the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2024
Earth observation reveals reduced winter wheat growth and the importance of soil water storing capacity during drought
Abstract. Drought poses increasing challenges to global food production. Knowledge about the influence of drought on crop development and about the role of soil properties in drought risk analysis and mitigating drought impacts at the landscape level is important to guide climate change adaptation. Satellite earth observations can provide area-wide insights into crop growth processes that may help identify risk factors and quantify vulnerability to drought. Here, we evaluate the potential of Sentinel-2 to reveal interactions of plant-growth and soil parameters during variable weather conditions. As a case study, we assess winter wheat growth on 13 fields belonging to commercial farmers in southern Sweden in a dry year and a year with normal weather conditions. To track crop growth, green leaf area index (GLAI) was estimated from satellite imagery using a radiative transfer model. Proxies for winter wheat growth rate, peak GLAI, and the timing of peak GLAI were derived from the GLAI development at the single field level.
We then compared the crop growth proxies between the two years and across the fields and related them to measured soil properties. We found a lower growth rate, lower peak GLAI and earlier peak GLAI in the dry year compared to the year with normal weather conditions. An increase in peak GLAI in the dry year was also shown to be related to a higher growth rate, and this was not shown in the year with normal precipitation. Differences in crop development between years were large for some fields but small for other fields: suggesting that soil properties play a role in crop response to drought. We found that fields with a higher amount of plant available water capacity had better crop performance in the dry year and smaller relative differences in growth rate between the two years. The observed lower growth rate, lower peak GLAI, and earlier peak in the dry year compared to the year with normal weather conditions, demonstrate that satellite imagery can be used to quantify plant-soil-weather interactions at scales relevant to commercial farming. Our investigation serves as a first step towards supporting drought risk management, drought adaptation and communication activities on this important topic.
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RC1: 'Comment on egusphere-2024-1872', Anonymous Referee #1, 11 Sep 2024
General comments:
This study used a remote sensing product to timely assess crop growth at farm scale. Combining with in-situ measured field soil conditions this work evaluated the effects of soil properties and drought on crop growth. The results contribute to understanding and protecting food security under global warming. The manuscript is well motivated and nicely written. However, I believe the structure and content can be largely improved by focusing on the key novel results and keeping the language concise. There are several concerns that the authors should address to enhance the manuscript.
Main concerns:
- Neither soil properties affect plant water available water or soil water holding capacity affect crop drought stress is exactly new finding. I believe the novelty of this study needs to be clearly stated, such as GLAI at farm level can be used as a reliable indicator of crop growth and stress response to guide management.
- Are there yield data as well?
- Heat stress cannot be separated from this setting. Since accumulative temperature is one variable throughout the analysis, I would suggest including in the introduction and expanding in the discussion the impact of heat on wheat growth. Besides, dry years often have high temperatures too, especially that the DMI is associated with both precipitation and temperature.
- Please indicate field management (e.g., irrigation, fertilisation especially concerning manure and slurry, organic or conventional concerning herbicides) and cultivar for all sites in both years in M&M if the information is available. Only mentioning in the discussion that “minimised variation in these factors by selecting fields that were managed by the same farmer in 2018 and in 2021” does not address the uncertainties.
- I am glad that field management is finally brought up although in very end of the discussion (L318ff). I suggest expanding on the management practices especially that the authors highlighted them to contribute to climate change adaptation, considering there are studies on how management and tillage affect plant drought responses concerning crop performance including growth and yield. Although it is emphasised that commercially operated field studies are important, separating from field experiments and controlled potted experiments, the outcome from field research and potted experiments should not be dismissed. Instead, studies focusing on the difference among wheat cultivars (e.g., Tewolde et al. 2006), which is not directly comparable with the results presented in this study, are discussed. Several relevant studies to consider:
- Khan S, Anwar S, Shaobo Y, Gao ZQ, Sun M, Ashraf MY, Ren AX, Yang ZP. 2020. Soil water consumption, water use efficiency and winter wheat production in response to nitrogen fertilizer and tillage. PeerJ 8: e8892.
- Li YB, Hou RX, Tao FL. 2020. Interactive effects of different warming levels and tillage managements on winter wheat growth, physiological processes, grain yield and quality in the North China Plain. Agriculture Ecosystems & Environment 295: 106923.
- Sun, Q., A. K. Gilgen, R. Wittwer, G. von Arx, M. G. A. van der Heijden, V. H. Klaus, and N. Buchmann (2024), Drought effects on trait space of winter wheat are independent of land management, New Phytol, 243(2), 591-606.
- Wittwer, R. A., V. H. Klaus, E. Miranda Oliveira, Q. Sun, Y. Liu, A. K. Gilgen, N. Buchmann, and M. G. A. van der Heijden (2023), Limited capability of organic farming and conservation tillage to enhance agroecosystem resilience to severe drought, Agricultural Systems, 211, 103721.
