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Monitoring of wheat crops using the backscattering coefficient and the interferometric coherence derived from Sentinel-1 in semi-arid areas

Ouaadi, Nadia ; Jarlan, Lionel ; Ezzahar, Jamal ; Zribi, Mehrez ; Khabba, Saïd ; Bouras, Elhoussaine LU orcid ; Bousbih, Safa and Frison, Pierre Louis (2020) In Remote Sensing of Environment 251.
Abstract

Radar data at C-band has shown great potential for the monitoring of soil and canopy hydric conditions of wheat crops. In this study, the C-band Sentinel-1 time series including the backscattering coefficients σ0 at VV and VH polarization, the polarization ratio (PR) and the interferometric coherence ρ are first analyzed with the support of experimental data gathered on three plots of irrigated winter wheat located in the Haouz plain in the center of Morocco covering five growing seasons. The results showed that ρ and PR are tightly related to the canopy development. ρ is also sensitive to soil preparation. By contrast, σ0 was found to be widely linked to changes in surface soil moisture (SSM) during the first... (More)

Radar data at C-band has shown great potential for the monitoring of soil and canopy hydric conditions of wheat crops. In this study, the C-band Sentinel-1 time series including the backscattering coefficients σ0 at VV and VH polarization, the polarization ratio (PR) and the interferometric coherence ρ are first analyzed with the support of experimental data gathered on three plots of irrigated winter wheat located in the Haouz plain in the center of Morocco covering five growing seasons. The results showed that ρ and PR are tightly related to the canopy development. ρ is also sensitive to soil preparation. By contrast, σ0 was found to be widely linked to changes in surface soil moisture (SSM) during the first growth stages when Leaf Area Index remains moderate (<1.5 m2/m2). In addition, drastic changes in the crop geometry associated to heading had a strong impact on the C-band σ0, in particular for VH polarization. The coupled water cloud and Oh models (WCM) were then calibrated and validated on the study sites. The comparison between the predicted and observed σ0 yielded a root mean square error (RMSE) values ranging from 1.50 dB to 2.02 dB for VV and between 1.74 dB to 2.52 dB for VH with significant differences occurring in the second part of the season after heading. Finally, new approaches based on the inversion of the WCM for SSM retrieval over wheat fields were proposed using Sentinel-1 radar data only. To this objective, the dry above-ground biomass (AGB) and the vegetation water content (VWC) were retrieved from the interferometric coherence and the PR. The relationships were then used as the vegetation descriptor in the WCM. The best retrieval results were obtained using the relationship between ρVV and the AGB (R and RMSE of 0.82, 0.05 m3/m3 respectively and no bias). The new retrieval approaches were then applied to a large database covering a rainfed field in Morocco and 18 plots of rainfed and irrigated wheat of the Kairouan plain (Tunisia) and compared to other classical techniques of SSM retrieval including simple linear relationships between SSM and σ0. The method based on the WCM and the ρVV-AGB relationships also provided with slightly better results than the others on the validation database (r = 0.75, RMSE = 0.06 m3/m3 and bias = 0.01 m3/m3 over the 18 plots of Tunisia) but the simple linear relationships performed also reasonably well (r = 0.62, RMSE = 0.07, bias = −0.01 in Tunisia for instance). This study opens perspectives for high resolution soil moisture mapping from Sentinel-1 data over south Mediterranean wheat crops and in fine, for irrigation scheduling and retrieval through the assimilation of these new products in an evapotranspiration model.

