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Thirteen watersheds were selected in the Sudanese area of West Africa, nine of them situated in the transition zone between the Sudanese and the Sahelian area, the other four included in the humid tropical area, facing the Atlantic Ocean. The vegetation status was characterised over the 13 river basins using the Normalised Difference Vegetation Index (NDVI), computed from channels 1 and 2 of NOAA-AVHRR imagery, and the radiometric behaviour in the middle thermal infrared, channel 3. The frequency distribution of radiometric values, both NDVI and thermal infrared, within the watersheds clearly shows the effect of the dry season on the vegetation canopy. Thermal infrared data seem to be more efficient than NDVI data for characterising changes in density and vigour of the vegetation canopy, at least at that stage in the year. Channel 1 of AVHRR, being very sensitive to atmospheric conditions, introduces a masking effect on the NDVI temporal behaviour during the dry season. Also, radiometric behaviour in the middle thermal infrared band supplies information on bush fire activity, which is one of the main agents of vegetation degradation in this part of the world. The work presented shows that low resolution satellite imagery can be highly efficient for the monitoring of vegetation response to seasonal factors on a regional scale.

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Authors: GRÉGOIRE J, JRC Ispra (IT)
Bibliographic Reference: Article: Hydrological Sciences Journal, Vol. 35 (1990) pp. 323-337
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