- Please carefully check the references. I am not sure whether there are mistakes with citation tools or multiple papers were cited for the wrong reasons. For instance, I did not see LAI or GLAI data in Irfan Ullah et al. 2021and Mroz et al. 2023 so that it is unreasonable to state “some studies found no relationships between timing of peak GLAI and crop yield (Irfan Ullah et al. 2021; Mroz et al. 2023) (L282)”. Other instances include “GLAI at the heading stage of winter wheat has been shown to decrease with a high degree of soil compaction (Lipiec et al. 1991) (L56)” - LAI was used in the cited study. Are LAI and GLAI interchangeable? In the referred work, “Soil physical properties and growth of spring barley as related to the degree of compactness of two soils”, spring barley rather than winter wheat was studied.
- In the last paragraph of discussion 4.1, it reads very confusing as the terms are switching between many relevant but distinguished concepts, such as green (aboveground?) biomass as approximated by GLAI and total crop biomass, biomass and yield, growth rate and crop performance, timing of peak GLAI and heading stage. Please also consider clarifying plant available water capacity, soil water storing capacity, and soil water retention and avoiding interchanging the terms.
- Discussing the individual index of growth rate interpreted from GLAI, peak GLAI and timing of peat GLAI in this setting would only make sense after separating from the drought response (e.g., L283ff). You could consider statistical methods such as mixed effects models for such analysis.
Minor points:
- Certain content in the introduction can be rearranged, such as consider moving “The Sentinel satellites provide a high spatial resolution optical imagery of up to 10 metres… (L63ff)”, “Radiative transfer models describe the relationship between leaf and canopy traits and spectral properties of plants using physical principles…” to M&M.
- Please be consistent and use “peak GLAI” rather than only “peak” (e.g., temperature sum at peak GLAI) in text and figure legend.
- In Fig. 2, please explain the dashed line in the caption of legend.
- I wonder if Fig. 5 is essential to keep in the main text. It only serves to show one value. Consider move it to the supplement.
- Statements such as “Due to similar weather conditions (i.e., similar DMI) across all fields within a specific year, the varying crop responses to drought stress between fields imply that additional factors than the weather must have had an impact on crop development (L289ff)” does not bring any new information. Please keep the language concise throughout.
- There is a Tab. S1 and a Table S1 in the supplement. Please sort it out.
- Data availability should include the data used for analysis in this study too. Suggest including GLAI curves with the inferred slopes for all sites in the supplement.
Line-specific comments:
L13ff “Knowledge about …”: This sentence is very long and complicated. Please simplify or separate into shorter sentences.
*L17 “interactions of plant-growth and soil parameters during variable weather conditions…”: might be an overstatement for this study; the potential impact of soil properties on plant drought responses can be interpreted from the analysis, however not the feedback from the plant growth on the soil. Maybe some interactive effects of soil properties and weather conditions on crop growth but not clearly analysed. Besides, I am not sure about “variable weather conditions” by comparing two field seasons that are 3 years apart. Please revise this sentence.
L18 “a dry year and a year with normal weather conditions”: Suggest mentioning these are the year 2018 and year 2021.
L24 “An increase in peak GLAI in the dry year…": seems to be comparing among sites where differences should not be addressed as an increasement. Please revise.
L26 “suggesting that soil properties play a role in crop response to drought”: Before making this statement, differences among fields in e.g., field management, cultivar, and local climate need to be addressed. Same goes to L333ff in conclusion.
L27 “a higher amount of plant available water capacity…”: Should be “higher amount of plant available water” or something like “higher soil capacity for plant available water”; Unclear what “crop performance” refers to.
L28 “observed lower growth rate, lower peak GLAI, and earlier peak…”: suggest changing to “lower estimated growth rate”; please add “GLAI” after “earlier peak”.
L29ff: please add “in” before “the year” for “compared to the year”; “demonstrate that satellite imagery can be used to quantify plant soil-weather interactions at scales relevant to commercial farming” is not well supported by the results and analysis such as there is no “plant soil-weather interactions” nor it is quantified.
L30ff “Our investigation serves as a first step towards supporting drought risk management, drought adaptation and communication activities on this important topic”: I would strongly suggest being specific on what the “first step” is for, e.g., for the GLAI analysis using satellite imagery at field/farm level, than leaving it too broad, that e.g., involving communication activities would be a stretch.
L44ff “Soils with higher resilience to drought…”: What is soil resilience to drought? This sentence is unclear. Please revise.
L57 “there is still limited information about how soil properties affect crop development under various weather conditions”: I would say there has been more than limited studies which should be referred here to summarise the long-researched soil property impact on crop growth, although the results may vary by crop species, scales, and other environmental factors which should not be simply omitted.
L79 “around 40% of the cropping areas with winter wheat in Northern Europe had yields below the 10th percentile” I found this stat not very straightforward as it is not intuitive for what percentile 40% of the cropping areas should yield below. Suggest revising and improve clarity.
