, J.M.; Flanagan, D.C.; Alonso, C.V.; Binger, R.L.; Dabney, S.M., et al. Measurements and Models of Soil Loss Rates. Science 2000, 290, 1300-1301, doi:10.1126/science.290.5495.1300b.
5. Lin, B.-S.; Chen, C.-K.; Thomas, K.; Hsu, C.-K.; Ho, H.-C. Improvement of the K-Factor of USLE and Soil Erosion Estimation in Shihmen Reservoir Watershed. Sustainability 2019, 11, 355, doi:10.3390/su11020355.
6. Alves Sobrinho, T.; Gómez, J.A.; Macpherson, H.G. A portable rainfall and overland flow simulator Soil Use and Management 2008, 24, 163–170, doi:10.1111/j.1475-2743.2008.00150.x.
7. Gómez, J.A.; Nearing, M.A. Runoff and sediment losses from rough and smooth soil surfaces in a laboratory experiment. Catena 2005, 59, 253-266, doi:10.1016/j.catena.2004.09.008.
Author Response File: Author Response.pdf
Reviewer 3 Report
Main comments
The study compares different methods in assessing the USLE’s K-factor in a Brazilian tropical region. The study is important and relevant in terms of sustainable land-use and management. Particularly, it’s relevance stems from its relations to potential land degradation processes, with the correspondence potential deterioration in the provision of ecosystem services. The manuscript is very well written, and clearly delivers the study’s message for the readers. See below for few specific comments.
Introduction
Overall, the introduction section is clear and very well structured.
Line 49. ‘continental countries such as Brazil’: unclear. Usually, the term ‘continental’ is being used for continental climatic conditions. Brazil does not experience such climatic conditions. Please re-write.
It is unknown what are the study hypothesis. Following the study objective (lines 64-66), please specify the study hypothesis.
Materials and methods
The utilized methodology is very clearly detailed along this section.
In the Study Area sub-section, please provide details on lithology (‘basic rock’ is not enough).
Results and discussion
The results are clearly presented and thoroughly discussed. The tables, figures, and maps are very helpful.
Conclusions
In this section, you are not supposed to detail the methodology and results. Instead, you are only requested to very shortly describe the study insight. Accordingly, this section could be much shorter (up to approximately a half of its current length).
Author Response
RC3-01: Main comments
The study compares different methods in assessing the USLE’s K-factor in a Brazilian tropical region. The study is important and relevant in terms of sustainable land-use and management. Particularly, it’s relevance stems from its relations to potential land degradation processes, with the correspondence potential deterioration in the provision of ecosystem services. The manuscript is very well written, and clearly delivers the study’s message for the readers. See below for few specific comments.
Thank you for recognizing the importance of the topic described by our manuscript. We replied and solved, when possible, all the concerns and remarks found along the text to improve its comprehension and quality.
RC3-02: Introduction
Overall, the introduction section is clear and very well structured.
Line 49. ‘continental countries such as Brazil’: unclear. Usually, the term ‘continental’ is being used for continental climatic conditions. Brazil does not experience such climatic conditions. Please re-write. It is unknown what are the study hypothesis. Following the study objective (lines 64-66), please specify the study hypothesis.
The term ‘continental’ was used to express the country size. We changed the term to ‘large’, in order to clarify the meaning of the sentence. We added the study hypothesis before the objectives (P2L64) in the revised manuscript.
RC3-03: Materials and methods
The utilized methodology is very clearly detailed along this section.
In the Study Area sub-section, please provide details on lithology (‘basic rock’ is not enough).
We added further information about the rock type and its composition (P2L78).
RC3-04: Results and discussion
The results are clearly presented and thoroughly discussed. The tables, figures, and maps are very helpful.
We appreciated your comment.
RC3-05: Conclusions
In this section, you are not supposed to detail the methodology and results. Instead, you are only requested to very shortly describe the study insight. Accordingly, this section could be much shorter (up to approximately a half of its current length).
Following the Reviewer suggestion, we summarized the first paragraph of the conclusion, keeping only a brief description of the study for those readers who goes straight to the conclusion and also to remind the reader that read the entire paper all the pathway to reach the conclusion (P13L363).
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Please see the attachement
Comments for author File: Comments.pdf
Author Response
Please check the PDF file.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
The comments were well addressed and the manuscript is greatly improved.
Glad that the comments were useful
Regards
Tammo Steenhuis
