Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003
Introduction
The Sahel is a semi-rid region stretching approximately 5000 km across northern Africa from the Atlantic Ocean in the west to near the Red Sea in the east and extending roughly from 12°N to 18°N. This region forms an ecological transition between the Sahara desert to the north and the humid tropical savanna to the south (Le Houerou, 1980). The characteristic vegetation types constitute mixtures of grasslands, shrubs and thorny trees. In general, the precise geographic location of the Sahel is difficult to distinguish because these physical characteristics change over time and space (Le Houerou, 1980; Monod, 1986). Generally, the northern limit is defined by the 100–200 mm/year rainfall contour and the southern limit by the 400–600 mm/year as shown in Fig. 1a. The rainfall regime in the region is characterized large variations from year to year compounded by persistent long-term drought since the early 1970s. Since the mid 1960s this region has experienced a systematic decrease in rainfall and wide spread droughts affecting the larger area of Sub-Saharan Africa (Tanaka et al., 1975; Bunting et al., 1976; Nicholson, 1979; Lamb, 1982). These large variations and trends in rainfall have attendant impacts on vegetation dynamics and availability of food and fiber for the people of the region. The peak of the cumulative decreasing rainfall trend resulted in a large-scale drought during the 1982–1985 period resulting in large-scale food shortages and famine (Olsson, 1993; Glantz, 1994). The development and prosperity of the region has therefore been largely dependent on fluctuations in rainfall through the historical times. Over the last several years, the region has experienced an increase in rainfall compared to the amounts in the late 1960s, prior to the onset of the prolonged drought in the 1970s (Bell et al., 2000; Hulme et al., 2001; Nicholson, 2001). In this paper, we utilize time series satellite vegetation measurements from National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) sensor to examine the variability and trends of land surface conditions in the Sahel as represented by vegetation index data from July 1981 to December 2003.
Section snippets
Data and analysis methods
Analysis of seasonal and interannual vegetation dynamics and trends of Sahel region is based the normalized difference vegetation index (NDVI) data derived from measurements made by the AVHRR sensor aboard NOAA polar orbiting satellite series (NOAA-7, 9, 11, 14, 16). This index is calculated from AVHRR measurements in the visible and infrared bands aswhere and are the surface reflectances in the 550–700 nm (visible) and 730–1000 nm (infrared) regions of the
Annual mean rainfall and NDVI patterns
Fig. 2a and b show the mean annual evolution of rainfall and NDVI, respectively. The rainfall data is based on Global Precipitation Climatology Project (GPCP) data which blends satellite rainfall estimate with gauge network data (Huffman et al., 1995). Data shown here is averaged for the latitude band range from 12°N to 16°N spanning the entire width of the region from 15°W to 40°E. This illustration provides a generalized overview of the annual evolution of the Sahel rainy season, showing that
Summary and conclusions
Satellite measurements of Sahelian vegetation dynamics during the last 23 years have provided a comprehensive picture of the patterns of land surface interannual variation and trends. The persistence and spatial coherence of drought conditions during the 1980s is well represented by the NDVI anomaly patterns and corresponds with the documented rainfall anomalies across the region during this time period. The prevalence of greener than normal conditions during the 1990s to 2003, follows a
Acknowledgements
This work was funded in part through GEST under NASA/GSFC Cooperative Agreement with University of Maryland, Baltimore County, NCC5-494 and is part a contribution to interagency program in support of United States Agency for International Development/Famine Early Warning System (USAID/FEWS) on Food Security and Environmental Monitoring. Robert Mahoney and Jennifer Small assisted with the programming and the contributions of Dan Slayback are acknowledged.
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