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Satellite remote sensing of primary production in semi-arid Africa

Sjöström, Martin LU (2012)
Abstract (Swedish)
Popular Abstract in Swedish

Ökningen av växthusgaser i atmosfären har orsakat en global temperaturhöjning. Afrika anses vara en av de kontinenter som kan komma att bli mest utsatta av effekterna kring ökande temperaturer med mer frekventa torkor som följd. Satellitdata ger oss möjligheten att uppskatta biomassa över stora områden vilket i sin tur kan leda till att öka vår förståelse kring hur utbytet utav koldioxid, den viktigaste växthusgasen, ter sig i tid och rum. I denna avhandling har satellitbaserade metoder för uppskattning av bruttoprimärutbyte utav koldioxid i Afrika utvecklats och utvärderats. Vidare studerades även drivkrafter bakom satellitobservationer som visar att växtligheten i Sahel, en region som sträcker... (More)
Popular Abstract in Swedish

Ökningen av växthusgaser i atmosfären har orsakat en global temperaturhöjning. Afrika anses vara en av de kontinenter som kan komma att bli mest utsatta av effekterna kring ökande temperaturer med mer frekventa torkor som följd. Satellitdata ger oss möjligheten att uppskatta biomassa över stora områden vilket i sin tur kan leda till att öka vår förståelse kring hur utbytet utav koldioxid, den viktigaste växthusgasen, ter sig i tid och rum. I denna avhandling har satellitbaserade metoder för uppskattning av bruttoprimärutbyte utav koldioxid i Afrika utvecklats och utvärderats. Vidare studerades även drivkrafter bakom satellitobservationer som visar att växtligheten i Sahel, en region som sträcker sig från Senegal i väster till Sudan i öster, ökat sedan början av 1980-talet. Resultaten visade att en ökning i nederbörd var främsta orsaken till den ökade växtligheten observerad genom satellitbilder. Dessa resultat var dock baserade på lågupplöst data. En övergång till satellitdata med högre upplösning kan ge oss möjligheten att studera dessa förändringar och effekten utav dem i bättre detalj. Ett koncept kallat ”light use efficiency” användes som en teoretisk bas för att utvärdera och utveckla modeller för att uppskatta bruttoprimärutbytet utav koldioxid baserade på data från satellitsensorn Moderate Resolution Imaging Spectroradiometer (MODIS). På flera ställen i Afrika modellerades bruttoprimärutbytet av koldioxid relativt framgångsrikt genom vegetationsindex från MODIS. Dock förbättrades uppskattningarna av bruttoprimärproduktion när faktorer som tillgång till vatten, tillgång till ljus men även mängden bladyta per enhet markyta togs hänsyn till i modellen. En befintlig satellitbaserad modell utvärderades även över flera ekosystem i Afrika. Resultaten av denna utvärdering visade att modellen underskattade bruttoprimärproduktion över flera områden. Denna underskattning var främst påtaglig över torra områden och förklarades bland annat genom brister i parametersättningar som kontrollerar omvandlingen av mängden absorberat ljus till kol. För att förbättra uppskattningar av koldioxid från satellitdata krävs vidare utvärderingar bland annat med hjälp utav markbaserade spektrala mätningar. (Less)
Abstract
With the challenges Africa faces with respect to the predicted warming due to climate change, the continents role in the global carbon cycle has been increasingly recognized. Earth observing satellites have played a significant role in the study of vegetation, particularly in Africa where climate stations are sparse. Satellite data can provide the means to map plant primary production for monitoring of food and grazing resources and to elucidate the role of African regions in the global carbon cycle. In this thesis, remote sensing based methods for large area estimation of primary production have been evaluated, primarily in semi-arid African ecosystems, by using the light use efficiency concept. Furthermore, the degree to which... (More)
With the challenges Africa faces with respect to the predicted warming due to climate change, the continents role in the global carbon cycle has been increasingly recognized. Earth observing satellites have played a significant role in the study of vegetation, particularly in Africa where climate stations are sparse. Satellite data can provide the means to map plant primary production for monitoring of food and grazing resources and to elucidate the role of African regions in the global carbon cycle. In this thesis, remote sensing based methods for large area estimation of primary production have been evaluated, primarily in semi-arid African ecosystems, by using the light use efficiency concept. Furthermore, the degree to which environmental driving forces account for a satellite observed greening trend in the African Sahel (a semi-arid region located between the Sahara and the more tropical areas to the south) from 1982 to 1998 was investigated by using a dynamic global vegetation model (DGVM). The DGVM simulations re-enforce the hypothesis that the greening as observed by satellites is related to rainfall. Although this suggests that an increase in rainfall is the dominant causative factor of the greening, the analysis was based on aggregated data. Finer observations from the moderate resolution imaging spectroradiometer (MODIS) may be used to provide more detailed measures of primary production. An overall relationship was found between eddy covariance gross primary