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Contribution to journal
publication status
published
subject
keywords
Backscattering coefficient, C-band, Interferometric coherence, Semi-arid region, Sentinel-1, Surface soil moisture, Winter wheat
in
Remote Sensing of Environment
volume
251
article number
112050
publisher
Elsevier
external identifiers
  • scopus:85089745440
ISSN
0034-4257
DOI
10.1016/j.rse.2020.112050
language
English
LU publication?
no
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Funding Information: This work was conducted within the frame of the International Joint Laboratory TREMA ( https://www.lmi-trema.ma/ ). The authors wish to thank the projects: Rise-H2020-ACCWA (grant agreement no: 823965 ) and ERANETMED03-62 CHAAMS for partly funding the experiments. Nadia Ouaadi was awarded mobility research training grants from PHC TBK/18/61 and from the MISTRALS/SICMED program. We would like to thank also the Moroccan CNRST for awarding a PhD scholarship to Nadia Ouaadi. ESA is acknowledged for providing free products of Sentinel-1 and the authors are grateful to Theia for producing and distributing Sentinel-2 images corrected from atmospheric effects. Finally, we would like to acknowledge Dr. Omar Rafi, the owner of the “Domaine Rafi”, for his long-lasting support to our research activities. Funding Information: This work was conducted within the frame of the International Joint Laboratory TREMA (https://www.lmi-trema.ma/). The authors wish to thank the projects: Rise-H2020-ACCWA (grant agreement no: 823965) and ERANETMED03-62 CHAAMS for partly funding the experiments. Nadia Ouaadi was awarded mobility research training grants from PHC TBK/18/61 and from the MISTRALS/SICMED program. We would like to thank also the Moroccan CNRST for awarding a PhD scholarship to Nadia Ouaadi. ESA is acknowledged for providing free products of Sentinel-1 and the authors are grateful to Theia for producing and distributing Sentinel-2 images corrected from atmospheric effects. Finally, we would like to acknowledge Dr. Omar Rafi, the owner of the “Domaine Rafi”, for his long-lasting support to our research activities. Publisher Copyright: © 2020 Elsevier Inc.
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38e040e4-30f4-4765-b5fb-851130e4d147
date added to LUP
2023-01-04 09:50:41
date last changed
2023-10-04 11:16:32
@article{38e040e4-30f4-4765-b5fb-851130e4d147,
  abstract     = {{<p>Radar data at C-band has shown great potential for the monitoring of soil and canopy hydric conditions of wheat crops. In this study, the C-band Sentinel-1 time series including the backscattering coefficients σ<sup>0</sup> at VV and VH polarization, the polarization ratio (PR) and the interferometric coherence ρ are first analyzed with the support of experimental data gathered on three plots of irrigated winter wheat located in the Haouz plain in the center of Morocco covering five growing seasons. The results showed that ρ and PR are tightly related to the canopy development. ρ is also sensitive to soil preparation. By contrast, σ<sup>0</sup> was found to be widely linked to changes in surface soil moisture (SSM) during the first growth stages when Leaf Area Index remains moderate (&lt;1.5 m<sup>2</sup>/m<sup>2</sup>). In addition, drastic changes in the crop geometry associated to heading had a strong impact on the C-band σ<sup>0</sup>, in particular for VH polarization. The coupled water cloud and Oh models (WCM) were then calibrated and validated on the study sites. The comparison between the predicted and observed σ<sup>0</sup> yielded a root mean square error (RMSE) values ranging from 1.50 dB to 2.02 dB for VV and between 1.74 dB to 2.52 dB for VH with significant differences occurring in the second part of the season after heading. Finally, new approaches based on the inversion of the WCM for SSM retrieval over wheat fields were proposed using Sentinel-1 radar data only. To this objective, the dry above-ground biomass (AGB) and the vegetation water content (VWC) were retrieved from the interferometric coherence and the PR. The relationships were then used as the vegetation descriptor in the WCM. The best retrieval results were obtained using the relationship between ρ<sub>VV</sub> and the AGB (R and RMSE of 0.82, 0.05 m<sup>3/</sup>m<sup>3</sup> respectively and no bias). The new retrieval approaches were then applied to a large database covering a rainfed field in Morocco and 18 plots of rainfed and irrigated wheat of the Kairouan plain (Tunisia) and compared to other classical techniques of SSM retrieval including simple linear relationships between SSM and σ<sup>0</sup>. The method based on the WCM and the ρ<sub>VV</sub>-AGB relationships also provided with slightly better results than the others on the validation database (r = 0.75, RMSE = 0.06 m<sup>3</sup>/m<sup>3</sup> and bias = 0.01 m<sup>3</sup>/m<sup>3</sup> over the 18 plots of Tunisia) but the simple linear relationships performed also reasonably well (r = 0.62, RMSE = 0.07, bias = −0.01 in Tunisia for instance). This study opens perspectives for high resolution soil moisture mapping from Sentinel-1 data over south Mediterranean wheat crops and in fine, for irrigation scheduling and retrieval through the assimilation of these new products in an evapotranspiration model.</p>}},
  author       = {{Ouaadi, Nadia and Jarlan, Lionel and Ezzahar, Jamal and Zribi, Mehrez and Khabba, Saïd and Bouras, Elhoussaine and Bousbih, Safa and Frison, Pierre Louis}},
  issn         = {{0034-4257}},
  keywords     = {{Backscattering coefficient; C-band; Interferometric coherence; Semi-arid region; Sentinel-1; Surface soil moisture; Winter wheat}},
  language     = {{eng}},
  month        = {{12}},
  publisher    = {{Elsevier}},
  series       = {{Remote Sensing of Environment}},
  title        = {{Monitoring of wheat crops using the backscattering coefficient and the interferometric coherence derived from Sentinel-1 in semi-arid areas}},
  url          = {{http://dx.doi.org/10.1016/j.rse.2020.112050}},
  doi          = {{10.1016/j.rse.2020.112050}},
  volume       = {{251}},
  year         = {{2020}},
}