Long and short term of Fitri lake (Chad) monitoring based on HR satellite imagery : an unique source of information for the lake study and management
Résumé
Lake Fitri, located in the semi-arid zone of Chad within the Sahelian beltand, is a flat-bottomed lake with high intra- and inter annual variability linked to the variability of the West African monsoon, which is expressed in Lake Fitri by large fluctuations in the surface area of open water and the depth of the lake, on all time scales (from one season to several millennia). High resolution satellite data have been exploited to monitor the water extent over the last decades. Based on the Landsat data it was shown that in recent decades, since the droughts of the 1970s-19890s, the areas of open water and swamps have doubled and stabilized since the early 2000s. For the more recent period, ie since 2017 it is Sentinel2 resources, allow an higher revisit time of 5 days and this with a better spatial resolution, that have been exploited. From about 340 cloud free images, lake water extent have been generated wand then combined in occurrence products allowing to highlight the functioning of the flood plain; t.he LWE generating process being validated by comparison with Pleiaides VHR images. This Sentinel2 time series complete and precise the long term monitoring. In 2020 and 2022 it was possible to catch two major floods, represented respectively 1250 and 1180 km2. Images analyses highlights also the functioning of a unknown (or forget in the memorynot documented) of a Northern drainage pattern that is active over a short period at the flood peak period, allowing the inundation of flat shallow areas on the N and NW of the Fitri lake. A combination of the LWE with Sentinel3 altimetry data allowed to derive hypsometric curves initiating altimetry work on the lake in early December 2022. More generally, such regular monitoring of Lake Fitri flooding using multi-source satellite data, i.e. SWOT plus Sentinel2, in addition to rainfall, would provide an excellent indicator of climate trends in this little-studied region, but also the evolution of resources in a context of strong anthropic pressure on an ecosystem rich in biodiversity.but also of the pressure on resources in the context of population growth.