Evaluating the Accuracy of Contour Ridgeline Positioning for Soil Conservation in the Northeast Black Soil Region of China

Round 1

Reviewer 1 Report

 This manuscript study the orientation and position of contour ridging using AcrMap and Digital Elevation Models (DEMs) for estimating the reduction in soil erosion on sloped farmland after implementing contour ridging. In the manuscript, authors use Unmanned Aerial Vehicles(UAV) to obtain high-precision DEMs in the Sanjiang Plain of Heilongjiang Province, China. I think this is an interesting and useful topic. However, there are still doubts to be clarified as follows:

(1) The authors of the Web page do not match the authors of the PDF.

(2) The words in all Figures are not clearly visible.

(3)  It is suggested that a flowchart be added to the "Methodology" section.

Author Response

Dear reviewer:

We appreciate your comments on our draft “Accuracy of Ridgeline Positioning for Contour Ridging in Slope Farmland of the Northeast Black Soil Region of China”(sustainability-2917176). Those comments are valuable for our draft improvement. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Below are responses to your comments.

Q1: The authors of the Web page do not match the authors of the PDF.

Response: We would express our apology for your confusion. We confirmed that the authors of the PDF are correct. The authors in the PDF while not on the web page are students who haven’t got an academic email. We will try to get academic emails for all authors.

Q2: The words in all Figures are not clearly visible.

Response: We are grateful for the suggestion. All figures are reproduced to have larger word sizes. Hopefully words are visible enough.

Q3: It is suggested that a flowchart be added to the "Methodology" section.

Response: Thank you for the suggestion. A flowchart showing processes of contour ridging implemented, data collection as well as data analysis was added to "Methodology" section.

Once again thank you very much for your help and your future comments are expected.

Best regards,

Authors

Reviewer 2 Report

The article is well argued scientifically. I appreciate the explanation concerning soil estimated models is very clear. This is essentially a very technical and serious article.

However, ploughing plots along contour lines is a practice known to many farmers and is a matter of common sense. What does quantifying erosion really do in terms of agricultural yield durability, apart from during the designing ridgelines process, the turns should be made as smooth as possible ?

The usefulness of UAV overflights is not sufficiently explained, in particular what data is provided by the DEM in the quantitative erosion assessment when due to Contour Ridging. Why fly over these plots three times when a flying wing drone can cover a much larger area, with high-precision GPS control points for Z with dedicated software ?

Please argue also if the are some studies who mentionned UAV to quantify the erosion process during several years (cf bibliography)

Concerning other factors to limit the erosion process: the contribution of crop varieties and their adaptability to black soils is not addressed. The question of soil richness and the use of inputs is not addressed. (cf bibliography)

Author Response

Dear reviewer:

We appreciate your comments on our draft “Accuracy of Ridgeline Positioning for Contour Ridging in Slope Farmland of the Northeast Black Soil Region of China”(sustainability-2917176). Those comments are valuable for our draft improvement. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Below are responses to your comments.

Q1: However, ploughing plots along contour lines is a practice known to many farmers and is a matter of common sense. What does quantifying erosion really do in terms of agricultural yield durability, apart from during the designing ridgelines process, the turns should be made as smooth as possible?

Response: Thank you very much for your suggestion. Since the average topsoil in Northeast China black soil region is around 30 cm, and research about crop yields from topsoil removal experiments shows that a horizon depth of 25 cm may be a threshold for sustainable soil productivity in eroded soils, soil conservation measures that could maintain yield and reduce soil erosion to soil loss tolerance is critical for sustainable agriculture in this region. We add sentences in section of “4.1 Soil and Water Conservation Effects of Contour Ridging”as belows: “Investigation of 290 observations of crop yields from topsoil removal experiments worldwide demonstrated that a horizon depth of 25 cm may be a threshold for sustainable soil productivity in eroded soils [34]. Given that the average depth of black topsoil is less than 30 cm due to serious soil erosion [35], contour ridging could be crucial for sustainable agriculture in Northeast China because it could ensure agricultural yield while reducing soil erosion to soil loss tolerance.”

