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Revisiting the coupling between NDVI trends and cropland changes in the Sahel drylands : A case study in western Niger

Tong, Xiaoye; Brandt, Martin; Hiernaux, Pierre; Herrmann, Stefanie M.; Tian, Feng LU ; Prishchepov, Alexander V. and Fensholt, Rasmus (2017) In Remote Sensing of Environment 191. p.286-296
Abstract

The impact of human activities via land use/cover changes on NDVI trends is critical for an improved understanding of satellite-observed changes in vegetation productivity in drylands. The dominance of positive NDVI trends in the Sahel, the so-called re-greening, is sometimes interpreted as a combined effect of an increase in rainfall and cropland expansion or agricultural intensification. Yet, the impact of changes in land use has yet to be thoroughly tested and supported by empirical evidence. At present, no studies have considered the importance of the different seasonal NDVI signals of cropped and fallowed fields when interpreting NDVI trends, as both field types are commonly merged into a single ‘cropland’ class. We make use of the... (More)

The impact of human activities via land use/cover changes on NDVI trends is critical for an improved understanding of satellite-observed changes in vegetation productivity in drylands. The dominance of positive NDVI trends in the Sahel, the so-called re-greening, is sometimes interpreted as a combined effect of an increase in rainfall and cropland expansion or agricultural intensification. Yet, the impact of changes in land use has yet to be thoroughly tested and supported by empirical evidence. At present, no studies have considered the importance of the different seasonal NDVI signals of cropped and fallowed fields when interpreting NDVI trends, as both field types are commonly merged into a single ‘cropland’ class. We make use of the distinctly different phenology of cropped and fallowed fields and use seasonal NDVI curves to separate these two field types. A fuzzy classifier is applied to quantify cropped and fallowed areas in a case study region in the southern Sahel (Fakara, Niger) on a yearly basis between 2000 and 2014. We find that fallowed fields have a consistently higher NDVI than unmanured cropped fields and by using two seasonal NDVI metrics (the amplitude and the decreasing rate) derived from the MODIS time series, a clear separation between classes of fields is achieved (r = 0.77). The fuzzy classifier can compute the percentage of a pixel (250 m) under active cultivation, thereby alleviating the problem of small field sizes in the region. We find a predominant decrease in NDVI over the period of analysis associated with an increased area of cropped fields at the expense of fallowed fields. Our findings couple cropping abandonment (more frequent fallow years) with positive NDVI trends and an increase in the percentage of the cropped area (fallow period shortening) with negative trends. These findings profoundly impact our understanding of greening and browning trends in agrarian Sahelian drylands and in other drylands of developing countries characterized by limited use of fertilizers.

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Please use this url to cite or link to this publication:
author
publishing date
type
Contribution to journal
publication status
published
keywords
Cropland, Drylands, Fallowed fields, NDVI trends, Niger, Phenology, Sahel
in
Remote Sensing of Environment
volume
191
pages
11 pages
publisher
Elsevier
external identifiers
  • scopus:85011277287
ISSN
0034-4257
DOI
10.1016/j.rse.2017.01.030
language
English
LU publication?
no
id
1a570315-de3e-4772-9d4a-e5890a4f7a9a
date added to LUP
2018-06-08 14:24:52
date last changed
2018-06-12 12:01:59
@article{1a570315-de3e-4772-9d4a-e5890a4f7a9a,
  abstract     = {<p>The impact of human activities via land use/cover changes on NDVI trends is critical for an improved understanding of satellite-observed changes in vegetation productivity in drylands. The dominance of positive NDVI trends in the Sahel, the so-called re-greening, is sometimes interpreted as a combined effect of an increase in rainfall and cropland expansion or agricultural intensification. Yet, the impact of changes in land use has yet to be thoroughly tested and supported by empirical evidence. At present, no studies have considered the importance of the different seasonal NDVI signals of cropped and fallowed fields when interpreting NDVI trends, as both field types are commonly merged into a single ‘cropland’ class. We make use of the distinctly different phenology of cropped and fallowed fields and use seasonal NDVI curves to separate these two field types. A fuzzy classifier is applied to quantify cropped and fallowed areas in a case study region in the southern Sahel (Fakara, Niger) on a yearly basis between 2000 and 2014. We find that fallowed fields have a consistently higher NDVI than unmanured cropped fields and by using two seasonal NDVI metrics (the amplitude and the decreasing rate) derived from the MODIS time series, a clear separation between classes of fields is achieved (r = 0.77). The fuzzy classifier can compute the percentage of a pixel (250 m) under active cultivation, thereby alleviating the problem of small field sizes in the region. We find a predominant decrease in NDVI over the period of analysis associated with an increased area of cropped fields at the expense of fallowed fields. Our findings couple cropping abandonment (more frequent fallow years) with positive NDVI trends and an increase in the percentage of the cropped area (fallow period shortening) with negative trends. These findings profoundly impact our understanding of greening and browning trends in agrarian Sahelian drylands and in other drylands of developing countries characterized by limited use of fertilizers.</p>},
  author       = {Tong, Xiaoye and Brandt, Martin and Hiernaux, Pierre and Herrmann, Stefanie M. and Tian, Feng and Prishchepov, Alexander V. and Fensholt, Rasmus},
  issn         = {0034-4257},
  keyword      = {Cropland,Drylands,Fallowed fields,NDVI trends,Niger,Phenology,Sahel},
  language     = {eng},
  month        = {03},
  pages        = {286--296},
  publisher    = {Elsevier},
  series       = {Remote Sensing of Environment},
  title        = {Revisiting the coupling between NDVI trends and cropland changes in the Sahel drylands : A case study in western Niger},
  url          = {http://dx.doi.org/10.1016/j.rse.2017.01.030},
  volume       = {191},
  year         = {2017},
}