衛星データに見るチャド湖の水域および植物域の変化

抄録

Lake Chad is located in the Sahelian zone along southern boundary of Sahara desert. Expansion and reduction of water area were repeated several times since the prehistoric period. The authors found that the recent Lake Chad water area observed from satellite data is far smaller than that drawn on several maps. The authors investigate a change of Lake Chad water and vegetation area in recent ten and several years using LANDSAT and NOAA data. The mosaicked LANDSAT/MSS image of 1973 and seven NOAA/AVHRR data during the period 1982-1988 are obtained to analyze the distribution change of vegetation.
Water and vegetation area analyzed from the VI (Vegetation Index) and NVI (Normalized Vegetation Index) obtained from each satellite data shows the decreasing tendency year by year. The close relationship between this change and the recent climatic data, such as annual precipitation, relative humidity and atmospheric temperature was recognized. This result may contribute to a monitoring of the present desertification in Africa.
Concluding remarks;
1) The closed relationship was seen between the change of the area covered with water and/or vegetation and the annual precipitation change.
2) The area decrease corresponds to the decrease of relative humidity in the area and also the increase of the atmospheric temperature.
3) The decrement of the area reaches 40% during 10 years from 1973 to 1982. The area of water and vegetation in 1988 became 30% or less of that in 1973.
4) The recent Lake Chad is in a minimum size during the past 200 years.