Water Use Efficiency of Maize (Zea mays L.) Crop under Selected Soil and Water Conservation Practices along the Slope Gradient in Ruzizi Watershed, Eastern D.R. Congo

Round 1

Reviewer 1 Report

Comments:

Interesting work with well-explored data and in a practical way.  As I work in agricultural areas with no slope, and clay texture, therefore without limitations for water retention, I did not know these techniques of “damming” water in areas with slope and sandy soil. Very simple effective and interesting.

Rainfall between 142 and 289 mm is limiting for plant growth. I miss a treatment with more water like 600 mm, not to simulate rainfed cultivation but to have an high irrigation treatment in terms of water replacement and identify potential productivity even with the other limitations.

It would be interesting to have a figure showing the historical average of rainfall throughout the years in the target area, showing the dates of planting, harvesting and the main phenological phases, which would allow identifying the possibility at which stage the plants would normally suffered water stress.

Yields above 2 t/ha were interesting in a dryland area and with these precipitations and probably other limiting factors.

Line 251. The calculation of Water Use Efficiency also seemed very interesting to me, not needing to measure photosynthesis or evapotranspiration as is normally done in the physiological evaluations of WUE.

Would it be possible to make a Pearson correlation between the parameters in the different soil conservation treatments?

In the discussion, a comparison of the differences and similarities between the cited papers and the present manuscripts was lacking.

 Line 26: Explain briefly WUE.

 

Line 40: Key word Zea mays is already in the title.

Line 40: I would use water stress instead of water deficiency not to be confused with water use efficiency.

Line 47: Are these 40% in the same biome or similar conditions?

Line 52: Any survey of the percentage of rainfed or irrigation in the target region?

Line 62: Any paper using as a strategy genotypes seleciton (genetics) for drought tolerance?

Line 63: Papers that measured WUE in tropical and drought conditions also with bimodal rainfall regime similar to the manuscript.

LINE 65: Any paper testing silicium and/or Super absorbent polymers for increasing WUE or scape from drought?

Line 106: What about drought during grain filling?

Line 113. A figure showing precipitation during the plant growth (and phenological stage) could show which phenological phase is most affected by drought in the target region.

Line 113. Any measurement of soil moisture during experiment? A curve of water retention would help during discussion.

Line 229.  Shoots already means above ground biomasses. Unecessary repetition.

Line 234. Enter the exact cycle for each genotype.

Line 242. All plots had the grains in the same humidity or there were corrections?

Line 466. Compare the paper described by Udom and Kamalu [42] checking differences and similarities with the present manuscript.

Line 482. Compare the paper described by [40, 50] checking differences and similarities with the present manuscript.

Same comments for line 492, Zhang et al. [45],  Line 501 [1],   Line 513 [53, 54,55],  Line 517 [1, 56],  ...... and others

... In summary, for several papers cited, the differences and similarities of the citations with the present manuscript should be further explored.

Line 589. Start the conclusions indicating in which treatments the humidity was below or close to the wilting point.

Line 621.     Only 16 out of 65 references were published in the last 4 years. Search for the most recent references.

