Application of AVHRR to monitoring a climatically sensitive playa. Case study: Chott el Djerid, southern Tunisia.

The importance of monitoring changes in the levels of lakes within endorhei c basins using remotely sensed data as a means of assessing changes in regi onal aridity is noted. Large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patte rns, and which commonly do not have extensively managed catchments. To expl ore the application of high temporal frequency monitoring of salt playas us ing remote sensing, the Chott el Djerid, a large salt playa situated in sou thern Tunisia was targeted. A short time series of 39 Advanced Very High Re solution Radiometer (AVHRR; resolution 1.1 km at nadir) images of the Chott el Djerid (spanning 36 months between 1987 and 1990) were compiled along w ith climate information from a weather station at Tozeur. Using image histo gram manipulation, lake areas were extracted from the time series. A good l evel of agreement was observed between recorded rainfall events and the pre sence of surface water on the playa, and for a limited sample of large floo d events it was found that there were significant relationships between rai nfall, evaporation and estimated lake areas (r(2) = 98.5, p < 0.001). Overa ll, these data suggest that contemporary lake formation is largely controll ed by temporal changes in effective precipitation within the basin. In addi tion, it was found that the coefficient of variation of the time series, an d a combination of temporal reflectance profiles extracted from it, could b e used to give a direct indication of which sedimentary surfaces on the pla ya are affected by large flood events, and the extent to which these events may be preserved within the recent sedimentary record at these sites. Copy right (C) 1999 John Wiley & Sons, Ltd.