Relationship between altimeter data and areal extent of water in the Lake Chad Basin

Yearly seasonal fluctuations occurring in the Lake Chad Basin are triggered by the river volume discharges from the Chari river system. The seasonal 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 methods. Remote sensing offers an alternative to examining seasonal lake fluctuations. The feasibility of relating lake radar altimeter data with satellite derived horizontal areal extent of water resulting from water level fluctuations is examined using TOPEX/POSEIDON altimeter data and spectrally unmixed ASTER images of June 2000 and December 2001. A correlation coefficient (r2= 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 images as known variables. The images were thresholded to display inundated areas only. Water endmember fractions were converted to ground measurements and changes in horizontal area of water between the two time periods were compared with the derived altimeter values. The areal extent of inundation in June and December were 529.91km2and 509.01 km2respectively with a net loss in surface areal extent of water of 21 km2. 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.