Author Response

Reviewer 1
General comment:
Interesting work with well-explored data and in a practical way. As I work in agricultural areas with no slope, and clay texture, therefore without limitations for water retention, I did not know these techniques of “damming” water in areas with slope and sandy soil. Very simple effective and interesting.
General response:
Dear reviewer,
We are grateful for sparing part of your precious time to add value to our manuscript. Many thanks as well for supporting its relevance. All the concerns raised in the paper have been addressed accordingly and the revised manuscript is now much improved.
Comment 1: Rainfall between 142 and 289 mm is limiting for plant growth. I miss a treatment with more water like 600 mm, not to simulate rainfed cultivation but to have a high irrigation treatment in terms of water replacement and identify potential productivity even with the other limitations. It would be interesting to have a figure showing the historical average of rainfall throughout the years in the target area, showing the dates of planting, harvesting and the main phenological phases, which would allow identifying the possibility at which stage the plants would normally suffer water stress.
Response 1: Thank you for the suggestion. Figure 2 within the manuscript shows the rainfall distribution along the crop growth stages for the three seasons. Growth stages were defined according to the FAO protocol. The lines 207-212 in the method section present these growth stages. Since we do not have a treatment on irrigation, we have added it to recommendations for future studies in the region.
Comment 2: Yields above 2t/ha were interesting in a dryland area and with these precipitations and probably other limiting factors.
Answer Response 2: Dear Reviewer, thank you for mentioning that aspect. Two tonnes per hectare is still low compared with maize yield potential worldwide and those from the areas of the country not experiencing water deficit. However, in the dry zone of Sub-Saharan Africa and under the conditions of small-scale farmers, the result seems to be quite good, especially under extreme climatic conditions. Based on FAO data, a good commercial grain yield in an irrigated field is currently between 6-9 tons per hectare. In Ruzizi's plain, beyond water scarcity issues, it is clear that we should look for varieties and soil nutrition aspects. Nevertheless, it was not the aim of this research. We agree that other factors can influence the yield and we made such recommendation at the end of the revised manuscript. 
Comment 3: Line 251. The calculation of Water Use Efficiency also seemed very interesting to me, not needing to measure photosynthesis or evapotranspiration as is normally done in the physiological evaluations of WUE.
Would it be possible to make a Pearson correlation between the parameters in the different soil conservation treatments?
Response 3: Dear Reviewer, in lines 262-263, we explained how we performed a multiple regression preceded by a Pearson correlation matrix. This correlation allowed us to reduce the independent variables to avoid the problem of collinearity in the multiple regression. These results are presented in section 3.3.2. and 3.3.3.
Comment 4: In the discussion, a comparison of the differences and similarities between the cited papers and the present manuscripts was lacking.
Response 4: This concern was addressed accordingly in the revised manuscript. 
Comment 5: Line 26: Explain briefly WUE.
Response 5: Dear Reviewer, the definition of WUE is later provided in the introduction (line 78). Redefining it in the abstract was challenging as we are limited by the number of words. Our abstract is already longer than the journal requirement. 
Comment 6: Line 40: Keyword Zea mays is already in the title.
Response 6: The comment has been addressed accordingly
Comment 7: Line 40: I would use water stress instead of water deficiency not to be confused with water use efficiency.
Response 7: Water deficiency is a physical parameter. In contrast, Water stress refers to the impact of high-water use (either withdrawals or consumption) relative to water availability or deficiency. This is why we preferred the concept of water deficiency over water stress. 
Comment 8: Line 47: Are these 40% in the same biome or similar conditions?
Response 8: The sentence has been slightly modified to dissipate confusion from readers. The author of the quoted statement referred to the proportion of drylands covered by maize at the SSA level. The difference in maize response is 10-60% loss across environmental and agriculture production systems. 
Comment 9: Line 52: Any survey of the percentage of rainfed or irrigation in the target region?
Response 9: Maize production in the Ruzizi plain region is 100% rainfed. But line 52 is in relation with line 44, which shows the proportion in the semi-arid area of Sub-Saharan Africa.
Comment 10: Line 62: Any paper using as a strategy genotypes selection (genetics) for drought tolerance?
Response 10: We have recommended the genotype selection for future studies.
Comment 11: Line 63: Papers that measured WUE in tropical and drought conditions also with bimodal rainfall regime similar to the manuscript.
Response 11: The WUE was measured using approaches developed in drought tropical conditions. Piccinni et al. 2009 developed the Kc coefficient in south Texas, typical tropical dryland. Also, most of the comparisons we did were based on research from tropical ecosystems, especially the Sub-Saharan African drylands.
Comment 12: LINE 65: Any paper testing silicium and/or Super absorbent polymers for increasing WUE or scape from drought?
Response 12: Dear Reviewer, we appreciate the emphasize on that aspect in this research. Literature exists on using super polymers. However, our research was purely based on existing soil and water conservation practices in SSA ecosystems. Therefore, it may be challenging to consider it in the current study. 
Comment 13: Line 106: What about drought during grain filling?
Response 13: These authors have shown that there is no significant effect of water stress during dough to physiological maturity while others have shown that water stress at grain filling stage leads to significant yield reduction. This aspect is widely discussed across the discussion section of this paper.
Comment 14: Line 113. A figure showing precipitation during the plant growth (and phenological stage) could show which phenological phase is most affected by drought in the target region.
Response 14: This information is already provided by Figure 2. 
Comment 15: Line 113. Any measurement of soil moisture during experiment? A curve of water retention would help during discussion.
Response 15: The soil moisture is translated in the Soil water stored which was measured daily and results are provided at Figures 6 and 8. It has also been discussed in the discussion section. 
Comment 16: Line 229. Shoots already means above ground biomasses. Unnecessary repetition.
Response 16: Addressed accordingly.
Comment 17. Line 234. Enter the exact cycle for each genotype.
Response 17: Dear reviewer, this information was already provided in section 2.2.
Comment 18: Line 242. All plots had the grains in the same humidity or there were corrections?
Response 18: Harvest was done at maturity and then sundried to 13% moisture for all treatments and plots. This information has been added to the revised manuscript. 
Comment 19: Line 47wQbNPTDJp9hMYdvogK2hAUiHsGeiybwaWe36bwtRQ3UTpYV7YuZ8FV5j9nauFCWwcjM6dTzpL5s2N79Rp5unwdMvc8ZKUwith the present manuscript.
Response 19: Addressed accordingly
Comment 20: Line 482. Compare the paper described by [40, 50] checking differences and similarities with the present manuscript. Same comments for line 492, Zhang et al. [47wQbNPTDJp9hMYdvogK2hAUiHsGeiybwaWe36bwtRQ3UTpYV7YuZ8FV5j9nauFCWwcjM6dTzpL5s2N79Rp5unwdMvc8ZKUseveral papers cited, the differences and similarities of the citations with the present manuscript should be further explored.
Response 20: Addressed accordingly
Comment 21. Line 589. Start the conclusions indicating in which treatments the humidity was below or close to the wilting point.
Response 21: We have added the information related to soil water stored per treatment in conclusion.
Comment 22. Line 621. Only 16 out of 65 references were published in the last 4 years. Search for the most recent references.
Response 22: We agree that around 37.5% of the literature has been published in the last five years. However, it is necessary to point out that research on soil and water conservation in sub-Saharan Africa, especially in drylands, was popular from 2000 to 2015 during the Millennium Development Goals (MDGs). In the period of the MDGs, researchers have focused more on the assessments of climate change impacts in sub-Saharan Africa. Most references used in this paper were published in the last ten years. We searched the Web of Science and Scopus to select these articles. Some have been added in the manuscript. 