production (GPP, the total amount of carbon assimilated through photosynthesis by plants) and the MODIS enhanced vegetation index (EVI). However, the incorporation of measurements of photosynthetically active radiation (PAR), and a water availability factor to EVI improved relationships with eddy covariance GPP. Site specific relationships to GPP with all of these variables included were found to be explained by peak leaf area index (LAI) and long-term mean annual rainfall. In addition, the MODIS GPP model was evaluated over African sites. This model estimates GPP according to the light use efficiency approach and assumes a fixed maximum rate of carbon assimilated per unit PAR absorbed (epsilon max) for different biomes. MODIS GPP correlated well with eddy covariance GPP for some sites and seasonality was generally well captured. However, GPP was underestimated for the drier sites. The underestimations were concluded to be mainly due to low values of epsilon max but can also be due to the linear interpolation of fraction of absorbed photosynthetically active radiation (FAPAR) across unreliable values. Continued efforts of land surface validation are needed to improve models driven by satellite data over semi-arid ecosystems. To better link estimates of carbon with reflectance properties, installation of radiometric instruments at eddy covariance sites would be beneficial. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Prince, Stephen, Department of Geography, University of Maryland, College Park, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
remote sensing, primary production, vegetation index, savanna, grassland
pages
122 pages
publisher
Department of Physical Geography and Ecosystem Science, Lund University
defense location
Pangea, Geocentrum II, Sölvegatan 12, Lund University, Department of Physical Geography and Ecosystem Science
defense date
2012-02-03 10:00
ISBN
978-91-85793-22-8
language
English
LU publication?
yes
id
26943429-d089-4d3c-9ffa-6844b8a887f6 (old id 2270067)
date added to LUP
2012-01-11 13:53:21
date last changed
2016-09-19 08:45:04
@misc{26943429-d089-4d3c-9ffa-6844b8a887f6,
  abstract     = {With the challenges Africa faces with respect to the predicted warming due to climate change, the continents role in the global carbon cycle has been increasingly recognized. Earth observing satellites have played a significant role in the study of vegetation, particularly in Africa where climate stations are sparse. Satellite data can provide the means to map plant primary production for monitoring of food and grazing resources and to elucidate the role of African regions in the global carbon cycle. In this thesis, remote sensing based methods for large area estimation of primary production have been evaluated, primarily in semi-arid African ecosystems, by using the light use efficiency concept. Furthermore, the degree to which environmental driving forces account for a satellite observed greening trend in the African Sahel (a semi-arid region located between the Sahara and the more tropical areas to the south) from 1982 to 1998 was investigated by using a dynamic global vegetation model (DGVM). The DGVM simulations re-enforce the hypothesis that the greening as observed by satellites is related to rainfall. Although this suggests that an increase in rainfall is the dominant causative factor of the greening, the analysis was based on aggregated data. Finer observations from the moderate resolution imaging spectroradiometer (MODIS) may be used to provide more detailed measures of primary production. An overall relationship was found between eddy covariance gross primary production (GPP, the total amount of carbon assimilated through photosynthesis by plants) and the MODIS enhanced vegetation index (EVI). However, the incorporation of measurements of photosynthetically active radiation (PAR), and a water availability factor to EVI improved relationships with eddy covariance GPP. Site specific relationships to GPP with all of these variables included were found to be explained by peak leaf area index (LAI) and long-term mean annual rainfall. In addition, the MODIS GPP model was evaluated over African sites. This model estimates GPP according to the light use efficiency approach and assumes a fixed maximum rate of carbon assimilated per unit PAR absorbed (epsilon max) for different biomes. MODIS GPP correlated well with eddy covariance GPP for some sites and seasonality was generally well captured. However, GPP was underestimated for the drier sites. The underestimations were concluded to be mainly due to low values of epsilon max but can also be due to the linear interpolation of fraction of absorbed photosynthetically active radiation (FAPAR) across unreliable values. Continued efforts of land surface validation are needed to improve models driven by satellite data over semi-arid ecosystems. To better link estimates of carbon with reflectance properties, installation of radiometric instruments at eddy covariance sites would be beneficial.},
  author       = {Sjöström, Martin},
  isbn         = {978-91-85793-22-8},
  keyword      = {remote sensing,primary production,vegetation index,savanna,grassland},
  language     = {eng},
  pages        = {122},
  publisher    = {ARRAY(0x821b2d8)},
  title        = {Satellite remote sensing of primary production in semi-arid Africa},
  year         = {2012},
}