RELATIONSHIP BETWEEN ALTIMETER DATA AND AREAL EXTENT OF WATER IN THE LAKE CHAD BASIN

摘要

Yearly seasonal fluctuations occurring in the Lake Chad Basin are triggered by the river volume discharges from the Chari river system. The seas onal inundation is an important economic activity as it provides impetus for fishing and recessional farming activities. Interannual lake fluctuations lead to dry and wet years with negative and positive impacts on the economic activities of the area. Monitoring the hydrological dynamics of the lake is difficult using conventional m ethods. Remote sensing offers an alternative to ex amining seasonal lake fluctuations. The feasibility of relating lake radar altimeter data with satellite derived horizontal areal exten t of water resulting f rom wate r lev el f luctuations is exam ined using TOPEX/POSEIDON altimeter data and spectra lly unmixed ASTER i mages of June 2000 and December 2001. A correlation coef ficient (r~2 = 0.8545) was established between Lake Chad altimeter data and Chari river volume discharge. The results were used to derive altimeter values of the lake, using the dates of the ASTER i mages as known variables. The im ages were thresholded to display inundate d areas only. Water endm ember fr actions were converted to ground m easurements and changes in horizontal area of water between the two tim e periods were compared with the derived altim eter values. The areal extent of inundation in June and December were 529.91km~2 and 509.01 km~2 respectively with a net loss in surface areal extent of water of 21 km~2. Altimeter values of -0.594 m and 0.63 m for June and December indicates an inconclusive relationship between altimeter data and horizontal extent of water. External factors such as farming and fallowing the interdunes are thought to modify the relationship between the horizontal areal extent of the inundation and the altimeter lake level data.