Abstract
Problem statement: The Normalized Difference Vegetation Index (NDVI) is the most extensively used satellite-derived index of vegetat ion health and density. Since climate is one of the most important factors affecting vegetation conditi on, satellite-derived vegetation indexes have been often used to evaluate climatic and environmental c hanges at regional and global scale. The proposed study attempted to investigate the temporal vegetat ion dynamics in the whole Africa using historical NDVI time-series. Approach: For this aim, 15 day maximum value NDVI composites at 8 km spatial resolution produced from the NASA Global Inventory Mapping and Monitoring System (GIMMS) had been used. They were derived from data collected da ily by NOAA AVHRR satellites. The AVHRR NDVI GIMMS dataset was freely available and gives global coverage over an extensive time period. First of all, the selected NDVI base data had been geometrically pre-processed and organized into a historical database implemented in order to grant t heir spatial integration. Starting from this archiv e, monthly and yearly NDVI historical time-series, ext ended from 1982-2006, had been then developed and analysed on a pixel basis. Several routines had e been developed in IDL (Interactive Data Language programming tool) with the purpose of applying suit able statistical analysis techniques to the histori cal information in the database in order to identify th e long-term trend components of generated NDVI time-series and extract vegetation dynamics. Specif ic tests had been then considered in order to defin e the validity of results. Results: The existence of clear regional trends of NDVI, bot h decreasing and increasing had been showed, which helped to highlight areas subject, respectively to reduction or increase in vegetation greenness. Conclusion: As the relationship between the NDVI and vegetatio n productivity was well established, these estimated long-term trend components may be also, with much more caution, related to historical and ongoing lan d degradation or improvement processes.
Highlights
Land degradation and desertification have been major global issues during the last years because of their adverse impact on agronomic productivity, the environment and their effect on food security and the quality of life[1,2]
In this study we propose the first results of a study aimed at investigating, with proper robust statistical techniques, the temporal vegetation dynamics in the whole Africa using historical Normalized Difference Vegetation Index (NDVI) time-series
NDVI measurements range between -1 and +1; in practice, the measurements generally range between -0.1 and +0.7
Summary
Land degradation and desertification have been major global issues during the last years because of their adverse impact on agronomic productivity, the environment and their effect on food security and the quality of life[1,2]. According to Eswaran[2], about 33% of the global l and surface (42 million km2) is subject to desertification and, in particular, the semi-arid to weakly arid areas of Africa are vulnerable, as they have fragile soils, localized high population densities and generally a lowinput form of agriculture. Land degradation and desertification are strictly related to a reduced potential productivity or utility of plants and land, undesirable alterations in the biomass, soil deterioration, resulting from various factors, including natural or climatic variations and human activities[1,2,4].
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