Effects of the dry season on the vegetation canopy of some river basins of West Africa as deduced from NOAA-AVHRR data

Thirteen watersheds were selected in the Sudanese area of West Africa. Nine of them are situated in the transition zone between the Sudanese and the Sahelian area, three contributing to the Senegal river and six to the Niger river. The other four are included in the humid tropical area, facing the Atlantic Ocean. The vegetation status was characterized over the 13 river basins using the Normalized 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 characterizing 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. Depending on which climatological zone the river basins are in, they react in a specific way in terms of changes in the vegetation canopy status. Also radiometric behaviour in the middle thermal infrared band, channel 3 of AVHRR, supplies information on bush fire activity. This process is one of the main agents of vegetation degradation in this part of the world. Here again, results show an evident ecological zoning. The work presented here tries to show that low resolution satellite imagery can be highly efficient for the monitoring of vegetation response to seasonal factors on a regional scale. Vegetation dynamics or changes represent an indispensable set of data for hydrological modelling. They are an input for: (a) rainfall-runoff models; (b) ground water recharge studies; and (c) evaluation of evapotranspiration, soil erosion and sediment transport.