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Assessing Sahelian vegetation and stress from seasonal time series of polar orbiting and geostationary satellite imagery: Limitations and new potentials

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

  • Jørgen Lundegaard Olsen
Vegetation in the semi-arid Sahel has been widely studied using remote sensing data acquired from satellite based instruments. Due to the size of the region and limited ground based monitoring networks, remote sensing is the only feasible approach for quantifying spatial and temporal variation of vegetation
across the region. The vegetation growth is mainly water restricted and since annual cumulative rainfall varies from year to year, so does the conditions for vegetation growth.
Much of the current knowledge of Sahelian vegetation status and long term development is based on commonly applied vegetation remote sensing methodology. The objective of this Ph.D. study has been to examine limitations of these methods and to investigate the potential for improvement.Currently the most widely applied use of remote sensing data for vegetation analysis is based on reflectance data in the red and near infrared (NIR) spectral regions, acquired from instruments onboard polar orbiting platforms. The red spectral region is characterized by strong chlorophyll absorption and the NIR spectral region is reflected by healthy vegetation controlled by the spongy mesophyll layer in green leaves. Red and NIR reflectance data can be combined into the normalized difference vegetation index (NDVI). NDVI have been found related to important biophysical parameters, such as Net Primary Production (NPP) and vegetation cover, which will be covered in more detail in the introduction and in the individual papers. The red and NIR data from the Advanced Very High Resolution Radiometer (AVHRR) instruments are the only suitable time series of data available for large scale studies that goes back to the 1980s, where widespread droughts were experienced in the Sahel. From the AVHRR NDVI data a greening of the Sahel have been identified since the 80s and attributed to increasing trends in annual rainfall for large parts of the region.
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The Dahra test site and the data acquired there are presented in detail in the fourth paper, and subsets of the data are also presented in the third paper for investigating SEVIRI NDVI and SIWSI. Together with the AMMA Gourma super site in Mali and the Demokeya test site in Sudan, this is one of the few Sahelian sites
providing observations of ecosystem properties and hydrological and meteorological variables over many years. The site was established in 2002 and data from 2002 to 2012 are presented in paper four. Variation in annual NPP is strongly related to annual rainfall, but neither greening nor browning trends were found during the 10 year time period. Inter-annual variation in plant species composition is found related to the start of the rainy season, and productivity was affected by dominant species. Especially for years dominated by the planophile herb Zornia glochidiata, the biomass production was found different from other years. This difference was not observed in in situ NDVI or fraction of absorbed photosynthetic active radiation (FAPAR), thus challenging dryland productivity models. NDVI and FAPAR was linearly correlated in
until peak growing season, but during senescence NDVI decreased while a large fraction of FAPAR was still absorbed. This shows limitations in using EO based NDVI for estimating FAPAR over a full growing season. The decade long in situ time series are unique and valuable for assessing ecosystem response of Sahelian
savanna to climate and environmental change, and provide much needed opportunity for validating Earth observation data.
OriginalsprogEngelsk
ForlagDepartment of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen
Antal sider190
StatusUdgivet - 12 jun. 2014

Bibliografisk note

Ph.d.-grad opnået ved mundtligt forsvar 29. august 2014

ID: 122992811