Q2: The usefulness of UAV overflights is not sufficiently explained, in particular what data is provided by the DEM in the quantitative erosion assessment when due to Contour Ridging.

Response: We apologize that how data from DEM is used for quantitative erosion assessment was not clear in the draft. The contour ridging changed ridge direction, which means slope gradient along ridge direction would also be changed. Slope gradient along contour ridgeline was calculated using DEM and contour ridgeline, and slope gradient along downslope ridgeline was calculated using DEM and downslope ridgeling. The reduction of slope gradient along ridgeline caused by contour ridging could reflect soil loss reduction. In the draft sentences of “The slope gradient along ridge would be reduced when traditional downslope ridging is converted to contour ridging. The reduction in slope gradient along ridge can be calculated from high-precision DEM and changes in ridge direction, and this reduction can also represent the magnitude of soil erosion reduction” were added. If the above explanations are not clear or your question is not fully answered, please let us know. Thank you!

Q3: Why fly over these plots three times when a flying wing drone can cover a much larger area, with high-precision GPS control points for Z with dedicated software ?

Response: We apologize that our draft caused your confusion. This study conducted a total of three drone flights, one for each of the three plots, not three flights per plot.

Q4: Please argue also if the are some studies who mentionned UAV to quantify the erosion process during several years (cf bibliography)

Response: Thank you very much for your suggestion. We referenced some studies about using UAV to quantify erosion process as follows: “The high-precision DEM obtained by drones can reflect the fine elevation of the earth surface [26], thus using multi-temporal DEMs from drones to investigate the soil erosion process has been widely applied [27, 28].”

Q5: Concerning other factors to limit the erosion process: the contribution of crop varieties and their adaptability to black soils is not addressed. The question of soil richness and the use of inputs is not addressed. (cf bibliography)

Response: Thank you very for your crucial suggestions which are very important for contour ridging application. Contour ridging tends to cause waterlog in the field, and corn might be more appropriate for contour ridging. For soils with bad infiltration capacity, higher slope gradient along ridge is important for discharge. We discuss the crop selection and slope gradient modification for different soils as follows: “The reduction in slope gradient along ridge also implies that surface runoff would be difficult to discharge, which may cause waterlogging within slope farmland. Given that soybeans are susceptible to waterlogging [33], contour ridging might be more appropriate for corn. Additionally, for soils with low infiltration rates, such as vertosols, increasing the slope gradient along ridge to enhance the capacity for surface runoff discharge is advised; for soils with high infiltration rates, such as sandy soils, decrease the ridge orientation slope to reduce the capacity for surface runoff discharge could be appropriate.” In section of “Soil and Water Conservation Effects of Contour Ridging”

Once again thank you very much for your help and your future comments are expected.

Best regards,

Authors

Reviewer 3 Report

The introduction provides a clear overview of the importance of the Northeast black soil region in China's grain production and the issue of soil erosion, though additional context on current soil erosion prevention methods and why contour ridging is promising would be beneficial. The methodology is well-described, but further details on how CORS RTK measured XYZ coordinates and potential limitations would enhance rigour. The results summarize key findings, such as slope gradient and positional offsets, though the inclusion of statistical measures would bolster robustness. Practical recommendations for contour ridging improvement are offered, valuable for soil erosion prevention efforts. The language is clear, but consistency and avoidance of jargon could improve accessibility. Including future research directions would help readers grasp broader implications and inspire further investigation. Overall, the manuscript effectively communicates the study's objectives, methods, findings, and implications, though incorporation of suggested enhancements could enhance clarity, comprehensiveness, and impact.

I suggest undergoing a thorough grammar check.

