Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003

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Abstract

Remotely sensed measurements from NOAA-AVHRR expressed as normalized difference vegetation index (NDVI) have generated a 23-year time series appropriate for long-term studies of Sahel region. The close coupling between Sahelian rainfall and the growth of vegetation has made it possible to utilize NDVI data as proxy for the land surface response to precipitation variability. Examination of this time series reveals two periods; (a) 1982–1993 marked by below average NDVI and persistence of drought with a signature large-scale drought during the 1982–1985 period; and (b) 1994–2003, marked by a trend towards ‘wetter’ conditions with region-wide above normal NDVI conditions with maxima in 1994 and 1999. These patterns agree with recent region-wide trends in Sahel rainfall. However taken in the context of long-term Sahelian climate history, these conditions are still far below the wetter conditions that prevailed in the region from 1930 to 1965. These trend patterns can therefore only be considered to be a gradual recovery from extreme drought conditions that peaked during the 1983–1985 period. Systematic studies of changes on the landscape using high spatial resolution satellite data sets such as those from LANDSAT, SPOT and MODIS will provide a detailed spatial quantification and description of the recovery patterns at local scale.

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 asNDVI=(ρnir-ρr)/(ρnir+ρr),where ρr and ρnir 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.

References (59)

  • A. Giannini et al.

    Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales

    Science

    (2003)
  • M.H. Glantz

    Drought, desertification, and food production

  • Griffiths, J.F., 1972. Climates of Africa, World Survey of Climatology, vol. 10. Elsevier, Amsterdam, pp....
  • B.L. Henricksen et al.

    Growing period and drought early warning in Africa using satellite data

    International Journal of Remote Sensing

    (1986)
  • J.U. Hielkema et al.

    Rainfall and vegetation monitoring in the Savanna Zone of the Democratic Republic of Sudan using the NOAA advanced very high resolution radiometer

    International Journal of Remote Sensing

    (1986)
  • B.N. Holben

    Characteristics of maximum-value composite images for temporal AVHRR data

    International Journal of Remote Sensing

    (1986)
  • A.R. Huete et al.

    Investigation of soil influences in AVHRR red and near-infrared vegetation index imagery

    International Journal of Remote Sensing

    (1991)
  • G.J. Huffman et al.

    Global precipitation estimates based on a technique for combining satellite-based estimates, rain gauge analysis, and NWP model precipitation information

    Journal of Climate

    (1995)
  • M. Hulme et al.

    African climate change: 1990–2100

    Climate Research

    (2001)
  • C.F. Hutchinson

    Use of satellite data for famine early warming in sub-Saharan Africa

    International Journal of Remote Sensing

    (1991)
  • S. Janicot et al.

    Summer Sahel-ENSO teleconnection and decadal time scale SST variations

    Climate Dynamics

    (2001)
  • C.O. Justice et al.

    Monitoring the grasslands of the Sahel using NOAA AVHRR data: Niger 1983

    International Journal of Remote Sensing

    (1986)
  • C.O. Justice et al.

    The effect of water vapor on normalized difference vegetation index derived for the Sahelian region from NOAA AVHRR data

    International Journal of Remote Sensing

    (1991)
  • P.J. Lamb

    Sahelian Drought

    New Zealand Journal of Geography

    (1980)
  • P.J. Lamb

    Persistence of Sub-Saharan drought

    Nature

    (1982)
  • D. LeCompte et al.

    Wettest rainy season in 30 years across African Sahel. Special Climate Summary 94/2, 5

    (1994)
  • H.N. Le Houerou

    The rangelands of the Sahel

    Journal of Range Management

    (1980)
  • S.O. Los

    Calibration adjustment of the NOAA AVHRR normalized difference vegetation index without recourse to channel 1 and 2 data

    International Journal of Remote Sensing

    (1993)
  • A. Lotsch et al.

    Coupled vegetation-precipitation variability observed from satellite and climate records

    Geophysical Research Letters

    (2003)
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