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Investigating vegetation changes in the African Sahel 1982-2002 : a comparative analysis using Landsat, MODIS and AVHRR remote sensing data

Sjöström, Martin (2004) In Lunds universitets Naturgeografiska institution - Seminarieuppsatser
Dept of Physical Geography and Ecosystem Science
Abstract
There has been much debate concerning the concept of degradation and desertification in semi-arid lands, in particular semi-arid Sub-Saharan Africa, during the last two decades. However, recent findings suggest a consistent trend of increasing satellite-derived vegetation greenness in much of the African Sahel as interpreted from Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) data for the period 1982-1999. The Sahel has suffered several devastating droughts and famines during recent decades and an increasing trend in NDVI could be interpreted as a vegetation recovery from the severe droughts of the 1980s, as preliminary analysis indicate an increase in rainfall during this period.... (More)
There has been much debate concerning the concept of degradation and desertification in semi-arid lands, in particular semi-arid Sub-Saharan Africa, during the last two decades. However, recent findings suggest a consistent trend of increasing satellite-derived vegetation greenness in much of the African Sahel as interpreted from Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) data for the period 1982-1999. The Sahel has suffered several devastating droughts and famines during recent decades and an increasing trend in NDVI could be interpreted as a vegetation recovery from the severe droughts of the 1980s, as preliminary analysis indicate an increase in rainfall during this period. This study includes an analysis of spatial and temporal trends of vegetation, covering the years 1982-2002, for areas in the western, eastern and central parts of the African Sahel in order to try to verify and explain the observed increasing trends in NDVI. By implementing two change detection techniques, visual interpretation and change vector analysis of high resolution satellite sensor data, it was observed that vegetation patterns differed, with recent year satellite imagery showing higher amounts of vegetation. It was, however, observed through comparison of phenological activity between the years of the acquired imagery that seasonal differences exist, most probably due to different climatic conditions preceding the recordings. Linear trend regressions of NOAA NDVI and rainfall data was analysed, and separately show increasing trends. The relationship between these two was also established by regression. The observed recent trends in vegetation activity cannot entirely be explained by increasing rainfall but rather as a combination of driving forces. (Less)
Abstract (Swedish)
Populärvetenskaplig sammanfattning: Det så kallade Sahelbältet, sträcker sig från Atlanten i väst till Röda Havet i öst och gränsar mellan Sahara i norr och de mer tropiska områdena i söder. Regionen är torr med oregelbunden nederbörd och anknyts ofta till begreppen ökenspridning och markförstörelse. I kontrast mot spekulationer kring dessa begrepp visar nyligen utförda studier på en signifikant positiv trend i Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) över stora delar av Sahel mellan åren 1982-1999. NDVI är ett satellitbaserat vegetationsindex som används för att få en uppfattning om vegetationens mängd och dess tillstånd. Den påvisade positiva trenden kan tolkas som en... (More)
Populärvetenskaplig sammanfattning: Det så kallade Sahelbältet, sträcker sig från Atlanten i väst till Röda Havet i öst och gränsar mellan Sahara i norr och de mer tropiska områdena i söder. Regionen är torr med oregelbunden nederbörd och anknyts ofta till begreppen ökenspridning och markförstörelse. I kontrast mot spekulationer kring dessa begrepp visar nyligen utförda studier på en signifikant positiv trend i Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) över stora delar av Sahel mellan åren 1982-1999. NDVI är ett satellitbaserat vegetationsindex som används för att få en uppfattning om vegetationens mängd och dess tillstånd. Den påvisade positiva trenden kan tolkas som en återhämtning av växtlighet efter de återkommande perioder av torka som drabbade Sahel under 70- och 80-talet. I syfte att verifiera och förklara den observerade ökningen i NDVI, ingår i denna studie analyser av både rumsliga och temporala trender mellan åren 1982-2002 för områden i östra, centrala och västra Sahel. Den rumsliga delen innefattas av visuell tolkning samt en radiometriskt baserad förändringsanalys kallad ”change vector analysis”. Dessa metoder tillämpades på högupplöst (30 x 30 m) satellitdata. Resultaten visar på en ökad växtlighet, då det förekommer mer vegetation i senare års satellitdata. Dock visade jämförelser av fenologisk aktivitet mellan de analyserade bildparen att säsongsskillnader existerade. Analys av tidsserier från Global Inventory and Mapping Studies (GIMMS) NDVI samt nederbörd visade på en positiv trend mellan åren 1982-2002. I flera studier har man funnit ett samband mellan NDVI och nederbörd, således tillämpades en regressionsanalys för att undersöka hur väl dessa två korrelerade med varandra. Resultaten pekar på att den observerade ökningen i NDVI inte helt och hållet kan förklaras med ökad nederbörd. Ökningen tros istället vara ett resultat av en kombination av faktorer. (Less)
Please use this url to cite or link to this publication:
author
Sjöström, Martin
supervisor
organization
alternative title
Undersökning av vegetationsförändringar i Sahel 1982-2002
year
type
H1 - Master's Degree (One Year)
subject
keywords
marklära, desertification, semi-arid, Sahara, physical geography, geomorphology, pedology, cartography, climatology, geomorfologi, naturgeografi, kartografi, klimatologi
publication/series
Lunds universitets Naturgeografiska institution - Seminarieuppsatser
report number
106
language
English
id
1333072
date added to LUP
2005-10-11
date last changed
2011-12-12 16:40:41
@misc{1333072,
  abstract     = {There has been much debate concerning the concept of degradation and desertification in semi-arid lands, in particular semi-arid Sub-Saharan Africa, during the last two decades. However, recent findings suggest a consistent trend of increasing satellite-derived vegetation greenness in much of the African Sahel as interpreted from Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) data for the period 1982-1999. The Sahel has suffered several devastating droughts and famines during recent decades and an increasing trend in NDVI could be interpreted as a vegetation recovery from the severe droughts of the 1980s, as preliminary analysis indicate an increase in rainfall during this period. This study includes an analysis of spatial and temporal trends of vegetation, covering the years 1982-2002, for areas in the western, eastern and central parts of the African Sahel in order to try to verify and explain the observed increasing trends in NDVI. By implementing two change detection techniques, visual interpretation and change vector analysis of high resolution satellite sensor data, it was observed that vegetation patterns differed, with recent year satellite imagery showing higher amounts of vegetation. It was, however, observed through comparison of phenological activity between the years of the acquired imagery that seasonal differences exist, most probably due to different climatic conditions preceding the recordings. Linear trend regressions of NOAA NDVI and rainfall data was analysed, and separately show increasing trends. The relationship between these two was also established by regression. The observed recent trends in vegetation activity cannot entirely be explained by increasing rainfall but rather as a combination of driving forces.},
  author       = {Sjöström, Martin},
  keyword      = {marklära,desertification,semi-arid,Sahara,physical geography,geomorphology,pedology,cartography,climatology,geomorfologi,naturgeografi,kartografi,klimatologi},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lunds universitets Naturgeografiska institution - Seminarieuppsatser},
  title        = {Investigating vegetation changes in the African Sahel 1982-2002 : a comparative analysis using Landsat, MODIS and AVHRR remote sensing data},
  year         = {2004},
}