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Remote sensing of vegetation dynamics in drylands : Evaluating vegetation optical depth (VOD) using AVHRR NDVI and in situ green biomass data over West African Sahel

Tian, Feng LU ; Brandt, Martin ; Liu, Yi Y. ; Verger, Aleixandre ; Tagesson, Torbern LU ; Diouf, Abdoul A. ; Rasmussen, Kjeld ; Mbow, Cheikh ; Wang, Yunjia and Fensholt, Rasmus (2016) In Remote Sensing of Environment 177. p.265-276
Abstract

Monitoring long-term biomass dynamics in drylands is of great importance for many environmental applications including land degradation and global carbon cycle modeling. Biomass has extensively been estimated based on the normalized difference vegetation index (NDVI) as a measure of the vegetation greenness. The vegetation optical depth (VOD) derived from satellite passive microwave observations is mainly sensitive to the water content in total aboveground vegetation layer. VOD therefore provides a complementary data source to NDVI for monitoring biomass dynamics in drylands, yet further evaluations based on ground measurements are needed for an improved understanding of the potential advantages. In this study, we assess the capability... (More)

Monitoring long-term biomass dynamics in drylands is of great importance for many environmental applications including land degradation and global carbon cycle modeling. Biomass has extensively been estimated based on the normalized difference vegetation index (NDVI) as a measure of the vegetation greenness. The vegetation optical depth (VOD) derived from satellite passive microwave observations is mainly sensitive to the water content in total aboveground vegetation layer. VOD therefore provides a complementary data source to NDVI for monitoring biomass dynamics in drylands, yet further evaluations based on ground measurements are needed for an improved understanding of the potential advantages. In this study, we assess the capability of a long-term VOD dataset (1992-2011) to capture the temporal and spatial variability of in situ measured green biomass (herbaceous mass and woody plant foliage mass) in the semi-arid Senegalese Sahel. Results show that the magnitude and peak time of VOD are sensitive to the woody plant foliage whereas NDVI seasonality is primarily governed by the green herbaceous vegetation stratum in the study area. Moreover, VOD is found to be more robust against typical NDVI drawbacks of saturation effect and dependence on plant structure (herbaceous and woody compositions) across the study area when used as a proxy for vegetation productivity. Finally, both VOD and NDVI well reflect the spatial and inter-annual dynamics of the in situ green biomass data; however, the seasonal metrics showing the highest degree of explained variance differ between the two data sources. While the observations in October (period of in situ data collection) perform best for VOD (r2 = 0.88), the small growing season integral (sensitive to recurrent vegetation) have the highest correlations for NDVI (r2 = 0.90). Overall, in spite of the coarse resolution, the study shows that VOD is an efficient proxy for estimating green biomass of the entire vegetation stratum in the semi-arid Sahel and likely also in other dryland areas.

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author
; ; ; ; ; ; ; ; and
publishing date
type
Contribution to journal
publication status
published
subject
keywords
LPRM, Plant structure, Satellite passive microwave, Saturation effect, Semi-arid, Vegetation species composition, Woody cover
in
Remote Sensing of Environment
volume
177
pages
12 pages
publisher
Elsevier
external identifiers
  • scopus:84959517665
ISSN
0034-4257
DOI
10.1016/j.rse.2016.02.056
language
English
LU publication?
no
id
cd956bce-8cc6-41ff-b3d0-96a93701679a
date added to LUP
2018-06-08 13:50:45
date last changed
2020-10-07 05:55:00
@article{cd956bce-8cc6-41ff-b3d0-96a93701679a,
  abstract     = {<p>Monitoring long-term biomass dynamics in drylands is of great importance for many environmental applications including land degradation and global carbon cycle modeling. Biomass has extensively been estimated based on the normalized difference vegetation index (NDVI) as a measure of the vegetation greenness. The vegetation optical depth (VOD) derived from satellite passive microwave observations is mainly sensitive to the water content in total aboveground vegetation layer. VOD therefore provides a complementary data source to NDVI for monitoring biomass dynamics in drylands, yet further evaluations based on ground measurements are needed for an improved understanding of the potential advantages. In this study, we assess the capability of a long-term VOD dataset (1992-2011) to capture the temporal and spatial variability of in situ measured green biomass (herbaceous mass and woody plant foliage mass) in the semi-arid Senegalese Sahel. Results show that the magnitude and peak time of VOD are sensitive to the woody plant foliage whereas NDVI seasonality is primarily governed by the green herbaceous vegetation stratum in the study area. Moreover, VOD is found to be more robust against typical NDVI drawbacks of saturation effect and dependence on plant structure (herbaceous and woody compositions) across the study area when used as a proxy for vegetation productivity. Finally, both VOD and NDVI well reflect the spatial and inter-annual dynamics of the in situ green biomass data; however, the seasonal metrics showing the highest degree of explained variance differ between the two data sources. While the observations in October (period of in situ data collection) perform best for VOD (r<sup>2</sup> = 0.88), the small growing season integral (sensitive to recurrent vegetation) have the highest correlations for NDVI (r<sup>2</sup> = 0.90). Overall, in spite of the coarse resolution, the study shows that VOD is an efficient proxy for estimating green biomass of the entire vegetation stratum in the semi-arid Sahel and likely also in other dryland areas.</p>},
  author       = {Tian, Feng and Brandt, Martin and Liu, Yi Y. and Verger, Aleixandre and Tagesson, Torbern and Diouf, Abdoul A. and Rasmussen, Kjeld and Mbow, Cheikh and Wang, Yunjia and Fensholt, Rasmus},
  issn         = {0034-4257},
  language     = {eng},
  month        = {05},
  pages        = {265--276},
  publisher    = {Elsevier},
  series       = {Remote Sensing of Environment},
  title        = {Remote sensing of vegetation dynamics in drylands : Evaluating vegetation optical depth (VOD) using AVHRR NDVI and in situ green biomass data over West African Sahel},
  url          = {https://lup.lub.lu.se/search/ws/files/45734135/Tian_et_al_2016.pdf},
  doi          = {10.1016/j.rse.2016.02.056},
  volume       = {177},
  year         = {2016},
}