L122ff “The GLAI was derived from the radiative transfer model PROSAIL following the approach described in Graf et al. (2023)… We randomly generated combinations of leaf and canopy parameters according to a uniform or Gaussian distribution (Table S1)”: Table S1 (Tab. S1 in supplement) is identical to Table 5 in Graf et al. 2023 using the same parameters beside GLAI. I am not familiar with this approach, but I wonder if Graf et al. 2023 and two other papers cited by Graf et al. 2023 (Wocher et al. 2020 and Danner et al. 2021) on the parameters in the table should be cited here as well.
- Wocher M., Berger K., Danner M., Mauser W., Hank T. RTM-based dynamic absorption integrals for the retrieval of biochemical vegetation traits Int. J. Appl. Earth Obs. Geoinf., 93 (2020), Article 102219, 10.1016/j.jag.2020.102219
- Danner M., Berger K., Wocher M., Mauser W., Hank T. Efficient RTM-based training of machine learning regression algorithms to quantify biophysical & biochemical traits of agricultural crops ISPRS J. Photogramm. Remote Sens., 173 (2021), pp. 278-296, 10.1016/j.isprsjprs.2021.01.017
L200 Fig. 3 (c): I am confused by this figure that it seems the number of fields presented here don’t sum up to 13.
L249 Fig. 6 (c): Apologies for the clumsy question - as there are 5 circles in the plots, do they correspond to 10, 20, 30, 40, and 50 or 55? Why is the 5th circle having a smaller gap than the other ones?
L268 “Reduced wheat biomass during drought has been shown in earlier studies (Villegas et al. 2001; Zhang et al. 2018), and according to Villegas et al. (2001), the decrease in biomass und droughtis mainly due to lower growth rate”: please correct this sentence.
L273ff “highlighting the importance of faster growth to mitigate drought impacts”: this statement can only be drawn given specific drought severity, duration, and timing which I found missing in the discussion.
L283 “not unambiguous”: please revise.
Fig. S1: Suggest including long-term climate variability e.g., shown as error bar.
Citation: https://doi.org/10.5194/egusphere-2024-1872-RC1 -
RC2: 'Comment on egusphere-2024-1872', Anonymous Referee #2, 20 Sep 2024
This study shows how Sentinal-2 signals respond to winter wheat growth during drought under different soil types. The paper touches a very interesting and important topic which could add a new insight to the contribution of satellite products for field level purposes. Although the study starts with a nice story and introduction and a very logical database to address this particular issue, but it lacks a lot of critical information and data when it comes to presents and use them for interpretation. This made the interpretation of the results difficult and to some level impossible. Therefore, in my opinion, the paper must be improved substantially by providing more levels of supporting information. There are several concerns that the authors should address to enhance the manuscript.
- What is the novelty of this research? The fact that the different responses of two fields to drought could be related to soil properties is not novel. The author should make it clear how these different signals could be attributed to soil at larger scale and how this analysis lead us to landscape level. Or make the novelty of the analysis very clear.
- The authors showed only two extreme fields (Fig. 3) to present the responses of different fields. However, it is still not clear how this varies in other fields. I strongly recommend that the author provides the results for all 13 fields in the supplementary with information of soil data on each subplot. Without this, we cannot understand how the results of field which had different responses are related to soil.
- Another important lack is related to measurements of soil field capacity. The authors have measured field capacity at -10 KPa for all fields. However, based on Table S1, the clay content varies between 10 and 58%. Measuring FC at -10 KPs is valid for sandy soils and not for clayey soils which is measured at -33 KPa. So this has resulted in a wrong procedure for determining the plant available water capacity that biases the results and conclusions.
- The paper lacks a lot when it comes to presenting management data. It is not clear to what level these variations could be related to management factors. When was planting dates in different years and fields? Are there different cultivars? If they are different, how they differ in terms of phenological development? What are fertilizations? What are disease effects? All these information must be clearly provided and at the end reflected in the interpretation of results. Without this, it was difficult to me to make interpretation.
- The response of crop to drought depends on time of drought and the time of temperature increase. At which phenological stage did drought happen? I think temperature increase play a critical role here. I think here more detailed information on the stage of growth and onset of drought and temperature increase are required. The paper provides zero information on climatic variation between two years. Were the severity of drought and temperature increase the same in all years?
- The authors used the concept of temperature sum. Do they mean growing degree days? If yes, what were the base temperature?
- 3a. The crop response are significantly different even in April. This cannot be related to drought. I think the author should clearly justify using climate data that the drought already started in April. Otherwise these difference is related to factors which was overseen.
- In line 23, the author mentioned that “we found a lower growth rate, lower peak GLAI and earlier peak GLAI”. This is surprising to me, because the fig.3 shows that the peak occurs at about the same time.
- The author showed different vegetation responses in different years, however it is still not clear to what level it is related to yield data. How much these two responses did influence the final yield?
- 5 is confusing. Why the correlation was shown for only growth rate and peak GLA? What about the other?
- Line 314,”in soil properties changing over time”. This is surprising to me, soil properties do not change over 2-3 years which makes such interpretation less convincing.
Citation: https://doi.org/10.5194/egusphere-2024-1872-RC2
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