Author Response

Dear reviewer:

We appreciate your comments on our draft “Accuracy of Ridgeline Positioning for Contour Ridging in Slope Farmland of the Northeast Black Soil Region of China”(sustainability-2917176). Those comments are valuable for our draft improvement. Based on the instructions provided in your letter, we uploaded the file of the revised manuscript. Below are responses to your comments.

Q1: The introduction provides a clear overview of the importance of the Northeast black soil region in China's grain production and the issue of soil erosion, though additional context on current soil erosion prevention methods and why contour ridging is promising would be beneficial.

Response: We agree with you that contour ridging’s advantage is important for its future application. Sentence of “Compared with other slope farmland soil conservation measures such as subsoiling or crop rotation, contour farming could maintain agricultural productivity without additional agricultural machinery, making it an appropriate soil conservation strategy for slope farmland in this region.” Were added into section of Introduction.

Q2: The methodology is well-described, but further details on how CORS RTK measured XYZ coordinates and potential limitations would enhance rigour.

Response: Thank you very much for your suggestion. We added sentences about RTK precision and details about how to use RTK to measure ground elevation. Full sentences are as follows: “The verification point’s coordinates (XYZ) were measured using the CHCNAV RTK in a fixed solution state (10 mm + 1 ppm horizontally and 20 mm + 1 ppm vertically). During measurement, the bottom of the RTK pole was placed in the middle of the furrow on the ground and kept level. Since the RTK pole might penetrate the topsoil under its weight, ensuring that the bottom of the RTK pole remained on the surface of the topsoil was important, thereby guaranteeing the elevation measurements represent the surface elevation.” in the section of 2.3 Data Collection.

Q3: The results summarize key findings, such as slope gradient and positional offsets, though the inclusion of statistical measures would bolster robustness.

Response: we added sentences as follows: “Most of the verification point offsets for these three plots were under 1.5 m. For plot A, verification point offsets between 0 and 0.3 m, 0.3 and 0.6 m, 0.6 and 1.5 m occupied 25%, 64% and 11% of total points, respectively. Such values of plot C were 32%, 31%, and 29%, respectively, and the left 9% were between 1.5 and 3.5 m. For plot B, around one-third of the total points’ offsets were larger than 1.5 m.” And We apologize that our draft caused your confusion. This study conducted a total of three drone flights, one for each of the three plots, not three flights per plot. Paragraph orders were also modified to clearly state statistical measures.

Q4: The language is clear, but consistency and avoidance of jargon could improve accessibility.

Response: Thank you very much for your suggestion. We rechecked through the draft to keep consistency. We applied “sloped farmland” to “Sloped farmland”, and “contour farming” to “contour ridging”, as well as others. Jargons were replaced with common words.

Q5: Including future research directions would help readers grasp broader implications and inspire further investigation.

Response: Thank you very much for your crucial suggestions which are very important for draft improvement. We added a net section of “Implications for Future Works”. Here are the details: “The spatial distribution of baseline, as well as simulated ridgelines, were obtained using ArcGIS and high-precision DEM in this research. Indeed, the spatial distribution of slope gradient along contour ridgelines could also be obtained based on the spatial distribution of these simulated ridgelines. The research revealed that the positional offsets of ridgelines varied within the slope farmland. Consequently, discrepancies between the measured and simulated values of slope gradient along contour ridgelines might exist and exhibit spatial heterogeneity. The effect of spatial variability of slope gradient along contour ridgelines on the redistribution of soil moisture, nutrients, and sediments needs to be further demonstrated. Besides, investigating the impact of the discrepancies between the measured and simulated values of slope gradient along contour ridgelines on the model simulations of the aforementioned processes is worth exploring.”

Q6: I suggest undergoing a thorough grammar check.

We rechecked the grammar thorough the draft.

Once again thank you very much for your help and your future comments are expected.

Best regards,

Authors

Round 2

Reviewer 2 Report

Thanks to the authors for their pertinent and well-argued answers.