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript assessed soil water conservation practices in improving water use efficiency of Maize (Zea mays L.) along the slope gradient in Ruzizi Watershed, Eastern D.R. Congo. The authors found tied ridges showed potential in improving maize WUE and yield in water deficit conditions that characterize the Ruzizi Plain and could be promoted to improve maize productivity among smallholder farmers. The research is interesting and has an important practical value. 

Some suggestions are as follows:

 

1. Accuracy of calculated ET, SWC, LAI, WUE need to be discussed in the discussion section.

2. Interactions between SWC practices and slope gradient as well as interaction between season, SWC and slopes need to be discussed in the discussion section.

3. I would like to see discussions about SWC practices in other tropical semi–arid areas in the discussion section.

4. The authors found tied ridges showed potential in improving maize WUE and yield in water deficit conditions. You’d better strengthen explanations in the aspect of the mechanisms.

Author Response

Reviewer 2

General comment This manuscript assessed soil water conservation practices in improving water use efficiency of Maize (Zea mays L.) along the slope gradient in Ruzizi Watershed, Eastern D.R. Congo. The authors found tied ridges showed potential in improving maize WUE and yield in water deficit conditions that characterize the Ruzizi Plain and could be promoted to improve maize productivity among smallholder farmers. The research is interesting and has an important practical value. 

General Response

Dear Reviewer, thank you for sparing part of your precious time to review and add constructive comments to our manuscript. We are thankful as well for your support to its relevance. We have extensively considered the comments and changes have been in the manuscript.

Some suggestions are as follows:

Comment 1. Accuracy of calculated ET, SWS, LAI, WUE need to be discussed in the discussion section.

Response 1: Addressed accordingly. A section in the discussion have been added.

Comment 2. Interactions between SWC practices and slope gradient as well as interaction between season, SWC and slopes need to be discussed in the discussion section.

Response 2: Dear reviewer, this information was already provided in discussion from line 555 to 578. The discussion is based exclusively on interaction effect.

Comment 3. I would like to see discussions about SWC practices in other tropical semi–arid areas in the discussion section.

Response 3: Dear reviewer, as previously said, lines 522-580 are based on experiences in Subsaharan Africa. Tied ridges appear to be a practice more used in Africa. This is why we are explaining based on what has been reported in SSA. However, many Chinese researchers have been using a modified tied ridge in recent years, and results are also commented. The full section 4.3. is explaining all the differences based on the experience in the SSA and other drylands.  

Comment 4. The authors found tied ridges showed potential in improving maize WUE and yield in water deficit conditions. You’d better strengthen explanations in the aspect of the mechanisms.

Response 4. Dear reviewer, the explanation is provided in line 563-574.

 

Author Response File: Author Response.docx