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

Abstract

The importance of monitoring changes in the levels of lakes within endorheic basins using remotely sensed data as a means of assessing changes in regional aridity is noted. Large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patterns, and which commonly do not have extensively managed catchments. To explore the application of high temporal frequency monitoring of salt playas using remote sensing, the Chott el Djerid, a large salt playa situated in southern Tunisia was targeted. A short time series of 39 Advanced Very High Resolution Radiometer (AVHRR; resolution 1·1 km at nadir) images of the Chott el Djerid (spanning 36 months between 1987 and 1990) were compiled along with climate information from a weather station at Tozeur. Using image histogram manipulation, lake areas were extracted from the time series. A good level of agreement was observed between recorded rainfall events and the presence of surface water on the playa, and for a limited sample of large flood events it was found that there were significant relationships between rainfall, evaporation and estimated lake areas (r2 = 98·5, p < 0·001). Overall, these data suggest that contemporary lake formation is largely controlled by temporal changes in effective precipitation within the basin. In addition, it was found that the coefficient of variation of the time series, and a combination of temporal reflectance profiles extracted from it, could be used to give a direct indication of which sedimentary surfaces on the playa are affected by large flood events, and the extent to which these events may be preserved within the recent sedimentary record at these sites.