International audienceNumerous natural and anthropogenic factors have caused soil salinization, land surface degradation, and desertification in Keriya County in China's Xinjiang region. Information from multi-temporal remotely sensed data such as the Soil Salinity Index (SSI) has contributed significantly to an understanding of these environmental changes. The approach to calculating SSI is based on the spectral bands of Landsat Thematic Mapper (TM) or Enhanced Thematic Mapper Plus (ETM+). A soil salinity map of the Keriya County area was produced from Landsat ETM+ images, with an overall accuracy of 72.73% and kappa coefficient of 0.6689. The analysis of the recent evolution of the oasis of Keriya County was carried out by coupling climatic and socioeconomic data with information derived from multi-temporal remotely sensed data such as the SSI, Normalized Difference Vegetation Index (NDVI), and different land use classes. Such analysis appeared to be very useful in identifying and monitoring changes occurring in the oasis ecosystem and for understanding the consequences of human-induced land degradation processes
<p>The Sahel, located at the southern edge of the Sahara is defined as the transitional boundary of the Sahara, whose aridification since the late Holocene has profoundly affected landscapes, human occupation and ecosystems. Current climate change, characterized by an increase in temperature, raised the idea of aridification and the disappearance of surface water in the Sahelian zone. However, in central Sahel, the trend towards increasing open water surfaces seems to be confirmed over the last decade, particularly during extreme rainfall episodes. Here, we show the current changes of Lake Fitri, a remnant of the mega-lake Chad during the Holocene 6,000 years ago, whose seasonal flooding has reached unequaled levels in the last 50 years. This terminal and shallow lake and spreading out over a vast flat sedimentary plain irrigates a variable surface area. During exceptional rainy years such as the current one (2020-2021), the waters from the Batha river basin (main tributary of Lake Fitri), in the Sahelian zone, can reach the Chari catchment area (main tributary of Lake Chad) by passing through a succession of basins.</p><p>The diachronic analysis of Landsat images combined with ground-based observations recorded during the year 2020-2021 will be compared with the previous available time-series, in particular the monthly series already analysed for the year 2015 (Yalikun and al., 2019). The results confirm the trend towards the extension of the lake and the "outflow" of its waters to the west. These observations raise many questions related to the definition of a lake and its precise shoreline in this context. We will also specify the consequences in terms of the recomposition of the social-ecological system (changes in land use, recomposition of activity systems and access to natural resources, governance issues). Finally, we will discuss the timing of the changes observed in the short (annual, decade), medium (25 years), long and very long term (Holocene), on the comparability of these changes and on the elements to be considered in order to envisage the future evolution of such sahelian social-ecological system.</p>
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